Foraging ecology of Reddish Egrets in the Laguna Madre of Texas.

摘要

The Reddish Egret (Egretta rufescens) is highly restricted in its distribution, relying primarily on coastal wetland habitats along the Gulf Coast. There is an estimated 2,000 breeding pairs in the United States making it our rarest species of heron. Much remains unknown about the foraging ecology of Reddish Egrets, making it difficult to identify important foraging areas. I studied Reddish Egret foraging behavior and foraging success in the Laguna Made of Texas from March 2008-April 2010. I also assessed the temporal dynamics of Reddish Egret foraging habitat availability throughout the annual cycle. Foraging Reddish Egrets were recorded with a 1-40x camcorder and environmental variables including wind speed (m/s), light intensity (lux), water depth (cm), and percent seagrass coverage were also recorded. Foraging habitat was delineated using benthic habitat characteristics, bathymetry, and tidal readings in the Laguna Madre. Eighty-six percent of Reddish Egrets foraged in water depths from 5-20 cm and in areas with 10% seagrass coverage. After-hatch-year Reddish Egrets were more successful foragers than hatch-year individuals. They also spent more time foot-stirring and walking whereas hatch-year birds spent more time standing. Group foraging Reddish Egrets were more efficient foragers and had a higher capture rate than those foraging solitarily. Reddish Egrets in groups spent more time foot-stirring whereas individual foragers spent more time standing and wing-flicking. All four environmental variables influenced foraging success, but did not account for much variation in foraging behavior. Foraging habitat availability fluctuated between 1,200 and 14,000 ha throughout the annual cycle with only 115 ha available ≥ 50 percent of the time. Large amounts of stable foraging habitat (10,000-12,000 ha) were available from August-October when post-breeding adults and hatch-year individuals were present. Foraging habitat fluctuated widely during the breeding season from March-June. Large areas of foraging habitat may be required to ensure enough habitat is available at any time to support the Reddish Egret population in Texas.