Factors Affecting Nesting Success in Riparian Bird Communities

Abstract

Nest and egg successes are documented for open-nesting bird species in a variety of riparian habitats in central Iowa [USA]. In most species, nest success was higher during the nestling period than during the incubation period. Causes of nest failure in order of decreasing importance were predation by birds, snakes or small mammals; predation by large mammals; desertion; cowbird parasitism; natural disasters. Relationships between nesting outcome and the following variables are analyzed statistically: adult weight, date, nest height, nest concealment, vegetation form supporting the nest (support life-form) and habitat type. Nest failure resulting from predation by large mammals and parasitism by cowbirds [Molothrus ater] was greater among smaller-sized birds. The percentage of nests successfully fledging young increased with nest height. Losses from natural disasters differed among the support life-forms. Relationships among the factors that may affect nesting outcome are compared statistically to determine possible interactions. Mean body weight, which differed according to nest support life-forms, decreased as the season advanced. Body weight also was inversely related to nest concealment. Concealment differed among nest supports and was greater later in the breeding season. Concealment also was inversely related to nest height. The distribution of nests in the support life-forms changed during the season. Species more generalized in selecting nest substrates had lower nest success.