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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 groundsin Trinidad. Living Bird 6:123-140. studies. B. GERARD, M. 1967. Factors affecting earthworms in pastures.J. Anim. Ecol. 36:235-252. 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. 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