Landscape influence on the quality of heron and egret colony sites

Abstract

We evaluated landscape associations related to heron and egret colony site selection and the productivity of successful great blue heron (Ardea herodias) and great egret (Ardea alba) nests. The study was based on annual observations (1991–2005) at 45 colony sites known to be active within 10 km of historic tidal marshes of northern San Francisco Bay. The analyses focused on a priori models analyzed within 1, 3, 5, 7, and 10 km of colony sites, using the areal extents of several NOAA land cover types (Landsat images, 2000–2002), number of wetland patches, and total wetland edge as predictor variables. A comparison of landscape characteristics surrounding colony sites with those surrounding randomly selected, unoccupied sites revealed the primary importance of estuarine emergent wetland and open water within 1 km of colony sites. Increased productivity in successful great blue heron nests was associated with more estuarine emergent wetland, open water, and low-intensity development, and less grassland, but was not differentially related to the extent of habitat available within any particular distance from colony sites. The productivity in successful great egret nests was associated with variation in habitat extent at larger spatial scales, especially within 10 km of heronies, with nests producing more young at sites surrounded by more estuarine emergent wetland and low-intensity development, less open water and palustrine emergent wetland, and more patches of wetland habitat. To estimate landscape foraging patterns, we used aircraft to track the flights of great egrets departing from heronries and used the observed flight distances, colony sizes, and the regional distribution of wetland habitat to model regional foraging densities. Results suggested that increasing the extent of wetland feeding areas for herons and egrets might improve reproductive performance in colony sites up to 10 km away, increase foraging by herons and egrets in created or restored wetlands within 3–10 km of sites, and enhance nest abundance at colony sites within 1 km of restoration sites. Regional maps based on the distribution of colony-sites and predictions of landscape influences on colony site selection, nest productivity, and foraging dispersion, suggested areas potentially suitable for colonization.