Selection of human-influenced and natural wetlands by Great Egrets at multiple scales in the southeastern USA


Wetlands constructed or modified by humans (human-influenced wetlands [HIW]) constitute an increasing proportion of wetland habitat in the USA. It is unclear to what extent HIW (e.g., ponds, reservoirs, impoundments, aquaculture sites, and flooded agricultural fields) provide equivalent habitat for wading birds compared with the natural wetlands they are replacing or augmenting. We compared selection of HIW with natural wetlands by Great Egrets (Ardea alba) in 2 regions containing high proportions of wetlands (73% Louisiana [LA], 39% South Carolina [SC]) and similar proportions of HIW (4.3% LA, 4.5% SC). Great Egrets in LA (n = 11) and SC (n = 19) were tracked using satellite transmitters for up to 1 year to assess selection of home ranges and foraging sites. We also compared selection of flooded agricultural fields vs. natural wetlands as foraging sites from aerial surveys of untagged egrets in LA. In SC, tagged birds showed stronger selection for HIW than natural wetlands as foraging sites, driven by use of small man-made ponds (39.9% of foraging observations), but home ranges did not contain a disproportionate area of ponds. In LA, tagged birds showed no overall selection of HIW at either scale, but unmarked egrets showed strong selection for crayfish aquaculture ponds, especially during drawdown. Rice fields provided only a short window of opportunity for foraging Great Egrets and were not selected over nearby natural sites. Despite widespread availability of HIW in the southeastern USA, our results show that natural wetlands continue to provide the majority of foraging habitat for Great Egrets; however, some HIW types (aquaculture and small ponds) may be strongly selected.

The Condor