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Structure of a Wintering Dunlin Population

VIEWS: 9 PAGES: 9

  • pg 1
									The Condor 91562-570
0 The Cooper Ornithological    Society 1989




          STRUCTURE OF A WINTERING                               DUNLIN         POPULATION’

                                                    G. M.   Rmz2
ZooIogy Department, Universityof California, Berkeley, CA 94720 and BodegaMarine Laboratory, P.O. Box
                                      247, BodegaBay, CA 94923

                                       P. G. CONNORS AND S. E. GRIFFIN
                              BodegaMarine Laboratory, P.O. Box 247, BodegaBay, CA 94923

                                                   F. A. PITELKA
                     Museum of VertebrateZoology, Universityof California, Berkeley, CA 94720

              Abstract. We havedocumented          on
                                           structure two different     for
                                                                  scales the Dunlin (Calidris
                                       at
            aloinaj nonulationwinterine. BodeaaBav.Californiain 1986-1987.Within a niaht roost.
            si&rifi&ntly morejuvenile birdswerecap&redat the centervs. periphery      betweenoctober
                            In
            and December. addition,therewasa significant                in
                                                              difference themeansizeof birdsamong
            areasof the roostduringNovember,with birdsat the centerbeinglargest.
                         also
              Structure developed        duringthewinteron a scale                the
                                                                   whichseparated entirepopulation
            at Bodega                             In
                        Harbor into two groups. late winter, largenumbersof Dunlins left the area
            late in the day and returnedin early morning,apparently    roosting            As
                                                                                elsewhere. a result,
            the numberof Dunlins at the night roostdeclinedin winter much more than the daytime
            populationsize at Bodega      Bay. Theseroughlycrepuscular   movementswere not evident
            earlier,in thefall, and did not correspond       to             The
                                                      closely tidal cycles. resultant   fragmentation
            of the populationinto “resident” and “mobile” groups                  in
                                                                     wasreflected physiological dif-
            ferences between  groups.                             in
                                      Thosebirdswhichremained theharbordayandnight(residents)
            weighed                                                          in
                      less,had loweramountsof body fat, and weredelayed molt schedule March in
            relativeto (mobile)birdswhichleft at night.
              While the ecological               of         on
                                     significance structure both scales    remainsunknown,evidence
            from this and previousstudies            that
                                              suggests it may be a relativelycommonphenomenon
                                           for       of
            with importantimplications studies avian populations.
             Key words: Movement patterns;rooststructure;molt schedule;
                                                                      physiologicalcondition;
            Dun&; Calidrisalpina.

INTRODUCTION                                                fluence the distribution, activities, diet, and con-
There is abundant evidence that the dispersion              dition of smaller and younger individuals.
of wintering shorebirds (suborder Charadrii) is                Among shorebirds, various forms of spatial
not random with respect to age and sex. Lati-               structure have been found within local popula-
tudinal gradients and differencesin the timing of           tions of the Dunlin Calidris alpina (Have et al.
migration by age and sex have been well docu-                1984; B. Kus, pers. comm.), Bar-tailed Godwit
mented for many species(Pageet al. 1979, Myers              Limosa lupponica (Smith 1975), Eurasian Cur-
198 1, Morrison 1984, Pienkowski and Evans                  lew Numenius arquata (Townshend 1981), and
1984). Elements of spatial structure also emerge            Sanderling Calidris alba (L. M. Gordon and J.
on much smaller scales. The Eurasian Oyster-                P. Myers, pers. comm.). The mechanisms that
catcher Haematopus ostralegus, for example, is              structurethesepopulations may include age- and
known to form feeding and roosting flockswhose              sex-relateddifferencesin dominance, experience,
age and sex composition differ over very short              feeding efficiency, or vulnerability to predators.
distances (Goss-Custard and Durrell 1984,                   Here, we describe structure within a population
Swennen 1984). It appears that these local oys-             of shorebirdsthat differs substantially from pre-
tercatcher populations are structured by social             vious accounts.
dominance, whereby larger and older birds in-
                                                            STUDY SITE AND METHODS
                                                            Our primary study site was Bodega Harbor, lo-
                                                            catedat BodegaBay, California (Fig. 1). The har-
                                         16
  ’ Received29 July 1988. Final acceptance Feb-
ruary 1989.                                                 bor is approximately 320 ha in area, 70% of which
  * Present
          address: Smithsonian  EnvironmentalRe-            is exposedduring moderate low tides. A diverse
search Center,P.O. Box 28, Edgewater,MD 21037.              group of invertebrate speciesoccurs across the

                                                         15621
                                                        DUNLIN POPULATION STRUCTURE                 563



                                         N            SALMON                                   N


                                         ;T           CREEK     I                              f


             WALKER   CREEK




 PACIFIC   OCEAN




              10 km
                                                       OCEAN                                 1 km

FIGURE 1. Map of California coastbetween Bodega      FIGURE 2. Map of BodegaHarbor. Sulicornia marsh
Harbor and northern San Francisco Bay.               is shown as stippled area.

tidal sandflats and serves as the primary food       indicated that over 95% of the Dunlins present
resource for thousands of wintering shorebirds       within Bodega Harbor and on the two outer
(for further detail, seeStanding et al. 1975, Con-   beaches roosted in a small Salicornia marsh
nors et al. 1981, Ruiz 1987).                        (stippled area in the southeastcomer of the har-
   The closestsimilar tidal habitat for most wad-    bor, Fig. 2). We trapped birds at this roost site
er species from BodegaHarbor is at Ester0 Amer-      with mist nets from October to April 1985-1987.
icana, Dillon Beach, and Walker Creek delta,         Nets were set at three locations (each ca. 150 m
roughly 4-12 km southeast, bordering Bodega          apart), and birds were flushed out of the im-
Bay and Tomales Bay (Fig. 1). Farther south,         mediate surrounding vegetation. All birds were
other suitable habitat occursat the southern end     weighed to the nearest 0.5 g and banded im-
of Tomales Bay, within esteros the Point Reyes
                               on                    mediately at capture. The bill, wing, and tarsus
Peninsula, and on tidal flats of Bolinas Lagoon      were measuredfor eachbird, and birds were aged
and northern San Francisco Bay; these sites are      as either juvenile (first-year) or adult (older than
25-75 km from Bodega Harbor. To the north,            1 year) based on plumage as described in Page
no extensive coastal wetlands/sandflats occur        (1974). Most birds were individually color-band-
until Humboldt Bay (280 km away).                    ed and releasedwithin the harbor. On some eve-
   Approximately every 2 weeks from July 1983        nings, we held birds captive (in covered laundry
to June 1987 we counted the numbers of each          baskets that were kept in dark rooms)and weighed
shorebird speciesin Bodega Harbor within 1.5         them repeatedly throughout the night to deter-
hr of low tide. Counts were scheduledto coincide     mine rates of weight loss. Additional data were
with low tides between 0.4-0.7 m above mean          obtained from birds caught while feeding during
lower low water. During March 1987, we also          the day in 1985-1986.
countedDunlins at 2- to 4-hr intervals from dawn        We determined the presenceof color-marked
until dusk, three dates were chosen to examine       individuals within Bodega Harbor throughout
diurnal changesin abundance under different tid-      1986-1987 by using 15-60 x telescopes.From
al regimes, and the flux of birds entering and       February to April 1987, an effort was also made
leaving the harbor was monitored. Between two        roughly every 2 weeks to locate marked birds in
and four observers participated in counts, and       Tomales Bay and local fields where other shore-
four to eight individuals monitored bird move-       bird specieshad been observed previously. The
ments in and out of Bodega Harbor.                   pattern of prenuptial molt was recorded for both
   During evening high tides, our observations       marked and unmarked birds in March, assigning
564          G.   M. RUIZ   ET AL..


      70                                              among the three net sites. The middle net had
             11 NOVEMBER 1986
                                                      the most juvenile birds on four of the five nights
                                                      of trapping, comprising 62% of the entire sample
                                                      as opposed to 23% and 39% at the outside nets
                                                      (Table 1). A Friedman two-way analysis of vari-
                                                      ance indicates a significant difference (Friedman
                                                      statistic = 6.40, df = 2, P = 0.041) in the age
                                                      ratio of birds among nets when ranking the nets
              5 -   8 MARCH 1987
 3                                                    for each of the five nights; when pooling all five
 E    60                                              nights together, there was also a significant dif-
 ie                                                   ference in age distribution among net sites (x2 =
      50                                              8.71, df = 2, P < 0.02).
                                                         For November 1986, there was a significant
      40                                              difference in the size of birds among nets (F =
       -30                                     45
                                                      3.26, df = 2,92, P = 0.04) with larger birds being
                                                      most abundant at the center net. As already men-
                       of             vs.
FIGURE 3. Regression weightat capture cul-
men lengthfor Dunlins nettedin Bodega
                                    Harbor on         tioned, Dunlins are sexually dimorphic for size,
two separate
           nights.                                    so these results suggest that juvenile females
                                                      comprised a greater proportion of the birds at
                                                      the middle vs. outside nets. During the 1985-
a molt scorebasedon the appearanceof the dark-        1986 season,and later in the 1986-l 987 season,
ening breast (0 for white underparts to 5 for a       too few birds were captured at some net sites for
complete black patch).                                similar comparisons.
   Dunlins were also collected by shotgun while
feeding on exposedtidal flats in Bodega Harbor        ACTIVITY PATTERNS
during 1985-1986. These birds were used to ex-        The seasonal abundance of Dunlins in Bodega
amine diet, weight, and lipid content. For the        Harbor, as determined by daytime counts at low
latter, birds without their digestive tracts were     tide, is shown in Figure 4. Individuals begin to
dried at 60°C and extracted for 24 hr with hexane     return from breeding grounds in mid-October,
using a Soxhlet apparatus (Evans and Smith            the population size rapidly climbs to a peak of
 1975); lipid content was calculated as the differ-   3,000-6,000 individuals by late November, de-
ence between dry weight before and after ex-          clining soon thereafter to approximately 1,500-
traction.                                             2,000 birds until late March. The population de-
   We adjusted individual weights to take ac-         clinesthroughApril, and the last individuals leave
count of size variation among individual Dun-         in early May.
lins; we used analysis of covariance (Sokal and          At dusk, Dunlins coalescedinto flocks of lO-
Rohlf 198 1) to standardize weights, using ex-        500 birds which flew back and forth at the edge
posedculmen length asthe independent variable.        of the marsh vegetation, eventually breaking into
Dunlins are sexually dimorphic for size (Hayman       smaller groups which entered the marsh and
et al. 1986), and the regressionof weight on cul-     formed a dispersed roost. The nature of roost
men length explained roughly 25% of the vari-         formation changedseasonally;in late winter, we
ation in weight among individuals (Fig. 3). Using     observed flocks of 1O-300 birds coalesceat dusk
a bill length (exposed culmen) of 38.5 mm to          and then fly southeast out of the harbor from
separate males (X = 37.02, SD = 1.73, 12= 76)         February to April of both years. Trapping be-
from females (Z = 40.37, SD = 1.70, n = 68)           came much more difficult during this period, be-
collected at BodegaBay, the fit of this regression    causethe roost size diminished out of proportion
appearssimilar for both sexes; thus, the two sexes    to changesin the daytime population and fluc-
were not treated separately in the analyses.          tuated unpredictably.
RESULTS                                                  In March 1987, it appeared that a population
                                                      of approximately 200-400 birds remained all day
STRUCTURE WITHIN THE ROOST                            and night within BodegaHarbor, but most Dun-
Dunlins captured at the night roost in Bodega         lins present in the daytime roosted elsewhere,
Harbor were not distributed in a random manner        returning to the harbor on a daily basis. The
                                                            DUNLIN POPULATION STRUCTURE                          565

                                       of
TABLE 1. Variationin the agestructure Dunlinsamongcapture              in
                                                              locations Bodega Harborat night.Shown
                                                                    on
are the ratiosof juveniles(HY) to adults(AHY) for threenet locations eachof five differentnightsfrom
Octoberto December1986.

                               Net 1                           Net 2                         Net 3
                      HYIAHY            (n)           HYIAHY            00          HYlAHY                (n)

28 October             0.56             (25)           0.93                          0.69
11 November            0.62             I;:;           0.71                          0.44
13 November            0.58                            0.67             (40)         0.64                 (58)
23 December
25 November            0.67
                       0.39             (33)
                                        (31)           0.58
                                                       0.83             I::;         0.75
                                                                                     0.12              (::;
All five nights        0.23            (156)           0.62            (146)         0.39             (137)


majority of birds in this latter group entered the      flocksin the spring at BodegaHarbor. These mo-
harbor in large flocks near dawn, with smaller          bile groups of birds contained color-banded in-
groupscontinuing to arrive until early afternoon:       dividuals, indicating that many birds had roost-
at that time, the processwas reversed with small        ed in BodegaHarbor earlier in the year. Arriving
flocks leaving first, followed by a large exodus at     birds usually joined a resident flock of conspe-
dusk.                                                   cifics immediately and, therefore, could not be
   Figure 5 showsthe diurnal fluctuation of Dun-        identified as to age or sex.
lin abundance at Bodega Harbor for 3 days. The
magnitude of actual fluctuations, especially of         CONDITION OF BIRDS
evening departure, is underestimated due to ma-         The size-adjusted weights of Dunlins sampled
jor changesoccurring at times of darknessor low         during the day diverged from those at night as
light. Nevertheless, it can be seen that much of        the 1985-l 986 seasonprogressed   (Fig. 6A). From
the movement in and out of Bodega Harbor is             November to January, weights differed by only
crepuscular and did not closely track the tidal         l-2 g, but this difference increasedto 5 g in Feb-
cycle. In fact, birds were usually seen entering
the harbor at dawn regardlessof tidal condition,                  1                            ,     10 MARCH
and it was not unusual to observe flocks of de-
parting birds before and during midday low tides.
Most of the arriving and departing birds flew out
of Bodega Harbor, over Doran Beach, and fol-
lowed the eastern shore of Bodega Bay, but we
did not discover their destination during our vis-
its to surrounding coastal and inland sites.
                                                        B         1            H                     13 MARCt
                                                                                              .
   Dunlins often arrived in mixed flocks with           $ 2000.
Western Sandpipers (C. mauri). These mixed-             t
                                                        3
speciesflocks exhibited crepuscular movement            z 1000.
patterns that were similar to those of Dunlin           2              /?                             i
                                                        -




                                                                  4
                                                                       0700        1100        1500

                                                                               TIME OF DAY
                                                        PIGURE 5. Diurnal variationin thenumberof Dun-
               a3 NOV a4
             NOV              NOV a5 NOV a6
                                                        lins at BodegaHarbor for 3 days.Arrowsindicatethe
FIGURE 4. Number of Dunlins at Bodega Harbor            timesof low (L) and high(H) tides;firstcounton each
on low-tidecounts(1983-1987).                           day wastakenat dawn.
566         G. M. RUIZ   ET AL.



                                                           G
                                                           z 10       o NIGHT CAPTURE
                                                           =8         l DAY CAPTURE                e




                                                            3                4          8         12
                                                                        TIME SINCE    CAPTURE     (HRS)
                                                           FIGURE 8. Rate of weight lossfor Dunlins captured
                                                           at night (open circles) and during the day (closed cir-
                                                           cles). The dashed curve represents a least squares
            4                                      I
                                                           regressionfor all points.
                NOV         JAN        MAR
                CAPTURE      DATE   (1985-86)
                                                           ure 6B indicate that the differences observed in
FIGURE 6. The adjusted           (A)
                           weight andlipid weight
(B) of Dunlins from Bodega   Harbor duringthe 1985-        body weight actually reflect large differences in
1986season. adjusted
             For         weights, means
                                  the        and 95%       fat reserves:a 7- to 1O-gdifference in wet weight
comparison  intervalsare shown;the meansand 95%            for day vs. night sampleswas accompanied by a
confidence intervalsare givenfor lipid weights. Open       fourfold difference in fat reserves.
circlesdenotebirdsnettedat night, and closed   circles        The demographic composition of birds from
representdaytimecaptures.   [Sample      for
                                    sizes eachfig-
ure from left to right are as follows: Above = 36, 43,     day vs. night collections appeared very similar,
34, 28, 10, 15, 13, 17, 9, 23, 23, 43, 9; Below = 18, 8,   such that age or sex does not explain group dif-
8, 9, 8.1                                                  ferences. Since the regression of size on weight
                                                           does not appear different for males and females
ruary and to 7 g by April; in the latter compar-           (Fig. 3), sexis essentiallytaken into accountwhen
ison, this representsa significant difference (AN-         standardizing for size in the analysis of covari-
COVA, P < 0.05)despite the small sample size.              ante. That age is not responsible for observed
  Although lipid data are not available for day            differencesis seenin Figure 7, as adult birds were
and night sampleson the same date, data in Fig-            only slightly heavier than juveniles.
                                                              It also seems that diurnal weight fluctuations
                                                           cannot explain such large differences. Impor-
                                                           tantly, the magnitude of day vs. night weight
                                                           differenceswas small for three consecutive sam-
                                                           ples and then increased with no changesin the
                                                           site or time of collection. As the night sample
                                                           was collected routinely 2-3 hr after birds had
                                                           stoppedfeeding, the rate ofweight lossin captive
 3    65,                                              I   birds was not rapid enough to account for these
 n                                  1906 - 87              differences(Fig. 8). The weight loss of birds cap-
 P    60.
                                                           tured during the day and night suggests    that l-
 2
 5    55.                                                  2 g may be lost between sample times (see also
                                                           Lloyd et al. 1979, Pienkowski et al. 1679, Da-
      50.                                                  vidson 198 1); we suspectthis is an overestimate,
      451                                              I   becausethe stressand elevated temperature in-
                  NOV        JAN      MAR       MAY        volved in captivity may accelerateratesofweight
                         CAPTURE    DATE                   loss.
FIGURE 7. Adjusted weights for adult and juvenile
Dunlins netted from 1985 to 1987 at BodegaHarbor.          MOLT SCHEDULE
The mean and 9 5% comparisoninterval is shown sep-         For the period of 12-16 March 1987, the molt
arately for adults (AHY) and juveniles (HY) on each
                                                           scoresof marked and unmarked Dunlins feeding
sampling date. [Sample sizes for each figure from left
to right are as follows:Above = 26, 10, 17, 26, 17, 16,    in Bodega Harbor are summarized in Figure 9.
10, 5, 8, 9, 12, 11, 9, 14, 1.5,30; Below = 37, 16, 59,    The distribution of molt scores for unmarked
39, 30, 25, 42, 15, 39, 11, 53, 8, 44, 53, 17, 24.1        birds was bimodal with most birds relatively ad-
                                                         DUNLIN POPULATION STRUCTURE                        567


vanced in molt. Marked birds observed under                                      BODEGA HARBOR
                                                          60.                      0   BANDED   (N-99)
the same conditions had a unimodal distribution
                                                                                   =UNBANDED       (N489)
for molt score which correspondedto the lower
mode for the unmarked birds. Most marked birds
had been recently trapped in the night roost,while
unmarked birds included both resident Dunlins
and birds from the mobile population.
   Using Dunlins that were marked and released
at Walker Creek, a similar comparison of marked
vs. unmarked birds indicated that both were uni-
modal for a high molt score at this site (Fig. 9);
this was also the casefor marked and unmarked
birds at Bolinas Lagoon (N. Warnock and G.
Page, unpubl. data). It is apparent from the sim-                 0      1
ilarity in molt schedules between marked and                               MOLT SCORE
unmarked Dunlins at these nearby sites that the       FIGURE 9. Frequencydistribution of molt scores   for
differencesbetween Bodega Harbor groupswere           banded and unbanded Dunlins at BodegaBay (above)
not simply an artifact of being marked.               and Walker Creek (below). [Seetext for explanation of
                                                      molt scores.]
DISCUSSION
We have presented data that indicate spatial
structure in a population of shorebirds on two        population data from limited sampling. In the
scales. Within night roosts, Dunlins were dis-        present case, even relatively large sample sizes
tributed nonrandomly with respect to age and          (n > 100) gave us markedly different age ratios
sex. To our knowledge, structure at a roost has       depending on net location within the roost (Table
only been reported once for the Charadrii. In an       1). Such structure in any population will require
unpublished thesis, Whitlock (1979, as cited by       careful, spatially distributed, sampling schemes
Ydenberg and Prins 1984) found that adult Com-        to gain meaningful estimates of population data.
mon Redshanks (Tringa totanus) displaced ju-             Dunlins that fed in Bodega Harbor during the
veniles from downwind positions during the day.       day also exhibited structure, dividing into two
Such roost structure is probably widespread           groups. Activity data show that approximately
among shorebirds.It is well-known for other avi-      75% of the Dunlins left the harbor at night in
an groups (Orians 1961; Meanly 1965; Swing-           March 1987. A comparison of weights and molt
land 1977; Caldwell 1981; Weatherhead 1983,           schedulesindicates that day vs. night samples
 1985) and is thought to arise when particular        came from two statistically different groups,sug-
positions are more advantageous than others in        gestingthat respectivegroupswere somewhatco-
reducing the risk of predation or heat loss, with     hesive without much interchange between them.
accessto preferred sites being mediated by age-       Furthermore, we believe that the collection of
or sex-biased dominance (see especially Swing-        daytime birds may actually have underestimated
land 1977 and Weatherhead 1983). We doubt             weight differences between groups, due to the
that Dunlins’ positions within their dispersed        possiblepresenceofresident birds (oflow weight)
roost at Bodega Harbor carry meaningful costs         in daytime collections.
or benefits with respect to thermal regime, and          The Dunlins feeding in Bodega Harbor from
we have no data on the relative predation risk        November to January appearedto all remain and
by position. It seemsunlikely, however, that pre-     roost locally at night, although a quantitative
dation risk was higher at the center of the roost     confirmation of this pattern (such as the dusk-
area, or that the juvenile birds that occurredthere   dawn observations made in March) has not yet
would have aggressively    displaced older individ-   been made. Nonetheless, the relative lack of
uals. Thus, such explanations do not seem ap-         Dunlins roosting at Bodega Bay in March, the
plicable here.                                        sighting of many previously marked birds in the
   While we cannot explain the structure ob-          mobile flocks upon arrival, and the improbabil-
servedwithin the roost, we recognizeits practical     ity of missing an exodus of 75% of the Dunlin
importance for biologists attempting to obtain        population each day offer compelling evidence
568    G. M. RUIZ   ET AL.



for the changein roosting patterns between early     predation on tidal invertebrates, caused a dra-
and late winter.                                     matic decline in the abundance of shorebird prey
   Although crepuscular movement such as that        in Bodega Harbor (Ruiz 1987). It is not clear
exhibited by the mobile group is relatively com-     whether the existence and/or size of the mobile
mon for charadriforms at inland sites(Hamilton       group of Dunlins is a response to the resultant
1959, Swingbroad 1964, Brooke 1972, Atkinson         poor food resources.For example, Townshend
1976) and for passeriforms(Aldous 1944, ffrench      (198 1) has shown that in the Eurasian Curlew
1967, Davis and Lussenhoop 1970, Bray et al.         population at the Tees Estuary, United King-
1975, Weatherhead 1985) it appearsunusual for        dom, a small subgroup with a strong male bias
coastal waders, whose movements are usually          regularly used nearby fields to feed, and during
related to tidal cycle and the accompanying          severe weather, when prey availability was re-
changesin food availability (Burger et al. 1977,     duced, these birds returned to the estuary until
Connors et al. 198 1, Burger 1984, Myers 1984).      the weather relented. As utilization of such al-
For the few individual days examined quanti-         ternate habitat for feeding appears widespread
tatively, an associationwith tidal fluctuationswas   (as above), it seemsclear that many speciesbe-
weak, if present at all; on many other days, we      have like the Eurasian Curlew with some indi-
observed Dunlin flocks leaving Bodega Harbor         viduals specializingin the use of alternate habitat
at or before low tides. Nevertheless, body con-      and others using it only opportunistically. Thus,
dition and molt scheduledata indicate that dif-      the situation at BodegaBay may simply represent
ferences existed in the food resources experi-       an extreme example of opportunism in response
enced by mobile vs. resident birds, since mobile     to prolonged prey depletion.
individuals were in better condition despite a          While we observed similar population struc-
presumably greater energy expenditure in daily       tures to develop in the seasonalmovement pat-
flights. For this reason,movement schedules    may   terns of both Dunlins and Western Sandpipers,
be a response to superior food resources that        no comparable data are available prior to the
became available elsewhereon a schedulesome-         crab recruitment for these species.It is interest-
what independent of the tidal regime at Bodega       ing, however, that other species exhibited a qual-
Harbor. While it is possible that a lag in the       itatively similar pattern both before and after this
timing of tides at alternate coastal sites makes     event. From July to November/January 1983-
movement more profitable than remaining in            1987, Marbled Godwits roosted at night within
Bodega Harbor, this too should be predictable        the harbor. After midwinter, the entire roost flock
based upon tidal cycle and would not result in       departed from the harbor soon after forming at
the consistent crepuscular timing of movement.       dusk, flying south in the same direction as the
   We surmise that the mobile Dunlins were           departing Dunlins and returning at dawn. We
moving inland on a daily basis in late winter to     observed a similar seasonal pattern for Black-
feed in fields and nontidal drainage systems to      bellied Plovers (Pluvialis squatarolu), Willets
supplement their food intake, as observedin oth-     (Catoptrophorus semipalmatus), and Semipal-
er wader species(Atkinson 1976, Elphink 1979,        mated Plovers (Charudrius semipalmatus). The
MacLennon 1979, Townshend 198 1, Goss-Cus-           fact of crepuscular movement for other species
tard and Durrell 1984). Occasional inland sight-     before and after the crab settlement event sug-
ings of Marbled Godwits (L. $&a) that were           geststhat movement (and perhaps structure) of
color-marked at Bodega Harbor and of un-             the Dunlin population is a regular component of
marked Dunlins in flocks, and the absence of         Dunlin ecology at Bodega Harbor regardlessof
coastal sightingsof Bodega Harbor birds despite      annual changesin food conditions.
a substantial search effort in Tomales Bay and          As food resourcescontinue to recover in Bo-
Bolinas Lagoon, lend support to this possibility.    dega Harbor following the 1985 crab invasion,
Furthermore, the activity pattern of insect and      we can determine whether the formation of
annelid prey in this habitat may be nocturnal or     subgroups in the Dunlin population is a tem-
crepuscular(Gerard 1967, Dugan 198 1) and thus       porary, opportunistic responseto food resources
responsible for the observed timing of shorebird     or a relatively persistentfeature. The age and sex
movement.                                            composition of the mobile group, though elusive,
     In April 1985, an unusually large recruit-      may offer clues about the mechanism of for-
ment of Dungeness crabs, and their subsequent        mation and the ecological and evolutionary sig-
                                                             DUNLIN POPULATION STRUCTURE                     569


nificance of suchcomplex local population struc-             men&s arquata in mid-Cheshire England. Wader
ture. Further interpretation of this phenomenon              Study Group Bull. 26:31-35.
                                                         EVANS,P. R., AND P. C. SMITH. 1975. Studies of
must await new data along these lines. Never-                              at
                                                             shorebirds Lindisfame, Northumberland. II. Fat
theless,it is clear that spatial structure exists on         and pectoral muscles as indicators of body con-
several levels within shorebird populations and              dition in the Bar-tailed Godwit. Wildfowl 26:64-
must be given seriousconsideration in the sam-               76.
                                                         FFRENCH,R. P. 1967. The Dickcissel on its wintering
pling design and interpretation of ecological
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ACKNOWLEDGMENTS                                                             J.
                                                         GOSS-CUSTARD, D., ANDS.E.A. LE V. DIT DURRELL.
                                                              1984. Feeding ecology,winter mortality and the
                                           of
This work benefited from the assistance many peo-                                                      on
                                                             population dynamics of oystercatchers the Exe
ple. We wish to thank V. Chow, L. Gordon, C. Lund-           estuary, p. 190-208. In P. R. Evans, J. D. Goss-
mark, J. Maron, C. Schick, M. Sherman, N. Warnock,           Custard, and W. G. Hale [eds.], Coastal waders
and the Earthwatch volunteers for their help in the          and wildfowl in winter. Cambridge Univ. Press,
field. We also received excellent logistic support from      Cambridge.
the staff of the Bodega Marine Laboratory. G. Page, HAMILTON,W. J., III. 1959. Aggressivebehavior in
W. Sousa,N. Warnock, J. West, P. Williams, and two           migrant PectoralSandpipers.Condor 6 1:16l-l 79.
anonymousreviewers provided comments which im-           HAVE, T. M. VAN DER, E. NIEBOER,        AND C. BOERE.
proved the manuscript. This work was partially sup-           1984. Age-relateddistribution ofDunlin, Calidris
ported by grantsfrom the National ScienceFoundation          alpina, in the Dutch Waddenzee, p. 160-176. In
(#BSR 85 19201), the Dean Witter Foundation, and             P. R. Evans, J. D. Goss-Custard,and W. G. Hale
Earthwatch.                                                  [eds.], Coastal waders and wildfowl in winter.
                                                             Cambridge Univ. Press,Cambridge.
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