Effects of forest fragment size, nest density, and proximity to edge on the risk of predation to ground-nesting passerine birds

Many species of Neotropical migrant songbirds are experiencing population declines. Degradation of habitat on breeding grounds, particularly the fragmentation and isolation of forest habitats, has been suggested as an important factor in the decline of these populations. Using artificial ground nests containing quail and clay eggs, we compared risk of predation relative to the size of forest fragments, the proximity of nests to forest edge, and the density of nests. Because small predators rarely can damage or remove quail eggs, the loss of these eggs reveals activity by large predators. Small predators, however, often leave identifiable claw and tooth marks on artificial eggs, and we used such marks to assess the importance of small predators as nest predators. We found a negative correlation between predation rate and fragment size. Nest clustering increased predation by large predators but did not affect small predator activity. No significant edge effects on predation were apparent. The relative threat to nesting songbirds posed by small and large predators changed with forest fragmentation. Large predator activity increased as forests become more fragmented. Historically, interior forest-dwelling birds have been subjected primarily to small predator activity. The increased large predator activity we documented, against which songbirds have no defense, could be partially responsible for recent population declines.