The effects of patch shape and connectivity on nest site selection and reproductive success of the Indigo Bunting

Abstract

Habitat fragmentation and its associated effects have been blamed for the recent population declines of many Neotropical migratory bird species. Increased predation and parasitism resulting from edge-related effects have been implicated for poor nesting success in many studies, mostly of forest interior species. However, little attention has been devoted to disturbance-dependent birds. In this study, I examine how patch shape and connectivity in fragmented landscapes affects the reproductive success of disturbance-dependent bird species, specifically the Indigo Bunting (Passerina cyanea). I conducted my study in a landscape-scale experimental system of similar-area habitat patches that differed in connectivity and in shape. Shapes differed between edgy and rectangular forms, where edgy patches contained 50% more edge than rectangular patches. I tested whether edgy patches function as ecological traps for species with strong edge preferences, by leading them to select dangerous habitats. Indigo Buntings preferentially selected edgy patches over rectangular patches, but experienced significantly lower reproductive success in edgy patches early in the season. Although predation pressure intensified in rectangular patches late in the season, seasonal fecundity was still significantly lower in edgy patches, providing the first empirical evidence that edges can function as ecological traps for Indigo Buntings.

A second objective of my study was to evaluate the efficacy of conservation corridors for disturbance-dependent bird species. Conservation corridors have become a popular strategy to preserve biodiversity and promote gene flow in fragmented landscapes, but corridors may also have negative consequences. I tested the hypothesis that corridors can increase nest predation risk in connected patches relative to unconnected patches. Nest predation rates increased significantly in connected patches compared to unconnected rectangular patches, but were similar between connected patches and unconnected edgy patches. This suggests that the increase in predator activity in connected patches is largely attributable to edge effects incurred through the addition of a corridor. This is the first landscape-scale study to experimentally demonstrate the potential negative effects of conservation corridors.