Edge effect on nesting success of ground nesting birds near regenerating clearcuts in a forest-dominated landscape

Auk, The, Oct 2002 by Manolis, James C, Andersen, David E, Cuthbert, Francesca J

ABSTRACT.-Forest fragmentation has been implicated as a cause of population declines of several Neotropical migrant bird species. Fragmentation increases the amount of habitat edge, and reduced nesting success rates near forest edges are well documented in agricultural landscapes ("edge effects"). However, edge effects in predominantly forested landscapes, particularly those related to timber harvest, are poorly understood. This study examines nesting success of ground nesting birds in relation to clearcut edges in a forest-dominated landscape in north-central Minnesota. A total of 383 nests of seven species of ground nesting birds were found and monitored during 1992-1998. Ovenbird (Seiurus aurocapilus; n = 318) and Hermit Thrush (Catharus guttatus; n = 44) nests composed the majority of the sample. Predation accounted for 94% of all nest failures. Brown-headed Cowbird (Molothrus ater) parasitism was low (1.8% for all ground nests). Using proportional hazards regression, distance to nearest clearcut edge was the best predictor of nest failure. For all ground nests, nesting success was 0.18 at 0-100 m, 0.39 at 101-500 m, and 0.52 at 501-954 m from nearest clearcut edge. Source-sink modeling indicated that distances

RESUMEN.-La fragmentacion de bosque ha sido identificada como una causa de la disminucion de las poblaciones de varias especies de aves migratorias Neotropicales. La fragmentacion aumenta la cantidad de borde del habitat. La reduccion de las tasas de exito de nidificacion cerca de los hordes del bosque en paisajes agricolas ("efectos de borde") ha sido ampliamente documentada. Sin embargo, los efectos de borde en paisajes predominantemente arbolados, particularmente en aquellos relacionados a la cosecha de madera, se encuentran pobremente entendidos. Este estudio examina el 6xito de nidificacion de aves que anidan en el suelo en relacion a hordes de claros en un paisaje dominado por bosque en el norte-centro de Minnesota. Se encontraron un total de 383 nidos de siete especies que anidan en el suelo, los cuales fueron monitoreados durante el periodo de 1992-1998. Los nidos de Seiurus aurocapilus (n - 318) y Catharus guttatus (n = 44) compusieron la mayoria de la muestra. La depredacion dio cuenta de un 94% de todos los fracasos de los nidos. El parasitismo por Molothrus ater fue bajo (1.8% para todos los nidos en el suelo). Usando una regresion de riesgo propertional, la distancia al borde del claro mas cercano fue la variable que mejor predijo el fracaso de los nidos. Para todos los nidos del suelo, el exito de nidificacion fue 0.18 a 0-100 m, 0.39 a 101-500 m, y 0.52 a las 501-954 m del borde del claro mas cercano. Modelos "fuente-sumidero" indicaron que los habitat a una distancia

OVER THE PAST DECADE, forest fragmentation and related edge effects have received considerable attention for their potential role in population declines of forest-nesting Neotropical migratory birds. Robinson et al. (1995) reported a strong correlation between forest fragmentation and nest predation rates on several songbird species in the midwestern United States. They suggested that the extensive, relatively unfragmented forests of northern Minnesota, Wisconsin, Michigan, and Missouri may serve as reproductive "sources" for "sink" populations in small forest fragments elsewhere in the Midwest. Reproduction is so low in some forest fragments that populations are not self-sustaining; those sink populations depend on dispersal from source populations to perpetuate themselves (Donovan et al. 1995, Robinson et al. 1995).

Forest fragmentation may cause decreased nesting success because smaller forest fragments have a greater proportion of "edge" relative to forest interior than larger forest fragments. Edges-of ten distinct boundaries between forest and surrounding agricultural or residential land-- may represent "hostile" environments because of increased brood parasitism by Brown-headed Cowbirds (Molothrus ater) and predation (Askins 1995). Numerous studies have documented increased parasitism and predation near edges (summarized in reviews by Paton 1994, Andren 1995, Faaborg et al. 1995). However, the majority of those studies were conducted in agricultural landscapes. Fewer studies have examined reproductive success of birds in forest-dominated areas that are unfragmented by agriculture. Yet, forest-dominated areas, hypothesized to be population sources, are not entirely "unfragmented," as logging, development, and natural disturbances create a variety of edges and openings.

Thus, it is important to clearly define fragmentation in the context of forest-dominated landscapes. We prefer Lord and Norton's (1990) definition of fragmentation-simply the disruption of continuity of a habitat. This flexible definition allows a continuum of subtypes; from "enveloping fragmentation," where continuous forest is broken into isolated fragments, to "intrusive fragmentation," where gaps are created within continuous forest (Harris and Silva-Lopez 1992). Formation of gaps and fragments can be either temporary, as in the case of forest harvest or natural disturbance, or permanent (in a relative sense) as in housing development within extensive forest or isolation of small wood lots by agriculture. Permanent fragmentation can also occur on a geologic time-scale, as in the formation of lakes and wetlands or differing soil conditions (Lord and Norton 1990).