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 ac tivity areas. Several additional factors, including species interactions an d environmental change, can either mask or intensify the effects of human a ctivity. This stud). highlights the importance of considering these factors in concert with breeding biology when assessing the impacts of human distu rbance 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 a reas relative to distances from the WVA and compared the distribution of ne sts, nesting and fledging: success and breeding chronology among areas 2 y before and 2 y after construction of the WVA. We also evaluate tl whether a dult nest attendance patterns and chick behavior differed relative to dista nce from the WVA. The number of active heron nests and nest success of hero ns declined during the study but evidence that these declines were due sole ly to human disturbance is equivocal. These changes were most likely due to the interplay of habitat chang es (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 weath er contributed to nesting failures of cormorants.