Abstract
Populations of many shorebird species appear to be declining in North America, and food resources at stopover habitats may limit migratory bird populations. We investigated body condition of, and foraging habitat and diet selection by 4 species of shorebirds in the central Illinois River valley during fall migrations 2007 and 2008 (Killdeer [Charadrius vociferus], Least Sandpiper [Calidris minutilla], Pectoral Sandpiper [Calidris melanotos], and Lesser Yellowlegs [Tringa flavipes]). All species except Killdeer were in good to excellent condition, based on size-corrected body mass and fat scores. Shorebird diets were dominated by invertebrate taxa from Orders Diptera and Coleoptera. Additionally, Isopoda, Hemiptera, Hirudinea, Nematoda, and Cyprinodontiformes contribution to diets varied by shorebird species and year. We evaluated diet and foraging habitat selection by comparing aggregate percent dry mass of food items in shorebird diets and core samples from foraging substrates. Invertebrate abundances at shorebird collection sites and random sites were generally similar, indicating that birds did not select foraging patches within wetlands based on invertebrate abundance. Conversely, we found considerable evidence for selection of some diet items within particular foraging sites, and consistent avoidance of Oligochaeta. We suspect the diet selectivity we observed was a function of overall invertebrate biomass (51.2±4.4 [SE] kg/ha; dry mass) at our study sites, which was greater than estimates reported in most other food selection studies. Diet selectivity in shorebirds may follow tenants of optimal foraging theory; that is, at low food abundances shorebirds forage opportunistically, with the likelihood of selectivity increasing as food availability increases. Nonetheless, relationships between the abundance, availability, and consumption of Oligochaetes for and by waterbirds should be the focus of future research, because estimates of foraging carrying capacity would need to be revised downward if Oligochaetes are truly avoided or unavailable for consumption.
Highlights
Populations of many shorebird species appear to be declining throughout North America [1,2,3,4,5]
Relatively few studies have examined selection among diet, foraging site, and random sites, our results contrast others who suggest that shorebirds may select foraging sites based on food abundance [16,26,32,51,52]
We perceived individual wetlands in our study to be relatively homogeneous in terms of mudflat habitat and invertebrate availability, but we suspected that proximate cues, such as micro-topography or perceived predation risk, might have allowed shorebirds to select foraging sites that were more profitable than expected at random [52]
Summary
Populations of many shorebird species appear to be declining throughout North America [1,2,3,4,5]. The best way to affect shorebird fitness in this region is through management of habitat quantity and quality [2,5,6,7]. Supporting this notion is evidence that migration habitat quality can influence shorebird populations [8,9], and migratory patterns [10]. Because shorebirds may spend little time at individual stopover locations, their energy demands should require them to forage efficiently and opportunistically [11,12,13]. Optimal foraging theory may predict that animals would forage opportunistically when food resources are abundant [14,15], and we suspect food resources to be abundant at most highly used stopover locations [6,16]
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