The interplay of habitat change, human disturbance and species interactions in a waterbird colony

Potential responses to human disturbance at breeding colonies of waterbirds include reproductive failure, population declines and displacement from activity areas. Several additional factors, including species interactions and environmental change, can either mask or intensify the effects of human activity. This study highlights the importance of considering these factors in concert with breeding biology when assessing the impacts of human disturbance on wildlife. We studied the effects of a Wildlife Viewing Area (WVA) at Chatfield State Recreation Area, Colorado, on a nesting colony of great blue herons (Ardea herodias) and double-crested cormorants (Phalacrocorax auritus). We stratified the colony's nest trees into near, middle and far areas relative to distances from the WVA and compared the distribution of nests, nesting and fledging success and breeding chronology among areas 2 y before and 2 y after construction of the WVA. We also evaluated whether adult nest attendance patterns and chick behavior differed relative to distance from the WVA. The number of active heron nests and nest success of herons declined during the study, but evidence that these declines were due solely to human disturbance is equivocal. These changes were most likely due to the interplay of habitat changes (loss of 14 of 31 original nest trees by windfall), acquisition of heron nests by cormorants and human disturbance. We found no evidence that cormorants were adversely affected by the WVA in distribution of nests, nesting and fledging success, breeding chronology, adult nest attendance or chick behaviors. Habitat changes and adverse weather contributed to nesting failures of cormorants.