Composition and Digestive-Tract Dynamics

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Carcass                 Composition and     Digestive-Tract   Dynamics
of      Northern          Pintails Wintering Along  the Lower     Texas
Coast
        BARTM. BALLARD,'                   ResearchInstitute,
                       CaesarKlebergWildlife                TexasA&M                    Kingsville, 78363, USA
                                                                      University-Kingsville,      TX
                 E.
        JONATHAN THOMPSON,2   CaesarKleberg        ResearchInstitute,
                                            Wildlife                TexasA&M                   Kingsville, 78363, USA
                                                                             University-Kingsville,      TX
        MARK PETRIE,
            J.      Ducks Unlimited, PacificNorthwest
                                   Inc.,                               WA
                                                       Office,Vancouver, 98683, USA

        Abstract
          Wecollected341 northern   pintails(Anas  acuta)alongthelowerTexascoast, USA,to investigate              of
                                                                                                         dynamics carcassanddigestive
          tractcomponents   duringwinter helpassess theability thisregionto support
                                         to                       of                      wintering pintailpopulations.       reliedmore
                                                                                                                       Pintails
          on endogenouslipidand proteinreservesduring             of                           to
                                                             winter a dryyear thana normal wet year.Carcassfat remained         relatively
                       the
          stableduring wet winter;    however,  pintailscatabolized               65%
                                                                    approximately of their reservesbetweenarrival October
                                                                                              lipid                       in         and
                   at
          departure theend of February            the
                                          during drywinter.     Somatic protein mass alsodeclinedoverbothwinters pintails
                                                                                                                    as        catabolized
          up to 20%of theirmusclemass. Gizzard     atrophy explained most of thechangesinsomaticprotein           the
                                                                                                            during wet winter,   whereas
                                                        to
          catabolism breastmusclealso contributed changesin proteinmass during drywinter.
                     of                                                                   the                        digestivetractmass
                                                                                                          Ingesta-free
          was greatestin earlyDecember,and then declinedabruptly       through  February  during  both winters.
                                                                                                              Pintailsdepartedthe lower
          Texascoast in lateFebruary                            in
                                      approximately lighter bodymass thanwhentheyarrived autumn.
                                                      20%                                             in        Mid-continentpintails
                                                                                                                                    may
          frequently to winter southerly
                    opt          in           latitudeswheretheycan maintain   minimal endogenousreservesdue to themoderate      climate,
          limitedhuman               and
                        disturbance, relatively                 but
                                                   dependable, oftenlower-quality resources.
                                                                                      food             However,         consequencesof
                                                                                                                potential
          pintails                         with
                          springmigration reducedenergyreservesincludegreater
                 initiating                                                                        on
                                                                                         reliance springstagingand breeding      areasto
          meet theirnutrient                                                             on
                             requirements migration reproduction, arrival breedinggrounds,and reducedsurvival
                                            for           and                later                                                   and
          reproductive success. Nutrient                    of
                                        reservedynamics wintering,     mid-continent         supporttheneed forenhancedconservation
                                                                                      pintails
          of productive                                                             It
                        springstaging and breedinghabitatsfor this population. also providesadditional          concern over the loss of
          productive wintering alongthe westernGulfCoast.(JOURNAL WILDLIFE
                              sites                                            OF           MANAGEMENT       70(5):1316-1324; 2006)

        Key words
          Anas acuta, carcass composition,digestivetract,LagunaMadre,lipids,northern      protein,Texas, winter.
                                                                                   pintail,

Body mass decline in birds duringwinter has been explained                 precipitation (Miller 1986, Whyte et al. 1986, Heitmeyer
by 2 competing hypotheses. The energy-deficit      hypothesis              1988, Smith and Sheeley 1993). The extent of body mass
suggests that mass loss is a result of environmentalfactors,               loss or the inabilityto build nutrient reservesduring critical
such as limited food availabilityor decreasingtemperatures,                periodsmay have immediateor cross-seasonalimpactson an
which requirethe use of endogenous reservesto compensate                   individual. In fact, Raveling and Heitmeyer (1989) found
for reductions in energy acquisition or increased energy                   that pintail productiondeclined following dry winters with
demands (Owen and Cook 1977, Peterson and Ellarson                         reduced food availabilityin California,USA. Stored lipids
1979, Kaminskyand Ryan 1981, Miller 1986). Conversely,                     provide insulation and energy reserves to wintering birds
the endogenous-rhythm   hypothesisinfers that an endogenous                and can influence the probability of surviving during
mechanism regulatesthe size of stored reservesto optimize                  extremely low temperaturesor periods of negative energy
energy expenditure and survival (Reinecke et al. 1982,                     balance (King 1972, Raveling 1979). Additionally, females
Williams and Kendeigh 1982, King and Murphy 1985).                         periodicallyuse lipid reserves acquired during winter and
Support for endogenous control of body mass in wintering                   spring migration to compensate for their inability to meet
waterfowl has frequentlycome from studies of captivebirds
                                                                           daily nutrient requirementsfor reproductionon the breed-
that underwentpredictablefluctuationsin body mass despite
                                                                           ing grounds (Krapu1981, Esler and Grand 1994, Hobson et
readily available access to high-quality foods (Perry et al.               al. 2005). Furthermore,male ducks may rely on stored lipid
1986, Loesch et al. 1992). However, the magnitude of                       reservesto provide energy necessaryto defend their mates
change in body mass or deviation from the typicalpatternis                 (Krapu1981).
likely the result of proximateenvironmentalfactors such as                   Somatic lipid and protein catabolism during winter may
winter habitat quality (King and Farner1966). For instance,
                                                                           delay initiation of certain annual cycle events, such as
annualvariationin body condition of winteringwaterfowlis
                                                                           prebasic molt (Richardson and Kaminski 1992), which in
often correlatedwith winter habitat conditions, particularly               turn may influence timing of subsequent events. Delayed
food availabilityand quality in relation to the amount of
                                                                           clutch formation by females in poor body condition can
1E-mail:bart.ballard@tamuk.edu                                             result in reduced egg mass, smaller clutch sizes, and lower
2
 Present address: Ducks UnlimitedCanada, Edmonton,AL                       reproductive success (Krapu 1981, Eldridge and Krapu
T5S 1J3, Canada                                                            1988). Therefore, winter body condition of waterfowl may

1316                                                                                                  The Journal Wildlife
                                                                                                                of                *
                                                                                                                         Management 70(5)
have immediate or cross-seasonal influence on waterfowl        mild autumn and winter temperatures, averaging 14.2'C
fitness (Heitmeyer and Fredrickson 1981, Krapu 1981,           with lowest temperatures typically occurring in late
Haramis et al. 1986, Hepp et al. 1986, Nichols and Hines       December to earlyJanuary(National Oceanic and Atmos-
1987, Raveling and Heitmeyer 1989, Hobson et al. 2005).        pheric Administration [NOAA] 1999). Mild temperatures
  The Texas coast, USA, winters up to 78% of northern          compounded with strong coastal winds promote high
pintails (Anas acuta) in the Central Flyway (U.S. Fish and     evaporationrates throughout most of the year and influence
Wildlife Service 1999) with most birds wintering in rice-      seasonal availabilityof wetlands. Annual rainfall averages
producing areas (Texas Parks and Wildlife Department,          about 67 cm (Brown et al. 1977), with most precipitation
unpublisheddata). Rice agriculture  providesreadilyavailable   occurringin April and September.However, tropicalstorms
and abundant,high-energy foods to waterfowl (Fredrickson       and hurricanes can have large impacts on precipitation
and Taylor 1982, Miller 1987) and rice fields provide          patterns and wetland habitat conditions. Precipitation was
importanthabitatto pintails in severalmajorwintering areas     markedlydifferent between the 2 winters of this study. In
(Miller 1986, Cox and Afton 1997). However, large declines     1997-1998, rainfall was 133% of normal along the lower
in rice acreagein Texas, as well as predictedfuture declines   Texas Coast (30-yr average;  NOAA 1997, 1998, 1999). The
(Alston et al. 2000), compoundedwith considerableloss of       5 climate stations along the coast ranged from 20-58%
freshwaterwetlands adjacent to the coast (Moulton et al.       above averageprecipitationduringApril 1997-March 1998.
1997) may be reducing the capacityof western Gulf Coast        April-March of 1998-1999 had averageto dry conditions as
wintering areas to support historical numbers of pintails.     precipitationaveraged83%of normal,with 9 months during
Becauseadjacent   winteringareas(e.g., coastalLa., USA) have   the period experiencing below-normal rainfall. Based on
also experiencedconsiderable  wetlandloss (Dahl andJohnson     rainfall patterns, we refer to 1997-1998 as the wet winter
1991), options for pioneering new wintering areas may be       and 1998-1999 as the dry winter.
limited. Based on availability and proximity to the rice
prairies,pintails may use coastal estuariesand lagoons to a
                                                               Methods
greaterdegree with conversionof the remainingrice prairies     We conducted fieldwork under protocol certified by Texas
to other land uses. However, forced emigrationof pintails to   A&M University-KingsvilleInstitutionalAnimal Care and
coastal habitats where birds consume foods that have poor      Use Committee (approvalno. 1-97-41) and under federal
nutritional characteristics(Ballard et al. 2004) may have      (no. MB810027) and state (no. SPR-0697-888) scientific
unforeseen effects on survival and fecundity. Similarly, if    collection permits. We collected pintails from October
coastalhabitatsdo not providethe qualityor quantityof foods    through February1997-1998 and 1998-1999 throughout
necessary to maintain wintering pintails in optimal body       the Laguna Madre by shooting at estuarinefeeding sites or
condition, birds wintering in Texas may experience lower       along traditional flight corridors to avoid potential biases
survivalor greaterreductionsin fecundity,which will further    associatedwith collecting birds over decoys (Greenwood et
contributeto continentalpopulationdeclines in this species.    al. 1986). Although pair status may explain some of the
  The objective of this study was to investigate body mass,                in
                                                               variability body condition of winteringwaterfowl,we were
carcasscomposition, and digestive tract dynamicsof north-      unable to determinepair status for a large proportionof the
ern pintailswintering along the lower coast of Texas to help   birds sampled because of collection methods. Given the
assess the ability of this region to meet nutrient require-    open, expansivemudflatsthroughoutthe LagunaMadre, we
ments of this species during winter.                           collected 84% of the birds by pass shooting because of the
                                                               inability to approachforaging birds in most areas.
Study Area                                                       We weighed each specimenimmediatelyaftercollection to
We evaluatednutrientreservedynamicsof northernpintails         determine fresh body mass (1 g) and we took a series of
in the Laguna Madre along the lower Texas coast. The           external structuralmeasurementsto correct carcasscompo-
Laguna Madre is a shallow (generally <1-m-deep) coastal        nents for body size (Ankney and Afton 1988, Ankney and
lagoon that extends approximately208 km from Corpus            Alisauskas 1991). Measurements (0.01 mm) included bill
Christi Bay to Port Isabel. It ranges from 5-8 km wide.        width at widest point of the premaxilla,centralculmen from
Freshwaterinflow from mainland drainagesis limited and         intersection of skin and premaxillato the tip of bill nail,
evaporationtypicallyexceedsprecipitation,often resultingin     diagonal culmen from proximal tip of the posterior lateral
hyper-saline conditions. Salinities are generally >35 parts    lobe of the premaxillato bill nail, skull length from external
per thousand (ppt), but vary seasonallyand can reach >50                              to
                                                               occipital protuberance tip of bill nail, wing chord (1 mm)
ppt (McMahan 1968). The Laguna Madre has vast                  from wrist on bent wing to tip of the most distal primary,
meadows of sea grasses,with shoalgrass(Halodulewrightii)       tarsus length from proximal to lateral condyles of the
dominatingin most areas(Onuf 1996). Freshwater     wetlands    metatarsus, and middle toe length from base of nail to
adjacent to the Laguna Madre are important sources of          junction with metatarsus. We measuredkeel length from tip
dietary freshwater for waterfowl foraging in the lagoon        of cranialprocess to end of medial caudalprocess following
(Adair et al. 1996); however, during dry winters freshwater    removal of half of the pectoralis muscle during the final
can be limited and spatiallyconcentrated.                      necropsy.
  The climate of the region is semi-arid to subtropicalwith      We examined each specimen for contour plumage molt in

Ballard al. * Carcass Composition Pintails
      et                        of                                                                                     1317
9 major plumage regions comprised of 34 feather tracts                                                    or
                                                             Table 1. Sex and age (afterhatch year [AHY] hatch year [HY])
                                                             distribution northernpintailscollected from the southern Texas
                                                                        of
modified from Billard and Humphrey (1972). We assessed                         Oct-Feb 1997-1998 and 1998-1999.
                                                             Coast, USA,during
molt intensity using a grab-sampletechnique to expose each
feather sheath to determine the proportion of new feather                                         Month

growth (Titman et al. 1990). We identifiedgrowing feathers        Year     Sex Age Oct Nov Dec Jan Feb Total
as those with blood present in the calamus, or those in
                                                               1997-1998 F AHY 4             10      7    10     3    34
which only the emerging feather sheath was present. We                            HY 0         5     9     6     1    21
calculated molt intensity for each feather tract as the                     M AHY 7           15    19    12     7    60
percentageof incoming feathersfrom each grab-sample.We                            HY 2         7     7    11     3    30
determined molt intensity for each plumage region by           1998-1999 F AHY 2             20     20    21    10    73
                                                                                  HY 0         4     8     2     5    19
averagingacross all feather tracts comprising a region and                  M AHY 5          21     26    21   20     93
calculatedtotal molt scores as the averageintensity of the 9                      HY 1         2      1    2     5    11
plumage regions.
  We necropsied pintails to evaluate digestive-organ and
other muscle-mass dynamics. We determined age (i.e.,         score (PC1) of the correlationmatrixcan be interpretedas a
                                                             measureof body size with positive scores indicating above-
hatch-year vs. after-hatch-year)and sex of each pintail by
plumage characteristics (Carney 1992)      and corroborated average body size and negative scores indicating below-
with characteristics the bursaand gonads. We excised and
                     of                                      averagebody size (Pimental 1979, Alisauskas and Ankney
weighed (0.01 g) the left breast muscles (pectoralis     and 1987). To determine if a relationship existed between
                                                             carcasscomposition variablesand body size, we individually
supra-coracoideus), leg muscles (muscles attachedto the
                      left
tibiotarsus and metatarsus),and heart to examine somatic     regressedlipid, protein, and ash content (PROC REG; SAS
                                                             Institute 1999) on PC1. A carcasscomponent is influenced
protein dynamics.We removed and dissected the digestive
tract into the upper digestive tract (UDT; esophagus and     by body size if a significantrelationship(P < 0.05) is found
                                                             between the variable and PC1. For carcass components
proventriculus),gizzard, small intestine, caeca, and large
intestine. We determined lengths (1 mm) of the UDT,          influenced by body size, we used residuals from the
 small intestine, caeca, and large intestine on unstretched regression equations to derive a new value adjusted for
                                                             structuralsize. We generatedvalues correctedfor structural
 digestive tract components before removal of ingesta to
 reduce variation in measurementsassociatedwith elasticity   size using methodology describedby Ankney and Alisauskas
 of these organs. We measured mass (0.01 g) of each          (1991). Body, carcass,ash, and protein masses were related
                                                             to body size and required use of corrected values in
 digestive tract organ with its contents after removing any
 adhering fat, then emptied, washed, and patted dry each     subsequent analyses. Carcass fat was marginallyrelated to
                                                             structuralsize of males during the wet winter (P = 0.047).
 organ with a paper towel before reweighing. We removed                                                                size
 the liver and pancreasfrom the carcassand weighed them      However, correctingsomaticlipid reservesfor structural
                                                             may   be an over-correctionbecause the absolute mass of a
 separately(0.01 g). Following necropsies, we returned all
 excised organsand fat to the body cavity and froze them for bird'ssomaticlipids representsits usablelipid reserve,which
 later carcasscomposition analyses.                          may not have a relationshipwith body size (see Sedinger et
                                                             al. 1997). Thus, we used uncorrectedvalues of carcassfat in
   Subsequently, thawedand pluckedpintails.We weighed
                  we
 and chopped the plucked carcassinto approximately    2-cm-  subsequentanalyses.
 cubepieces and oven-driedto constantmassat approximately      We assessedscatterplots of all carcassand digestive-tract
 800C (Kerr et al. 1982). We weighed and then ground the     components across julian date for linear and nonlinear
 dried carcass to powder in an electric coffee grinder and   trends. Subsequently,we investigated trends in carcassand
 mixed by hand to ensure homogeneity. We defined carcass digestive tract parametersby sex and year using regression
 water as fresh body mass (excluding wet feather mass and     analysis(PROC REG; SAS Institute 1999).
 gastrointestinal contents) minus carcass dry mass. We        Results
 determined carcass composition by lipid extraction with
                                                 (Dobush et  Body Mass and Composition
 petroleumether in a modified Soxhlet apparatus              We collected 260 after-hatch-year(AHY) and 81 hatch-
 al. 1985), followed by ashingin a mufflefurnaceto determine
 protein and mineralcontent (Ankney and Afton 1988).         year (HY) pintails from the Laguna Madre during 1997-
                                                              1998 and 1998-1999 (Table 1). Because 45% of the
 Statistical Analysis                                         monthly sample sizes of HY birds were <2, we omitted
 Carcass components can show significant intraspecific all HY individualsfrom our analyses;thus, all resultsreferto
 variation in response to structural size (Alisauskas and     AHY birds (ad). Furthermore,there was no relationship(r2
 Ankney 1987, Ankney        and Afton 1988, Ankney and        < 0.09, P > 0.217) between carcassfat or protein and molt
 Alisauskas1991). To explain additionalvariationin somatic    scores of adult male or adult female pintails during any
 nutrient reserves, we used principal component analysis month by year combination.
 (PROC PRINCOMP; SAS Institute 1999) on the 8                   Body mass.-Ingesta-free body mass of male and female
 morphological measurements to correct nutrient-reserve pintails varied by month (F > 9.21, P < 0.001) and
 values for structural size. The first principal component    exhibited similar patterns between years (F < 1.36, P >

1318                                                                                 The Journal Wildlife
                                                                                               of                *
                                                                                                        Management 70(5)
                   1200                                                   250
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                                     fo n .   t2                                                         t-
                                                                                 -
                   10000
             u'     900                                                   150
             E
             T      800                                                   100-
                    700
                    600 A                                                  50        B
                    500                                                     0
                             OCT    NOV            DEC   JAN       FEB               OCT   NOV           DEC   JAN   FEB



                   30                                                     180
                   25
                20 _-170                                                  160
               20                                                         150 -
             E 15 -140                                         •

                  S10                                                     130-
                                                                          120-
                    5                                                                D
                                                                          110-
                    0.                                                    100
                            OCT    NOV             DEC   JAN       FEB               OCT   NOV           DEC   JAN   FEB

Figure 1. Trendin ingesta-freebody mass (A),somaticfat (B),percentfat (C),and somatic protein(D)of adultmale (0) and female(0) northern
       collectedalongthe lowercoast of Texas, USA,during (1997-1998:- - - - -) and dry(1998-1999:
pintails                                                 wet                                                  ) winters.


0.283). Pintails were heaviest during arrivalin October and                Somatic mineral.-Carcass ash remained stable during
body mass declined throughout winter (Fig. 1A). During                   the wet winter for adult females (r2 - 0.04, n - 34, P
the wet winter, averagebody mass was 12%(104.7 g) lighter                0.282), however,there was a moderatedeclining trend in the
for females and 15%(138.3 g) lighter for males in February               drywinter (r2- 0.07, n = 73, P= 0.027). Carcassash did not
                                                                                        =
than in October (r > 0.13, P < 0.040). During the dry                    display any   trends for males either year (P > 0.248). On
winter, body mass declined at a higher rate than during the              average, adult males contained 22% more carcassash than
wet winter as male (216.5 g) and female (168.8 g) pintails               adult females.
departedthe lower Texas coast about 20%lighter than their                  Somatic protein.-Both males and females catabolized
average arrival body mass (r2 > 0.44, P < 0.001). On                     proteinthroughoutwinter eachyear(Fig. 1D). Adult females
average, females were 58 g lighter and males were 89 g                   exhibitedsimilartrendsin carcass  proteindynamicseachyear,
lighter by the end of Februaryin the dry winter than in the              catabolizing  15% (wet winter: r2-0.24, n- 34, P= 0.003)
wet winter. Males were approximately22% heavier than                     and 19% (drywinter: r2 - 0.40, n = 73, P < 0.001) of their
females in October each year and 19% heavier in February.
                                                                         protein between October and the end of February.Adult
  Somatic fat.-Average lipid content differed by 18%                     males also catabolizedcarcassprotein throughout winter in
between years for females (wet winter = 164.4 g; drywinter
                                                                         both years (Fig. 1D). However, adult males retainedgreater
= 135.0 g; P -0.042), and by 28% for males (wet winter =                 lean mass throughoutthe wet winter, catabolizingonly 9%of
204.4 g; drywinter= 147.2 g; P < 0.001), but therewere no
                                                                         their protein mass, whereas during the dry winter they
year by month interactions (F < 2.33, P > 0.060). We                     catabolized 17% (Fig. 1D). On average,adult males carried
detected no trends in somatic fat for adult males or females
                                                                         20% more ash-free lean mass than females.
during the wet winter (r2 < 0.08, P > 0.109; Fig. 1B),
                                                                           Changes in mass of breast and leg muscles, gizzard, and
although there was a single female with extremely low fat                heart explained >69% of the change in somatic protein of
reservesin late October of the wet winter that had a large
effect on this result.When this individualwas removedfrom                pintails. Gizzard mass had the largest influence on changes
the analysis,there was a significant decline in lipid reserves           in protein mass for both males and females during the wet
of female pintails even during the wet winter (r2 = 0.18; P=             winter (i.e., had the most significant coefficient for the 4-
                                                                         variablemodel), whereas, atrophy of breast muscle had the
0.013). In the dry winter, pintails catabolized>63% of their
lipids by the end of Februaryas somatic fat declined for                 largest influence during the dry winter. During both years,
males (r2- 0.34, n = 93, P < 0.001) and females (r2=0.27,                each of these muscle groupsdisplayedsignificantcoefficients
n - 73, P < 0.001). Percent body fat remained relatively                 for carcassprotein.
stable during the wet winter for each sex, averaging                       During the wet winter, breast and heart mass remained
approximately   24% of carcassmass in October and dropping               stable acrosswinter for males and females (r2 < 0.05, P >
to about 21% by the end of February(Fig. 1C). During the                 0.082). However, mass of breast muscles decreased (r2 >
drywinter, %fat declined from about23% of carcassmass at                 0.31, P < 0.001) by approximately18% in both males and
arrivalto <13% by the end of February(r2 < 0.27, P <                     females by the end of Februaryin the dry winter (Fig. 2A).
0.001). In both years,females generallymaintaineda greater               Similarly,heart mass declined by 21% for females (r2= 0.11,
percentageof body fat than males throughout winter.                      n = 73, P= 0.004) and by 10%for males (2-= 0.05, n = 93, P

Ballard al. * Carcass Composition Pintails
      et                        of                                                                                              1319
                 110                                                           10

                 100l                                                          -Mi9
                                                                               9w                     mm
                  90
                                                                               81
            cn 80
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                               7o-
                S7700
                  560     A                                                    6-       B
                  50 1                                                         5
                          OCT . NOV               DEC         JAN   FEB                 OCT        NOV           DEC       JAN       FEB



                 24                                                            70
                 23-                                                           60
                                                                               6              w-
                 22 -
            co 21                                                    -a4mom
            E 20                                                              40
               19                                                             30
            '3 182
               17        c
                         C20                                                            D
               16'                                                             10
               15                                                                   -
                                                                                              I             I          I         I
                         OCT ..0 NOV             DEC          JAN   FEB                 OCT        NOV           DEC       JAN       FEB

Figure 2. Trendin mass (g)of breastmuscle(A),heart(B),leg muscle(C),and gizzard of adultmale(0) and female(0) northern
                                                                               (D)                                          collected
                                                                                                                     pintails
alongthe lowercoast of Texas, USA,during (1997-1998:- - - - -) and dry(1998-1999:-
                                          wet                                               ) winters.

= 0.027) between October and the end of Februaryduring                        Gut Morphology
the dry winter (Fig. 2B).                                                     Morphology of the digestive tract varied moderately
  Leg muscle mass did not exhibit a trend for males in the                    throughout winter for females, and more notably for males
wet winter (r2= 0.02, n= 60, P= 0.279), but decreasedfrom                     (Tables 2 and 3). Females displayed little variation in gut
19.2 g to 16.4 g (15%; r2 = 0.17, n - 34, P- 0.015) for                       morphology during the wet winter. Mass of the total
females (Fig. 2C). During the dry winter, leg muscle mass                     digestive tractexhibiteda quadratictrend for females during
reduced by 21% in females (r2 - 0.17, n - 73, P < 0.001)                      the dry winter and for males during both winters. Changes
and by 15% in males (2 - 0.18, n - 93, P < 0.001).                            in gizzard mass appearedto have the greatest influence in
  A quadratic function best described gizzard mass (Fig.                      the decline in mass of the total digestive tract for females,
2D). Increases in gizzard mass from October to early                          however, reductions in caeca and large intestine mass also
December occurredfor both sexes, followed by a significant                    contributedto the decline (Table 2). Length of the female
decline through February.Males showed strong declines in                      digestive tract increased during early winter, and then
gizzard mass both years (r2 > 0.43, P < 0.001). Gizzard                       decreasedthrough February    during the dry winter. Changes
mass in females was relativelystable in the wet winter (r2=                   in length of the gizzardand caecawere primarilyresponsible
0.15, n = 34, P= 0.078) and declined in the drywinter (r2 =                   for this quadratictrend. The pancreasand liver exhibited no
0.13, n = 73, P= 0.008).                                                      trends in mass either year for females.

                    between digestivetractorgansand Julian
Table 2. Relationship                                     date forfemalenorthern
                                                                               pintails        winter
                                                                                      throughout     alongthe lowercoast of Texas,
USA,during1997-1998 (wetyear)and 1998-1999 (dryyear).
                                               Wet year (1997-1998; n = 34)                                 Dry year (1998-1999; n = 73)

                                        Regression     line          r        P-value                Regression     line              r2     P-value

 Mass (g)
  Totaldigestivetract                66.7 -    0.09x                0.057      0.167              48.9 +   1.18x - 0.002x2           0.119    0.013
  Upperdigestivetract                 5.4 +    0.07x - 0.001x2      0.169      0.057               4.8 +   0.02x - 0.001x2           0.054    0.143
  Smallintestine                     13.4 -    0.02x                0.078      0.109              11.6 -   0.01x                     0.013    0.350
  Caeca                               1.2 +    0.0002x              0.001      0.909               0.9 +   0.01x - 0.0001x2          0.120    0.012
  Largeintestine                      1.5 -    0.002x               0.032      0.308               1.3 -   0.002x                    0.080    0.016
  Pancreas                            2.3 +    0.004x               0.064      0.148               2.3 +   0.002x                    0.045    0.072
  Liver                              17.5 -    0.01x                0.039      0.265              17.6 +   0.01x                     0.005    0.551
 Length(mm)
  Totaldigestivetract                1,882 +   1.61x                0.015      0.491          1,886 +      7.20x - 0.054x2           0.006    0.531
  Upperdigestivetract                  263 -   0.25x                0.238      0.003            267 -      0.06x                     0.028    0.157
  Smallintestine                     1,324 +   0.71x                0.007      0.644          1,267 +      5.37x - 0.039x2           0.074    0.071
  Caeca                                272 +   0.10x                0.013      0.522            231 +      1.52x - 0.011x2           0.146    0.004
   Largeintestine                     77.1 +   0.04x                0.019      0.443           75.7 -      0.03x                     0.012    0.365
  Gizzard                             61.2 -   0.04x                0.064      0.148           55.9 -      0.05x                     0.087    0.012


1320                                                                                                                      of
                                                                                                                The Journal Wildlife        *
                                                                                                                                   Management 70(5)
Table 3. Relationship
                    between digestive-tract
                                          organs and Juliandate for male northern
                                                                                pintails        winteralongthe lowercoast of Texas,
                                                                                       throughout
USA,during1997-1998 and 1998-1999.
                                     Wet year (1997-1998; n = 60)                          Dry year (1998-1999; n = 93)
                                Regression line           r          P-value         Regression line           r*         P-value
  Mass (g)
   Totaldigestivetract      89.9 + 0.17x - 0.004x2      0.493        <0.001       64.4 + 0.51x - 0.005x2     0.411        <0.001
   Upperdigestivetract      8.5 + 0.04x - 0.001x2       0.350        <0.001       6.2 + 0.04x - 0.0003x2     0.107         0.006
   Smallintestine           16.1 - 0.04x                0.172         0.001       14.4 - 0.02x               0.076         0.007
   Caeca                    1.6 - 0.003x                0.100         0.015       1.4 - 0.0003x              0.001         0.712
   Largeintestine           2.0 - 0.005x                0.166         0.001       1.7 - 0.003x               0.125         0.001
   Pancreas                 2.9 - 0.001x                0.003         0.683       2.9 - 0.002x               0.013         0.283
   Liver                    25.5 - 0.07x                0.280        <0.001       21.5 + 0.005x              0.002         0.643
  Length(mm)
   Totaldigestivetract      2,234 - 0.72x               0.004         0.636       2,196 + 1.47x              0.079         0.007
   Upperdigestivetract      292 - 0.08x                 0.021         0.279       310 - 0.10x                0.061         0.017
   Smallintestine           1,482 - 0.36x               0.001         0.782       1,454 + 1.36x              0.090         0.004
   Caeca                    309 - 0.28x                 0,018         0.306       287 + 0.28x                0.065         0.014
   Largeintestine           80.8 + 0.07x                0.013         0.399       78.1 + 0.02x               0.006         0.482
   Gizzard                  68.3 + 0.07x - 0.002x2      0.464        <0.001       60.4 + 0.22x - 0.002x2     0.424        <0.001


  Males displayed greater changes in gut morphology over            for spring migration in California (Miller 1986, Heitmeyer
winter than females during both years. Total digestive tract        1988).
mass was greatest during late autumn and declined abruptly            Whether body condition of pintails wintering along the
from early December through Februaryduring both years.              lower Texas coast was diminished enough to influence
All digestivetractorganswere lighterby the end of February,         survivalor reproductiveremains uncertain.Female pintails
except caeca in the dry winter (Table 3). Mass of the upper         rely on endogenous reservesaccumulatedduringwinter and
digestive tract and gizzardfollowed a similarquadratictrend         spring migration to meet nutrient requirements for
present in the total digestive tract each year. Small intestine,    reproduction(Krapu1981, Esler and Grand 1994). There-
caeca(wet winter only), and largeintestine of malesexhibited        fore, departing wintering grounds with reduced nutrient
linear reductionsin mass throughoutwinter (Table 3). Mass           reservesmay influence their ensuing reproductivesuccess if
of the pancreasremained stable throughout winter during             reliance on spring habitats is high and conditions of these
both years.Livermassdeclinedconsiderably      from Octoberto        habitats are poor. Additionally, prolonged migration that
the end of Februaryin the wet winter, but remained stable           results in later arrivalon breeding areas and delayed nest
during the dry winter. Total digestive tract length did not         initiation may influence breeding propensity and, for birds
change during the wet winter, but increased in length               that opt to nest, it likely reduces reproductivesuccess. It is
throughout the dry winter. The increase in length was               well establishedthat later nest initiations negativelyimpact
attributableto increasesin small intestine and caecalengths.        reproductivesuccess in northern pintails through progres-
The upper digestive-tractlength exhibited a declining trend         sively smallerclutch sizes (Flint and Grand 1996, Guyn and
during the dry winter, and the gizzard displayeda quadratic         Clark 2000), lower nest success (Flint and Grand 1996),
trend, peaking in length during mid-winter and declining            reduced brood survival (Guyn and Clark 1999), and a
considerablythrough February.                                       reducedpropensityto renest (Grand and Flint 1996). Most
Discussion                                                          pintails departedthe lower coast of Texas by early March,
                                                                    apparently earlier than pintails leaving California (Miller
Pintails catabolized lipid and protein across winter and            1986), the SouthernHigh Plains (Smith and Sheeley 1993),
departed the lower Texas coast with reduced nutrient                and the Rice Prairieregion immediatelynorth of the lower
reserves.Declining somatic nutrient reservesover winter is          coast of Texas (B. Ballard, unpublished data). Deficient
apparentlya consistent patternof nutrient-reserve dynamics          endogenous reserves and the reduction of flight range
for pintails throughout the mid-continent region (Thomp-            capabilitiesmay stimulate earliermigratorymovements and
son and Baldassarre1990, Smith and Sheeley 1993), but is            require a more protracted migration to include more
inconsistentwith pintails wintering in the Central Valley of        frequent stops to rebuild somatic reserves. Most pintails
California (Miller 1986). Pintails wintering in the Central         wintering in Texas probably do not experience major
Valley exhibited a pattern of mid-winter declines in body           ecological barriers (e.g., mountain ranges) during spring
mass followed by building of endogenous reservesprior to            migration and also have the opportunity to exploit
departure in spring. Thus, pintails depart coastal Texas            numerousstopover habitatsbefore arrivalon breeding areas
wintering areasweighing approximately    20% (-200 g) less          (Pedersonet al. 1989). Therefore, assumingthat these birds
than pintails departingwintering areasin California(Miller          can arriveon breeding areas in a timely manner and with
1986). Endogenous nutrients, particularly lipids, were              sufficient endogenous stores, there may be no advantageto
reduced at a time when birds increasedthem in preparation           build and maintain large nutrient reservesprior to migra-

Ballard al. * Carcass Composition Pintails
      et                        of                                                                                           1321
tion. The difference in winter fat accumulationfor pintails       priorto migratorymovementsoccurin preparation long- for
in California may be due to different migration strategies        distance movements or in response to deposition of
(e.g., possibly longer, nonstop flights across portions of the    premigratoryfat (Evans and Smith 1975, Gaunt et al.
Pacific Ocean or mountainous terrain in route to breeding         1990). The reductionin breast-musclemass during the dry
areas in Alas., USA, or western Canada). Additionally,            winter prior to departure further suggests that pintails
climatic conditions along the more northerly migration            probablydid not make large-scalemovements after depart-
routes in western North America are likely less predictable,      ing the lower Texas coast. Pintails departingwintering areas
making foragingopportunitiesless certain.Therefore,larger         along the lower Texas coast may rely on rice prairiehabitats
nutrient reserves are necessary to provide energy at times        immediately north along the central Texas coast to
when migratory stopover sites may be limited or widely            accumulateenergy prior to further migratorymovements.
dispersed or less likely to provide adequate foraging               Although the lower coast of Texas appearsto provide a
conditions. However, because carrying large reserves will         low-energy diet for winteringpintails (Ballardet al. 2004), it
increasethe transportcost, birds that have the opportunity        has other characteristicsthat would seem attractive to
to stop frequentlycan reducethe energeticcost of migration        wintering waterfowl, which may compensate for its
becausethey are able to cover shortersegmentswith smaller         relativelyenergy-poorfoods. For instance, the considerable
reserves(Alerstam et al. 2003).                                   size (200 km long and 5-13 km wide; Cornelius 1977) and
  Pintails in California (Miller 1986) and the Southern           shallow depths of the Laguna Madre provide abundant
High Plains (Smith and Sheeley 1993) did not catabolize           foraging and roosting habitat. Additionally, the primary
protein and relied extensivelyon stored fat duringwinter. In      food of pintailsin this region (shoalgrass)remainsstable and
contrast, pintails along the Texas Coast catabolized              predictable from year to year relative to foods in highly
significant amounts of protein during both wet and dry            dynamic freshwatersystems. Further, the moderate winter
winters. Protein catabolismmay be a responseto ameliorate         climate reduces the probabilityof cold stress and possibly
energy balance or to provide necessary amino acids when           eliminates the need for maintaining larger lipid stores that
protein intake is inadequate(Kendallet al. 1973). However,        are typical of birds wintering in more unpredictable
based on protein content of the winter diet of pintails in this   environments (Evans and Smith 1975). Disturbance and
region (Ballard et al. 2004) and the amount of protein            hunting pressure are also relatively light along the lower
needed to meet daily maintenancerequirements(2.03-2.88            coast of Texas compared to adjacent regions (Texas Parks
g based on equationsfrom Robbins 1993), pintails along the        and Wildlife Department, unpublished data), primarily
lower Texas Coast would only be requiredto consume 12.3-          because of large private land holdings that limit access.
33.6 g of food to meet daily maintenance requirements.            Therefore, pintails along the lower Texas coast may employ
Thus, protein catabolism was probably not in response to          a strategy to sustain themselves in a highly predictable
 amino acid deficiencies, but appearedto be in response to        environmentthroughoutwinter without accumulatingheavy
the negative energy balance. Further, pintails catabolized        fuel loads, thereby reducing their energy demands and risk
somatic fat and protein at relativelyconstant rates through-      of depredation.
out winter resulting in percent fat remaining stable.                Most of the variation in the digestive tract relates to a
 Reducing lean body mass concurrent with fat stores may            decreasein mass from earlywinter to the end of February.
be a strategyto maintain the effectivenessof the fat reserve      Atrophy of the gizzard explained much of the decrease in
(Reinecke et al. 1982). Advantages of reducing body mass           digestive tract mass, particularly females.The proportion
                                                                                                    for
include reducedenergy demandsfrom lower basal metabolic            of seeds in the diet can influence gizzard mass because
costs, lower activity energy expenditureto carrythe smaller        increased grinding action is required to breakdown hard
body mass, and potentiallyreducedrisk of predationbecause          seed coats (Thompson and Drobney 1996). However, seed
 of less time spent foraging to meet their energy needs            consumptiondid not decreasebetween winter and springfor
 (Biebach 1993). Further, reduced body mass may be a               either sex; thus, it cannot explain changes in digestive tract
 strategyfor some avian species to achievegreatermaneuver-         mass. Hypertrophy of the digestive tract can improve
 ability (relative to carrying heavy fuel loads) in order to       protein assimilation of the diet (Reinecke et al. 1982,
 decreasesusceptibilityto predators(Lima 1986).                    Austin and Fredrickson 1987, Thompson and Drobney
   The gizzard explained much of the reduction in protein          1996), but digestive-tractmass decreasedat a time when we
 during the wet winter;however,diet qualitydid not change,         expected birds to improve digestive efficiency.
 again supporting the notion that pintails were using this           Diet quality declined from winter to spring (Ballardet al.
 protein store to meet energy demands (Korschgen 1977).            2004), which was opposite of the expected trend during the
 Breast muscle had the largest influence on reductions in          dry winter if diet quality alone was influencing changes in
 protein during the dry winter, even though gizzard mass           gut morphology. Changes in diet quality (i.e., digestibility)
 declined more than in the wet winter. Greaterreductionsin         or volume of foods consumed can influence digestive-tract
 breast, leg, gizzard, and heart masses during the dry winter      morphology(Moss 1974, Ankney 1977, Kehoe and Ankney
 suggest that pintails were distributing protein catabolism        1985, Ankney and Scott 1988). In general, diet quality has
 across muscles and organs, possibly emphasizingthe degree         an inverse relationship to gut length and mass, whereas
 of negative energy balance. Increasesin flight-muscle mass        quantity of food consumed has a direct relationship.

1322                                                                                                of
                                                                                          The Journal Wildlife        *
                                                                                                             Management 70(5)
Therefore, it appearsthat diet quality did not play a major               dynamics of pintails in southern Texas, protection and
role in decreases in gut mass from December through                       management of freshwaterwetlands along the Texas coast
February.                                                                 would benefit this species. Providing a better-qualitydiet in
  The increase in gut length of males concurrent with                     areaswhere diet qualityis relativelypoor or providingenergy
reductions in digestive tract mass during the dry winter is
                                                                          during late winter when reductions in somatic fat and
puzzling and must have been the result of longer, thinner-                protein occur will enable pintails to rebuild endogenous
walled digestive tracts.The increasein length correspondsto               reservesto provide energy and nutrients for migration and
considerablereductionsin diet quality and may have been a
                                                                          reproductiveactivities.Protection and managementof these
combination of reducing mass while attempting to increase
                                                                          areasis particularly importantif they are criticalfor pintails
digestive efficiency. Because smaller organs reduce energy
                                                                          wintering along the lower Texas coast to acquire nutrients
expenditure,it is adaptivefor birds to maintain the smallest
functional organ size possible (Moss 1974). Mass of upper                 prior to making more extensive movements to northern
                                                                          staging and breeding areas. Further, the availability of
digestive-tract contents increased considerably in male
                                                                          quality habitat on wintering grounds may also partially
pintails from winter to spring during the dry winter; thus,
                                                                          mitigate for loss of habitat or drought on migratoryroutes.
correspondingincreases in total digestive-tract length was
probably due to increased food consumption. However,                      Acknowledgments
nutrient reserves continued to decline through the end of
February,and potential increasesin food consumption did                   Ducks Unlimited, Inc., the Caesar Kleberg Wildlife
not appearto influence these trends. Diet qualitywas likely               Research Institute, and Texas A&M University-Kingsville
so poor in late winter (Ballard et al. 2004) that increased               provided support for this project. We thank the King
food consumption was not enough to result in a positive                   Ranch, Inc., and Kenedy Ranch for allowing access to their
energy balance.                                                           properties. We are grateful to R. Heilbrun, S. Lee, J.
                                                                          McCloskey, and S. Perez for laboratoryassistance.We are
Management Implications                                                   indebted to R. Ballardfor dedicatedfield assistance.This is
Regardless of whether the endogenous-rhythm hypothesis                    manuscript04-118 of the CaesarKlebergWildlife Research
or the energy-deficit hypothesis explains body condition                  Institute.
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                                                                                                          The Journal Wildlife        *
                                                                                                                             Management 70(5)