Measuring Edge Effects on Nest Predation in Forest Fragments: Do Finch and Quail Eggs Tell Different Stories?
Niehaus, Amanda C., Heard, Stephen B., Hendrix, Stephen D., Hillis, Stephen L., The American Midland Naturalist
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 guttala) 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.
Recent declines in populations of neotropical migratory songbirds have received considerable attention from professional and amateur ornithologists and are believed to be related to widespread habitat loss and fragmentation in both breeding and wintering grounds (Terborgh, 1989; Askins et al., 1990; Robinson et al., 1995). Fragmentation not only isolates patches, but also increases the proportion of edge habitat (Hansen and Urban, 1992) which differs from the forest interior in ways that affect the nesting success of many bird species (Hawrot and Niemi, 1996). For example, mixed vegetative nest cover (Johnston and Odum, 1956) and higher abundance of arthropod prey at edges may enhance nesting success, but these advantages could be offset by higher densities of nest predators such as raccoons (Procyon lotor), foxes (e.g., Vulpes fulva), sciurids and corvids (Andren and Angelstam, 1988; Leimgruber et al., 1994).
Effects of edge on avian nest success have been hotly debated (Donovan el al., 1997) because of mixed results from experimental studies in a wide range of biomes, regions and forest types (Latta et al., 1995; Huhta et al., 1998; Wong et al., 1998). These mixed results may be due in part to the use in many artificial nest experiments of excessively large quail (Coturnix spp.) eggs (Major and Kendal, 1996; Ortega et al., 1998; Newton and Heske, 2001), which are 30-100% larger than the eggs of most neotropical passerines (Haskell, 1995a). This size difference matters because egg predators include small-mouthed mammals (e.g., shrews, mice, chipmunks and rats; Boag et al., 1984; Haskell, 1995b; Ettel et al., 1998), and these animals may have insufficient jaw gape to grasp a quail egg (Roper, 1992; Rangen et al., 2000). Indeed, small-mouthed mammals such as white-footed mice (Peromyscus leucopus, DeGraaf and Maier, 1996), cotton rats (Sigmodon hispidus; Ettel el aL, 1998), eastern chipmunks (Tamias striatus, Haskell, 1995b) and house mice (Mus musculus, Marini and Melo, 1998) are often unwilling or unable to consume quail eggs, even after fasting.
The use of large quail eggs in field experiments could produce exaggerated or even spurious edge effects if large mammals (e.g., skunks, raccoons and foxes) and corvids are more abundant at edges than interiors, but small mammals are not. …