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Nest-site selection and nest survival of early successional birds in central Pennsylvania.

Article Excerpt
Many neotropical migrant bird species in the northeastern United States are experiencing population declines (Sauer et al. 2008). Those using early successional habitats appear to be at particularly great risk, perhaps because of habitat loss due to forest maturation (Askins 1993, Litviatis 1993). Species nesting in shrubs also may experience higher rates of nest predation than those nesting on the ground or in trees (Martin 1992, 1993a). Nest predation can reduce recruitment to a population and affect population size (Martin 1992, Sherry and Holmes 1992). Information relating demographic parameters to habitat characteristics is essential for identifying high-quality habitats as conservation priorities, for examining causes of population change, and for predicting consequences of habitat change (Sherry and Holmes 1995). Studies addressing processes such as nest survival at the species-assemblage level provide important data for monitoring effectiveness of habitat management activities that affect entire communities.

Nest predation is the primary source of nest mortality for most bird species (Ricklefs 1969; Martin 1992, 1993b). Predation risk varies not only among habitat types but also at the microhabitat scale. Many species choose nest sites nonrandomly with respect to vegetation characteristics (Martin and Roper 1988, Holway 1991, Knopf and Sedgwick 1992), and some may preferentially select nest sites with lower predation risk (Martin 1992, 1998; Siepielski et al. 2001). Nest-site preferences may be adaptive, if heritable, when birds choose sites associated with low nest predation risk.

A number of studies have identified microhabitat characteristics that may be important for nest-site selection or nest survival, including foliage density and concealment (Martin 1992, Kelly 1993, Norment 1993, Johnson 1997), nest height (Best and Stauffer 1980, Wilson and Cooper 1998), substrate (Schmidt and Whelan 1999), and composition of vegetation surrounding nests (Martin and Roper 1988). We examined the relationship of these factors to nest-site selection and nest survival for an early successional bird community, focusing on individual species when possible. Our objectives were to identify: (1) which microhabitat characteristics were selected by early successional birds, (2) which characteristics were associated with lower predation risk, and (3) whether birds selected characteristics associated with lower predation risk.

Defoliation by gypsy moth (Lymantria dispar) larvae drastically altered the vegetation structure at our site during 1 year (2007) of the study. Nest predation may increase in defoliated areas in years following a gypsy moth outbreak (Thurber et al. 1994), but defoliation in closed-canopy forests may create more nesting substrate for shrubnesting birds by allowing more light to reach shrub layers (Bell and Whitmore 1997, 2000). However, few studies have addressed effects of defoliation on nest predation risk during outbreak years. We did not attempt to relate nest survival to defoliation directly but examined temporal changes in nest survival that may correspond to periods of heavy defoliation.

METHODS

The Barrens Grouse Habitat Management Area (GHMA), which is part of State Game Lands 176, is in Centre County, Pennsylvania in the Ridge and Valley province (Yahner 1993, 2003). It contains 136 square, contiguous 4-ha blocks in a sector treated by even-age management with each block consisting of four 1-ha (100 X 100 m) plots (A-D, labeled clockwise). We used the 76 blocks classified as mixed-oak (Quercus spp.) cover type because it is the most common cover type in Pennsylvania (McWilliams et al. 2004). Plots A, B, and C in each block were clearcut with retention of overstory trees (15-20 trees > 10 cm dbh/plot) in three cutting cycles during winters 1976-1977, 1986-1987, and 1999-2000, respectively, but D plots remained uncut. We used only C plots (76 ha total), which were at least 100 m apart and surrounded by older forest (Schill 2007).

Common overstory trees were white oak (Quercus alba), chestnut oak (Q. montanus), scarlet oak (Q. coccinea), northern red oak (Q. rubra), and pignut hickory (Catwa glabra). Major understory and shrub species were oak, red maple (Acer rubum), and blackberry (Rubus allegheniensis). Gypsy moth larvae defoliated much of the vegetation at the site in 2007. Severe defoliation began in early June, peaking during the third week of June. Most overstory trees (>90%) and saplings, particularly oak and maple, and large patches of the shrub layer were mostly or completely defoliated. A second leaf flush began in early July.

We located nests from mid-May to mid-July 2006 and 2007 using parental behavior and conducting searches of the shrub layer (Martin and Geupel 1993). We monitored nests of Chestnut-sided Warblers (Dendroica pensylvanica), Indigo Buntings (Passerina cyanea), and Field Sparrows (Spizella pusilla) in 2006; we monitored all nests ([less than or equal to] 1.6 m height) of early successional birds in 2007. Nests were checked every 3 days until evidence of fledging or predation was found. A nest was considered successful if at least one young fledged. We assumed a nest failed if evidence of predation was found or if the nest was empty before fledging was possible (Martin et al. 1997). We measured vegetation characteristics after young fledged or the nest failed (usually 3-6 days afterwards) at the nest site and within a 5-m radius of the nest, hereafter termed the nest patch, for only Chestnut-sided Warbler nests in 2006 and...

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