Measuring Edge Effects on Nest Predation in Forest Fragments: Do Finch and Quail Eggs Tell Different Stories?

Abstract

Experiments assessing rates of avian nest predation often find that nests near forest edges are at high risk of predation, suggesting the importance of forest fragmentation in recent population declines of ground-nesting passerines. However, the use of quail (Coturnix spp.) eggs in nest predation experiments may confound conclusions about edge effects because only large-mouthed predators are able to consume these relatively large eggs, but both large and small-mouthed predators consume smaller passerine eggs. We directly compared predation rates on artificial nests baited with quail eggs or with zebra finch (Poephila guttata) eggs; the latter are similar in size to the eggs of many neotropical passerines. In 1998 and 1999 we placed 392 artificial ground nests at edge and interior locations in two east-central Iowa forest fragments. Predation on these nests varied with egg type (quail or finch) and location (edge or interior) and there was a significant interaction between egg type and location: predation on quail eggs was greater at edges than in the interior, whereas finch egg predation was high in both edge and interior locations. Based on tooth imprints in clay eggs, we determined that large-mouthed predators were six times more active at edges, whereas activity of small-mouthed nest predators was evenly distributed between edge and interior locations. We suggest that the use of only quail eggs can exaggerate edge effects and that finch eggs or clay eggs used in conjunction with quail eggs in artificial nests can be used to estimate relative predation rates by large- and small-mouthed predators.