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.

Effects of Roads on Movements by Understory Birds in Mixed-Species Flocks in Central Amazonian Brazil

“…In temperate forests, edges of fragments or narrow corridors, such as roads or powerlines, may create ecological traps, attracting predators, parasitic cowbirds, or nonforest birds, especially in heavily fragmented landscapes (Gates & Gysel 1978;Ambuel & Temple 1983;Rich et al 1994;Robinson et al 1995). Along the road we studied, we do not know whether the main predators of mixed-species flock birds, probably Micrastur forest-falcons, are affected by edges.…”

Testing mechanistic explanations for mammalian predator responses to habitat edges

“…Hence, the possibility that the occurrence of mammalian predators in edge elements was the result of overflow from one habitat through the habitat edge into the second habitat (i.e., the spillover model; the matrix edge effect, sensu Lidicker 1999) was not supported during the second year. Although the spillover model has been supported by empirical data in several cases (Andrén and Angelstam 1988;Heske 1995), many studies have not been able to test this hypothesis because predation patterns have rarely been measured across the whole ecotone continuum (but see Gates and Gysel 1978;Ratti and Reese 1988).…”

Influence of fragmentation and disturbance on the potential impact of feral predators on native fauna in Australian forest ecosystems

“…Overseas studies have demonstrated that fragmentation of habitat results in large edge-toarea ratios and subsequent high predation rates by predators originating from outside the habitat patches and/or within the edge environment (Gates and Gysel 1978;AndrCn et al 1985;Wilcove 1985). For example, AndrCn et al (1985) found that predation on nests was positively correlated to the degree of fragmentation of the forest habitat.…”

“…There have been many studies investigating the rate of predation within the edge environment (e.g. Gates and Gysel 1978;AndrCn et al 1985;Wilcove 1985;AndrCn and Angelstram 1988;Maller 1988;Gibbs 1991;AndrCn 1992). AndrCn and Angelstam (1988) found that predation rates increased and levelled off 200-500 m from the edge, while Wilcove (1985) demonstrated that an edge-related increase in predation may extend upto 600 m into the forest.…”