Factors Affecting Nesting Success in Riparian Bird Communities

Nest and egg successes are documented for open-nesting bird species in a variety of riparian habitats in central Iowa.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 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.Nesting success probably has been documented more extensively than any other facet of avian breeding ecology (for reviews see Nice 1957, Ricklefs 1969), but few workers (Kendeigh 1942, Young 1949, Nolan 1963, Gates and Gysel 1978) have considered nesting success at the community level.Detailed analyses of factors affecting nesting outcome also are scarce, although several variables are known to contribute to nesting failure.Because the sites selected for nest placement vary in probability for success, mortality during nesting is an important force in the evolution of avian reproductive behavior.The objectives of this study were to document nesting success among open-nesting species in avian communities occupying riparian habitats, and then to evaluate factors affecting nesting outcome.We attempted to determine if certain nest-site characteristics were associated with certain mortality factors, irrespective of differences in the breeding behavior of the individual bird species.Hence, we focussed on the avian community, rather than any single species or a few closely related species.This approach is reasonable because nests of all members of the community represent potential prey for predators as well as hosts for the generalist brood parasite, the Brownheaded Cowbird (Molothrus ater).Also, all nests are subject to the rigors of weather and to other natural catastrophes.METHODS Chosen for study were 28 sites along Brushy Creek, Beaver Creek, and the Middle and South Raccoon rivers in Guthrie County, Iowa.These represented a range of riparian habitats from open hayfields to closed-canopy woodland.Procedures for establishing the study sites and the floristic composition of the various habitats are described in Stauffer and Best (1980).We found most nests by watching the behavior of breeding birds and by searching suitable vegetation.Additional nests were found while censuses were conducted.Nests were harder to find in treeless hayfields and dense forests than in areas with a semi-open woodland canopy.The field season extended from mid-April to mid-July during 1976 and 1977.We checked the status of active nests every four or five days before June, and every two or three days thereafter.We tried to avoid trampling or disturbing vegetation near the nest site.A mirror on a pole was used to view contents of high nests; data were restricted to nests within 8 m of the ground.At each visit, we recorded the number of eggs and(or) young, as well as any evidence of predation or cowbird parasitism.Nest predation where the nest bowl was torn apart and(or) the nest partly or entirely pulled from its original po-I1491