The Foraging Behavior of Semipalmated Sandpipers in the Upper Bay of Fundy Stereotyped Or Prey-Sensitive?’
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VOGEL Department of Biology, Colby College, Waterville, ME 04901, e-mail: [email protected] Abstract: Videotapes of migrant Semipalmated we explore the relationship of prey density and prey Sandpipers foraging in the upper Bay of Fundy were patchiness (measured by coefficient of variation) on analyzed to test for foraging behaviors sensitive to foraging behavior of Semipalmated Sandpipers prey density. Over a range of prey densities, both the (Calidris pusilla) which essentially prey on a single number of steps set-’ and probes set-’ increased species in the upper Bay of Fundy. The high visibil- with increasing prey density. However, the number ity and confiding nature of these shorebirds allowed of steps between probes was constant over the range us to videotape foraging behavior at close range, per- of prey densities observed. The average angle of di- mitting the acquisition of data on foraging behavior rectional change during foraging and the number of that cannot be gathered for many avian species. turns mini’ were constant despite large differences These data are used to test predictions of the rela- in prey patchiness. tionship of foraging behaviors to prey density and prey patchiness. Key words: SemipalmatedSandpiper, Calidris pu- Many scolopacid sandpipers, including Semipal- silla, foraging behavior, stop-over area, migration, mated Sandpipers, undertake migrations between arc- Buy of Fundy, Corophium volutator. tic breeding grounds and subtropical or tropical win- tering areas. The distances of these migrations place extraordinary energetic demands on the birds. In addi- Ornithological studies have contributed much to the tion, shorebirds have higher metabolic rates than ex- development of foraging theory (e.g., Tinbergen, pected based on other birds of similar mass (Kersten 1967, Davies, 1977, Krebs et al., 1977, Zach and and Piersma 1987). It is reasonable to expect that there Falls, 1977). However, the foraging behavior of should be strong selective pressure to maximize food many birds confounds testing many predictions be- intake at stop-over areas during migration. cause birds may be difficult to observe continuously Semipalmated Sandpipers nest in the low- to mid- for extended periods, may take a diverse array of prey arctic (Harrington and Morrison 1979, Gratto-Trevor and may thwart efforts to quantify their behaviors be- 1992). After nesting, the majority of central and east- cause of their rapid movements. In this contribution, em Canadian breeding birds wend their way to the upper Bay of Fundy. During an average stay of 15 days (Hicklin 1987), the sandpipers feed primarily on ’ ’ Received 11 July 1996. Accepted 25 October the abundant amphipod crustacean, Corophium volu- 1996. tutor (Hicklin and Smith 1979). These sandpipers SHORT COMMUNICATIONS 207 nearly double their weight from about 20-25 g to 40 these species are rarely eaten by C. pusilla (Hicklin g (Hicklin and Smith 1984) before departing on a and Smith 1979). nonstop, oceanic migration to Suriname. It is esti- Three stations were established along an intertidal mated that this 4,000 km flight requires 60-70 hr gradient on 3 August 1992, corresponding roughly to (Stoddard et al. 1983). the upper three stations used in 1986 in a previous Previous research in the upper Bay of Fundy has study (Wilson 1990). These stations were at the fol- shown that C. pusilla occur on intertidal flats where lowing tidal heights: 5.7 m, 4.9 m and 4.3 m above the abundance of Corophium is relatively high (Hick- mean low-water; successive stations were about 300 lin and Smith 1984, Wilson 1990, Matthews et al. m apart. Corophium densities drop precipitously be- 1992). Within a flat, birds are likely to forage any- low the 4.0 m mark and shorebirds avoid this area where as long as Corophium abundance is above a for foraging (Wilson 1990). threshold density; areas below this threshold are On 4-6 August 1992, shorebirds were videotaped avoided (Wilson 1990). at each of the stations using a Canon video camera For individual C. pusilla, the only aspect of forag- with a 10X telephoto lens. As the tide ebbs, the for- ing behavior that has been quantified is prey capture aging sandpipers follow the tideline out below the rate as a function of prey density (Wilson 1990). In level of Station 3 and then disperse broadly to feed this study, we quantify a number of other foraging throughout the intertidal zone. Birds were taped at behaviors of C. pusilla which were videotaped at the each station as each station was exposed. Once the height of fall migration in the upper Bay of Fundy. tideline had receded past Station 3, equal time was Analysis of these videotapes in slow-motion permit- spent videotaping at each site, moving among sta- ted the acquisition of data on behaviors that simply tions at least twice each low tide. As the tide rose, occur too quickly to quantify in field observations. each station was videotaped until the birds were forced to move to higher ground. Therefore, we ob- METHODS AND MATERIALS tained videotapes of the birds feeding immediately The study site was located on the western shore of after subaerial exposure, immediately before immer- the Minas Basin, Bay of Fundy in the village of sion, and at two different times during low tide ex- Avonport, Nova Scotia (Hicklin and Smith 1984). posure. All videotapes were obtained by standing at The intertidal flats have been the site of previous one point with the video camera mounted on a tri- work on the foraging behavior of Semipalmated pod. As flocks of birds flew in, they were videotaped. Sandpipers (Boates and Smith 1979, Gratto et al. The birds are remarkably tame and often foraged 1983, Hicklin and Smith 1984, Wilson 1988, 1989, within 4 m of the observer. 1990). The Avonport flats are one of the most im- On 6 August, five core samples of the sediment portant stop-over areas for C. pusilla in the upper Bay were taken from each of the three stations; the cores during the fall migration (Hicklin and Smith 1984). were no closer than 5 m from each other. Each core Although 34 species of shorebirds may be found on was 10 cm in diameter and was taken to a depth of the intertidal flats of the upper Bay of Fundy during 10 cm, well below the maximum depth of fall migration (Hicklin 1987), C. pusilla is the most Corophium. The sediment was sieved through a 500 common shorebird by far, with up to 2.2 million pass- pm screen. The residue with all retained organisms ing through the area (Mawhinney et al. 1993). Other on the screen for each core was preserved in 10% relatively common species are Short-billed Dowitch- formalin in seawater. ers (Limnodromus griseus), Least Sandpipers (C. The residue was later washed in freshwater and then minutilla) and Semipalmated Plovers (Charadrius sorted under a stereomicroscope at 12 X As the sand- semipalmatus). pipers strongly select Corophium larger than 4 mm The tidal range in the upper Bay of Fundy is the long (Peer et al. 1986), only Corophium exceeding 4 highest in the world. The tidal amplitude during neap mm were counted. Differences in abundance of the tides is 7.5 m and during spring tides is 15 m (Dohler large Corophium among the three stations were tested 1970). Because of the great tidal range, the intertidal for significance by analysis of variance and Scheffe area is expansive; near?y one-third of the bottom of post hoc pair-wise contrasts (Sokal and Rohlf 198 1). the Minas Basin is intertidal (Cranford et al. 1985). The videotapes were played at slow speed to ana- At Avonport, the horizontal ‘distance between the lyze foraging behavior. Most birds were not close high and low tide marks exceeds 1 km. enough to allow us to determine from the videotapes The intertidal sediment varies with tidal height, be- if a probe resulted in prey capture. However, several coming sandier in the lower portion of the intertidal other behavior patterns could be quantified from the zone.