Carcass Composition and Digestive-Tract Dynamics
of Northern Pintails Wintering Along the Lower Texas
BARTM. BALLARD,' ResearchInstitute,
CaesarKlebergWildlife TexasA&M Kingsville, 78363, USA
JONATHAN THOMPSON,2 CaesarKleberg ResearchInstitute,
Wildlife TexasA&M Kingsville, 78363, USA
J. Ducks Unlimited, PacificNorthwest
Office,Vancouver, 98683, USA
Wecollected341 northern pintails(Anas acuta)alongthelowerTexascoast, USA,to investigate of
tractcomponents duringwinter helpassess theability thisregionto support
to of wintering pintailpopulations. reliedmore
on endogenouslipidand proteinreservesduring of to
winter a dryyear thana normal wet year.Carcassfat remained relatively
stableduring wet winter; however, pintailscatabolized 65%
approximately of their reservesbetweenarrival October
lipid in and
departure theend of February the
during drywinter. Somatic protein mass alsodeclinedoverbothwinters pintails
up to 20%of theirmusclemass. Gizzard atrophy explained most of thechangesinsomaticprotein the
during wet winter, whereas
catabolism breastmusclealso contributed changesin proteinmass during drywinter.
of the digestivetractmass
was greatestin earlyDecember,and then declinedabruptly through February during both winters.
Texascoast in lateFebruary in
approximately lighter bodymass thanwhentheyarrived autumn.
20% in Mid-continentpintails
frequently to winter southerly
opt in latitudeswheretheycan maintain minimal endogenousreservesdue to themoderate climate,
disturbance, relatively but
dependable, oftenlower-quality resources.
food However, consequencesof
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
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)
Anas acuta, carcass composition,digestivetract,LagunaMadre,lipids,northern protein,Texas, winter.
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:email@example.com result in reduced egg mass, smaller clutch sizes, and lower
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
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
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
modified from Billard and Humphrey (1972). We assessed Oct-Feb 1997-1998 and 1998-1999.
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
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
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
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
1 10 0 "[ 00 --- m
fo n . t2 t-
u' 900 150
T 800 100-
600 A 50 B
OCT NOV DEC JAN FEB OCT NOV DEC JAN FEB
20 _-170 160
20 150 -
E 15 -140 •
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
560 A 6- B
50 1 5
OCT . NOV DEC JAN FEB OCT NOV DEC JAN FEB
co 21 -a4mom
E 20 40
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
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
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
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
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
The Journal Wildlife *
Table 3. Relationship
organs and Juliandate for male northern
pintails winteralongthe lowercoast of Texas,
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
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
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
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.
The Journal Wildlife *
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
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