Factors affecting predation on artificial nests in marshes

Artificial waterfowl and passerine nests containing chicken or quail eggs were used to examine the effects of breeding season date, nest concealment, and surrounding upland habitat type (high density urban, agricultural, and natural areas) on nest predation in 8 marshes in 1989 and 1990. Predation on waterfowl nests was lower than on passerine nests in agricultural marshes. Predation on waterfowl nests consistently increased seasonally in all marshes in both years, in spite of the increase in nest concealment resulting from growing vegetation. The increasing predation was likely a result of decreasing water depth that increased accessibility of waterfowl nests to terrestrial predators such as raccoons (Procyon lotor). Predation on passerine nests was generally high throughout the 2 seasons in agricultural marshes but increased seasonally in urban and natural marshes. We argue that this pattern was caused by predation by marsh wrens (Cistothorus palustris) that were breeding mostly in agricultural marshes throughout both seasons. In the natural marshes, the pattern of predation on passerine nests reflected increasing effect of mammalian predators, presumably resulting from decreasing water depth. Our results support the view that water depth plays the key role in reproductive success of marsh-nesting birds. Successful management of marsh-nesting birds thus requires maintenance of high water levels throughout the breeding season.