Ecology of spring and fall migrant shorebirds in the Playa Lakes Region of Texas

Date

1996-12

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

During spring and summer/fall migration, shorebirds rely on stopover areas to replenish energy reserves and fulfill nutrient requirements. The Playa Lakes Region (PLR) of west Texas may provide critical stopover habitats for migrant shorebirds. Little is known about migrant shorebirds during their stay in the PLK Knowledge about species compositions, migration chronologies, and use of habitat by migrant shorebirds in the PLR is critical to developing conservation and management strategies for migrant shorebirds. An examination of shorebird behaviors and feeding ecologies is needed to determine the importance of certain habitats and foods to migrant shorebirds during their stay in the PLR Moreover, an examination of shorebird behaviors and feeding ecologies may provide insight into possible limiting Actors to shorebird populations during migration. Playas also provide an opportunity to test various hypotheses concerning mechanisms that allow migrant shorebirds to coexist under the constraints of a variable environment as well as evaluate foraging strategies that migrant shorebirds may use in response to this variable environment. The objectives of this study were (1) to document shorebird abundances, migration chronologies, and use of habitat in playas on the PLR during spring and summer/fall migration; (2) to determine diurnal behavior of American avocet (Recurvirostra Americana) long-billed dowitcher (Limnodromus scolopaceus), least sandpiper (Calidris minutilla) and western sandpiper (C. mauri) during spring and summer/fall migration-these 4 species were selected because they were considered the most abundant shorebird species during both migrational periods and they represented a wide range of body sizes; (3) to determine spring and summer/fall diets and food preferences of American avocet, long-billed dowitcher, least sandpiper, and western sandpiper; (4) to determine mechanisms that allow American avocets, long-billed dowitchers, least sandpipers, and western sandpipers to coexist during migration in the PLR; and (5) to determine the foraging strategies used by American avocets, long-billed dowitchers, least sandpipers, and western sandpipers in response to the variable and ephemeral food resources they may encounter during migration.

A diverse group of 30 species of shorebirds used playas during spring and summer/fall, 1993-1994. The most abundant species during spring were American avocet, long-billed dowitcher, and Wilson's phalarope (Phalaropus tricolor), while the most abundant species during summer/fall were American avocet, long-billed dowitcher, long-billed curlew (Numenius americanus). stilt sandpiper (Calidris himantopus) and lesser yellowlegs (Tringa flavipes). Migration chronologies were distinct in spring with peak abundances occurring over 2-4 weeks and were protracted in summer/fell with peak abundances occurring over 5-8 weeks. In general, shorebirds used playas that contained sparse vegetation (<33% vegetation cover), adequate amounts of mudflat (10-15%) and shallow (< 4 cm depth) water (10-20%) habitats, and abundant invertebrate populations (> 1,000 invertebrates/m^2).

Time-activity budgets of American avocets, long-billed dowitchers, least sandpipers, and western sandpipers during spring and summer/fall revealed that feeding was the most important activity for least and western sandpipers, whereas feeding and sleeping/resting were important activities for American avocets and long-billed dowitchers. Least and western sandpipers spent 70-80% of their diurnal time feeding in spring and summer/fall, whereas long-billed dowitchers spent 57% of their time feeding in spring and 77% in summer/fall. American avocets spent similar amounts (47%) of time feeding in each season. This indicates that migrant shorebirds use playas to replenish energy and nutrient reserves.

Invertebrates were an important component in the diets of American avocets, long-billed dowitchers, least sandpipers, and western sandpipers. During spring, chironomids were the predominant invertebrate in the diets of all 4 species. The diets of all 4 species were more diverse in summer/fall. Diet differences between seasons were attributed to seasonal differences in invertebrate abundances and diversities. In general, all 4 species exhibited a wide range of selection patterns for invertebrates. Such patterns suggest that all 4 species use an opportunistic foraging strategy. Shorebird foraging activities appeared to have a minimal impact on invertebrate populations; shorebirds may not have used playas over a long enough time period to have an impact on invertebrate populations.

I used 4 niche dimensions (i.e., diet diversity, food size, foraging method diversity, and foraging habitat) to evaluate differences in niche space among the 4 species. Niche space differed among large species (American avocet), medium species (long-billed dowitcher), and small species (least and western sandpipers). The foraging habitat dimension was the most important resource dimension for segregating large, medium, and small species. Because least and western sandpipers occupied similar niche spaces, a temporal niche dimension may be important in segregating them

I determined foraging strategies of the 4 species by examining food selection patterns of each species (i.e., selection of foods based on quality of a food or abundance of a food). The foraging strategies of all 4 species were opportunistic. Avocets and dowitchers opportunistically consumed prey sizes in accordance with prey size availability, while least and western sandpipers were opportunists on small-prey items. The opportunistic approaches of these 4 species is likely a response to the variable and unpredictable food resources they encounter at stopover sites during migration.

Playas are important stopover sites for a diverse group of migrant shorebirds. Management of playas should focus on creating and maintaining vegetation cover < 33% and adequate mudflat (at least 10-15%) and shallow water (at least 10-20%) habitats. Because invertebrates are important foods of migrant shorebirds, management should also focus on enhancing invertebrate populations in playas. Gradual drawdowns and flooding of playas should be used to provide available habitat for shorebirds throughout the migration. Shallow disking and mowing can be used to create preferred habitat conditions and provide a detrital food base to enhance invertebrate populations.

Description

Keywords

Sandpipers, Shore birds -- Migration -- Texas, Playa -- Texas -- High Plains, Limnodromus, Shore birds -- Food

Citation