During spring and fall migration, shorebirds rely on stopover areas to replenish energy reserves and fulfill nutrient requirements. Most studies of stopover areas have focused on wetlands in the Northern Great Plains; little attention has been given to wetlands in the Southern Great Plains, especially the Playa Lakes Region (PLR). Our objectives were to determine migrant-shorebird species compositions, abundances, migration chronologies, use of habitats, and feeding ecologies in the PLR during spring and fall migration. More than 130 playa wetlands were surveyed for shorebirds in a 34,000-km2 area of western Texas. We selected American avocet (Recurvirostra americana), long-billed dowitcher (Limnodromus scolopaceus), least sandpiper (Calidris minutilla), and western sandpiper (C. mauri) as a subset of all shorebirds present to examine feeding ecologies; these 4 species were common species during both migration periods and represent a wide range of body sizes and guilds. We also evaluated the influence of sex on the feeding ecologies of the 4 species.
Thirty shorebird species used playa wetlands during spring and fall, 1993-94. The most abundant species during spring were American avocet, long-billed dowitcher, and Wilson's phalarope (Phalaropus tricolor), whereas the most abundant species during fall were American avocet, long-billed dowitcher, long-billed curlew (Numenius americanus), stilt sandpiper (Calidris himantopus), and lesser yellowlegs (Tringaflavipes). Migration chronologies of each species were distinct in spring with peak abundances occurring over 2-4 weeks and were protracted in fall with peak abundances occurring over 5-8 weeks. In general, most shorebird species selected playas that contained sparse vegetation (<25% vegetation cover), adequate amounts of mudflat (10- 15%) and shallow (<4 cm depth) water (10-20%) habitats, and higher invertebrate populations.
Invertebrates were the most important component in the diets of American avocets, long-billed dowitchers, least sandpipers, and western sandpipers, and diets varied little between males and females. In the spring, all 4 species consumed mostly chironomids, whereas in the fall, all 4 consumed a wider variety of invertebrates. I mportant invertebrate foods during the fall included chironomids, hydrophilids, leeches, planorbids, corixids, conchostracans, and hydracarinas. The 4 species also consumed more plant material (predominantly seeds) in the fall than in the spring. Differences in spring and fall diets of the 4 shorebird species were attributed to seasonal differences in invertebrate abundances and diversities; invertebrate abundances and diversities were higher in the fall than in the spring.
Shorebird diets were compared with availabilities of foods within and across individual playas. For most foods, overall selection patterns (i.e., selection across playas) were different from selection patterns within individual playas. In general, all 4 species exhibited a wide range of selection patterns for invertebrates as availability of invertebrates changed, suggesting that the 4 species used an opportunistic foraging strategy. Shorebird foraging appeared to decrease invertebrate populations in spring, but not in fall.
Management of playas in the PLR should focus on creating and maintaining sparse vegetation cover, and adequate mudflat (at least 10-15%) and shallow water (at least 10-20%) habitats. Because invertebrates are important foods of migrant shorebirds, management also should focus on enhancing invertebrate populations in playas. Gradual drawdowns of playas with deep water and flooding of dry playas should be used to provide available habitat for shorebirds throughout migration. Mowing and shallow disking can be used to create preferred habitat conditions and provide a detrital food base to enhance invertebrate populations. Managers that can manage only a few playas should consider managing playas during periods of maximum shorebird diversity in the PLR. During spring, maximum shorebird diversity occurred in late April and early May, whereas during fall, maximum shorebird diversity occurred in late August.