Fine-scale foraging site selection by nonbreeding white-faced ibis in managed wetlands of the northern San Joaquin Valley, California

I examined fine-scale foraging site selection by White-faced Ibis (Plegadis chihi) wintering in the Grasslands Ecological Area (the Grasslands), northern San Joaquin Valley, Merced County, California. The Grasslands contains the second largest population of nonbreeding ibis in California, in addition to large numbers of other wintering waterbirds. There were two components to this study. First, I compared habitat variables at White-faced Ibis foraging sites with paired, random locations in managed wetlands of the Grasslands (N = 60). Second, I contrasted the density and biomass of benthic macroinvertebrates between a subsample of bird foraging locations and random sites (N = 20). Within managed wetlands of the Grasslands, White-faced Ibis foraged in water depths shallower than random locations and closer to emergent vegetation 10 cm in height, compared to random locations (P 0.009). Within a shallow range of water depths (approximately 6-16 cm), ibis selected foraging locations with significantly higher chironomid and lower oligochaete biomasses, relative to random sites (P 0.005). I conclude that White-faced Ibis are constrained to forage in shallow water depths, where they selected sites based on invertebrate prey abundance. Moreover, I conclude that ibis foraged close to vegetation where prey abundance may be greater. Features of wetlands that provide suitable habitat for ibis and other waterbirds include shallow to medium water depths from late fall through late spring, and vegetative structure that supports benthic invertebrate populations.