Foraging Ecology of Fall Migrating Shorebirds in the Illinois River valley

U.S. Fish and Wildlife Services, Upper Mississippi River and Great Lakes Region Joint Venture Grant/Contract No: 301817G038Populations of many shorebird species appear to be declining in North America. Food resources at stopover habitats are considered limiting factors in many migratory bird species. Thus, investigations of foraging habitats are priorities of the Upper Mississippi River and Great Lakes Region JV (hereafter, JV) and the U.S. Shorebird Conservation Plan, but few researchers have examined foraging ecology of shorebirds in mid-migration locations of the JV region. To address these research needs we investigated body condition of, and foraging habitat and diet selection by, the following 4 species of shorebirds in the central Illinois River valley (IRV) during fall migrations 2007 and 2008: Killdeer (Charadrius vociferus), Least (Calidris minutilla)andPectoral (Calidris melanotos)sandpipers, and Lesser Yellowlegs (Tringa flavipes). We collected 153 shorebirds in 2007 and 133 in 2008, though not all individuals were suitable for inclusion in each analysis. Our results indicated that 3 of the 4 species were in good to excellent condition based on size-corrected body mass and fat scores. Killdeer had low fat scores in each year, but size-corrected body mass estimates were within published ranges. Shorebird diets were dominated by taxa from Orders Diptera (flies) and Coleoptera (beetles). Additionally, the contribution of Orders Isopoda (pillbugs), Hemiptera (true bugs), Hirudinea (leaches), Nematoda (round worms), and Fish (Gambusia spp.) to diets varied by shorebird species and year. We evaluated habitat selection in the context of Johnson (1980) by comparing aggregate percent dry mass of food items in shorebird diets and core samples. We evaluated third-order selection by comparing food abundance at shorebird collection sites to random sites, and fourth-order selection by comparing diets to food abundance at collection sites. Invertebrate abundances at shorebird collection sites and random sites were typically not significantly different, indicating no support for third-order selection. Conversely, we found considerable evidence of fourth-order selection for some forage taxa, and consistent avoidance of Oligochaeta. Our results of apparent diet selectivity were somewhat surprising given that shorebirds are typically considered opportunistic foragers. Additionally, other studies have reported that Oligochaeta were important foods, or that they may have been important but were underestimated in diets. Our results conflict with these findings; Oligochaetes were the most abundant invertebrates in our core samples, but were always under-represented in shorebird diets. Relationships between the abundance, availability, and consumption of Oligochaetes for and by waterbirds are unclear, but should be the focus of future research given that they are often the most abundant wetland invertebrate. If Oligochaetes are truly avoided or not available for consumption, estimates of foraging carrying capacity would need to be revised accordingly. We suspect that the diet selectivity we observed may have been a function of overall invertebrate biomass at our study sites. Several other studies have estimated invertebrate biomass for shorebird foraging habitats, but our study-period estimate (51.2 ± 4.4 (SE) kg/ha; dry mass) was as much as an order of magnitude greater than other reported estimates. However, our estimates were similar to those from other wetlands associated with the Illinois and Mississippi Rivers, perhaps indicating regionally abundant food resources. Regardless, we speculate that diet selectivity in shorebirds may follow tenants of optimal diet theory; that is, at low food abundances shorebirds may forage opportunistically, with the likelihood of selectivity increasing as food abundance or availability increases. Our study area included important habitats for migrating shorebirds, especially Chautauqua National Wildlife Refuge, which is a Western Hemisphere Shorebird Reserve Network site. However, we conducted a large portion of our research on other publicly and privately managed wetlands in the IRV floodplain. The value of these wetlands as shorebird staging areas is not well known, but all received considerable use by our target species. Understanding the combined contribution of both private and public wetlands in the region to stopover habitat for migrating shorebirds would provide critical information for conservation planning, allocation, and restoration. Finally, mid-continent stopover habitats are typically ephemeral to seasonal, but the current hydrology of the IRV can be highly variable within and among years. Regardless of ownership, most wetlands in the IRV are dewatered for moist-soil management in summer. However, the majority are not protected by large levees and may become completely inundated by unseasonal (i.e., summer or early fall) flooding that can eliminate shorebird foraging habitat in the region. Consequently, this flooding may drastically alter the ability of the region to meet annual conservation objectives. Our results suggest that the central IRV contains excellent habitat for fall-migrating shorebirds when water levels and management are appropriate, and we speculate the region may support a substantially larger population than previously thought. However, unpredictable hydrology may lead to seasonal habitat deficits, imposing additional migration stress on already depressed shorebird populations. Understanding the consequences of intra-and interannual variability in shorebird foraging habitat in the region will be important for targeted conservation planning and implementation.INHS Technical Report Prepared for U.S. Fish and Wildlife Services, Upper Mississippi River and Great Lakes Region Joint Ventur