Avian Nest Dispersion and Fledging Success in Field‐Forest Ecotones

Abstract

Observations of 21 species of open—nesting passerines breeding in contiguous field and forest habitats at Rose Lake Wildlife Research Area, Michigan, were made during 1974 and 1975. Data were collected on nest dispersion, clutch—size, and fledging success in relation to the field—forest edge. Losses of eggs or nestlings were attributed to predation, inclement weather, Brown—headed Cowbird (Molothrus ater) parasitism, nest desertion, hatching failure, and adult death. Each bird species seemed to have a preferred distance from the habitat discontinuity that was used as a nest site. Furthermore, nests were not uniformly distributed on the area. Over one—half of the nests were found within ±15 m of the habitat discontinuity. Seventy—five percent of the nests belonged to birds characteristic of mixed breeding habitats, i.e., birds requiring an open overstory canopy with elevated singing and observation perches and dense cover near the ground for nesting and feeding. These mixed—habitat species also accounted for the increase in avian species nesting near edges. Based on Kendall rank and partial rank correlation tests, increasing numbers of nests and the percentage of total clutches, <3 eggs were found to be negatively correlated with increasing distance from the habitat discontinuity. Correlation between fledging success and increasing distance from the edge was positive and highly significant. Of the several mortality factors investigated, predation and cowbird parasitism were found to be the most important. The increased predation rate with decreased distance from the edge was attributed primarily to a functional response to higher numbers of nests and a greater activity of potential nest predators in the vicinity of the habitat discontinuities. Our results indicate that habitat suitability decreases with increasing numbers of nests toward the narrow field—forest edges. Although such abrupt habitat discontinuities did attract a variety and abundance of birds characteristic of habitats with mixed life—form, these discontinuities seemed to function as "ecological traps" by concentration nests and thereby increasing density—dependent mortality. Ironically, the cowbird was also a victim of the increased predation rate. As these man—made forest edges are of recent origin, they are perhaps unrepresentative of the ecological niche in which these species evolved, and thus they may be poorly adapted to cope with the increased nest predation.