Forest fragmentation, the disruption in the continuity of forest habitat, is hypothesized to be a major cause of population decline for some species of forest birds because fragmentation reduces nesting (reproductive) success. Nest predation and parasitism by cowbirds increased with forest fragmentation in nine midwestern (United States) landscapes that varied from 6 to 95 percent forest cover within a 10-kilometer radius of the study areas. Observed reproductive rates were low enough for some species in the most fragmented landscapes to suggest that their populations are sinks that depend for perpetuation on immigration from reproductive source populations in landscapes with more extensive forest cover. Conservation strategies should consider preservation and restoration of large, unfragmented "core" areas in each region.

Fragmentation of breeding habitat in North America has been implicated in the decline of forestnesting, Neotropical migrant birds. We used a comparative approach to examine the effects of fragmentation on three forest-nesting migrants: Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo ofivaceus), and Wood Thrush (Hylocichla mustelina). We surveyed birds and monitored reproductive success on 28 study plots in fragmented and contiguous forests in two midwestern regions. Distribution of individuals between fragmented and contiguous forests appeared to vary among species and regions, but total nest failure was significantly higher in fragments than contiguous forests in both regions for all species (p = 0.053). We attributed greater nest failure to increased nest predation (p = O. 093) and increased brood parasitism by the Brownheaded Cowbird (Molothrus ater, p = 0. 009). In addition to greater total nest failure, partial nest faaure due Exito reproductivo de las aves migratorias en hhbitats fuente y sumidero Resutnen: La fragmentaci6n del hdbitat reproductivo en Norte Amdrica ha estado relacionada con la dismi-nuci6n de las ayes migratorias Neotropicales que anldan en los bosques. En este estudio usamos un mdtodo comparativo para examinar el efecto de la fragmentaci6n sobre tres ayes que anldan en los bosques: Seirus aurocapillus, Vireo olivaceus, y Hylocichla mustelina. Monitoreamos el dxito reproductivo en 28 parcelas en bosques fragmentados y continuos, en dos regiones geogrdtficas en el medio-oeste de los Estados Unldos. La distribuci6n de los individuos entre bosques fragmentados y continuos pareci6 variar entre especies y regiones, pero el fracaso total de los nidos fue stgntficativamente mdts alto en los fragmentos queen los bosques continuos, en ambas regiones para todas las especies (p = 0.053). Atribuimos los mayores fracasos de los nidos a la mayor depredaci6n (p = 0.093) y al mayor parasitismo por parte de Molothrus ater (P = 0.053). Ademdts del mayor fracaso total de los nldos, los fracasos parclales, debidos al parasitismo pot parte de Molo-1381 thrus ater, condujeron a una reducct6n en el ngmero de volantones de los hudspedes. St bidn las causas de la mortaltdad en los nidos parecteron ser especiflcas para carla especie, los fracasos torales y parctales de los nidos actuaron en forma conjunta reductendo el namero de descendientes por adulto en los fragmentos para las tres espectes. Usamos modelos stmples de crectmtento poblaclonal para evaluar la viabtlidad de las ires especies en hdbttats fragmentados y cont~nuos en ambas regiones. En general, las poblactones de los fragmentos parecteron ser poblaciones sumidero y las poblaclones en los bosques conttnuos parecieron ser poblaclones fuente. Asumiendo una mortaltdad constante durante el tnvterno, el crectmtento proyectado indtc6 queen ausencia de tnmigract6n, las poblactones de S. aurocapillus y V. olivaceus deberlan exttnguirse en los fragmentos en ambas regiones, y las poblaciones de H. mustelina deber~n mantenerse o declinar levemente en los fra...

Ecological processes near habitat edges often differ from processes away from edges. Yet, the generality of “edge effects” has been hotly debated because results vary tremendously. To understand the factors responsible for this variation, we described nest predation and cowbird distribution patterns in forest edge and forest core habitats on 36 randomly selected plots in three states in the midwestern United States. We tested four hypotheses that may explain the variation and mechanisms responsible for edge effects among the 36 plots: (1) the landscape context, (2) the local predator community, (3) the local bird (host–prey) community, and (4) the nest site microhabitat structure. We used artificial nests baited with quail and clay eggs to determine nest predation patterns and predators and used point count surveys to determine cowbird and host abundance in forest edge and forest core habitats. Raccoons, opossums, canids, and birds accounted for most predation of artificial nests. Neither local host abundance nor mean nest concealment of artificial nests significantly influenced nest predation rates in habitat edge or in habitat core. Nest predation was significantly greater in highly fragmented landscapes than in unfragmented landscapes and was significantly higher in edge habitats than in core habitats. However, detection of edge effects varied, depending upon landscape type. Higher predation rates in edge habitats were detected in highly and moderately fragmented landscapes, but not in unfragmented landscapes. Both mammalian and avian predator groups contributed to higher predation rates along edges in highly and moderately fragmented landscapes. Cowbird abundance was significantly related to host abundance, but the effect of hosts varied depending upon habitat type. In edge habitats, cowbird abundance was negatively associated with host abundance in all three landscapes studied. By contrast, cowbird abundance was positively associated with host abundance in core habitats. Once the effects of host abundance were removed, cowbird abundance in core habitat was greater in highly fragmented landscapes than in moderately and unfragmented landscapes, but did not differ between the latter two. In edge habitat, cowbird abundance did not differ between landscapes, but abundance in edges tended to be highest in the highly fragmented landscape and lowest in the unfragmented landscape. Cowbird abundance did not vary between edge and core habitat in any of the landscapes studied. We suggest that the first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale. Given geographic location, local factors such as host abundance and predator composition should be assessed.