Considerations for Conservation of Shrubland Birds in Early Successional Forest Habitat

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Title: Considerations for Conservation of Shrubland Birds in Early Successional Forest Habitat
Author: Shake, Corey Scott
Advisors: Dr. Christopher E. Moorman, Committee Chair
Abstract: Populations of many bird species associated with shrubland habitats are declining in the eastern United States, but incentive programs that restore shrubland or early-successional forest habitat on privately-owned land may help to ameliorate these declines. The habitat patches created by these programs are highly variable in size, shape, and surrounding habitat matrix, and it is unclear how these characteristics affect patch occupancy and nest survival of shrubland passerines. Our first objective was to determine how patch area, patch shape, and extent of forest cover in the surrounding landscape affect shrubland bird species’ occupancy of early-successional forest habitat patches and, for species that were area-sensitive, we sought to identify minimum area requirements. Our second objective was to determine if nest predation was higher at habitat edges, and whether patch vegetation structure or the landscape surrounding a patch influenced nest predation rates. To study patch occupancy, we surveyed 35 individual habitat patches in 2007 and 43 in 2008 for the presence of nine shrubland birds in North Carolina, USA. We then modeled individual patch occupancy probability of five of these species relative to patch area, patch shape, and % forest cover within 1 km of the patch. We documented evidence of area-sensitivity for yellow-breasted chat (Icteria virens) and prairie warbler (Dendroica discolor), and estimated minimum area requirements of 2.3 and 1.1 ha, respectively. Blue grosbeaks (Passerina caerulea) also were area-sensitive in irregularly-shaped patches. Predicted individual patch occupancy probability was >0.9 in patches ≥5.5 ha for all area-sensitive species. Shape index alone and proportion of forest cover were not important predictors of occupancy for shrubland birds. Restored shrubland and early-successional forest in agricultural landscapes can provide habitat for many shrubland birds, but patches should be >5 ha to maximize shrubland bird diversity. To study nest predation, we collected data on nests of five shrubland passerine species during the 2007 and 2008 breeding seasons in 12 early successional forest patches in North Carolina, USA. We used model selection methods to assess the effect of distance to cropland and mature forest edge on nest predation rates and accounted for other sources of variation, including temporal trends, nest stage, vegetation structure, and landscape context. For nests of all species combined, nest predation decreased with increasing distance to cropland edge, by nearly 50% at 250 m from the cropland edge. Nest predation of all species combined also was higher in patches with taller saplings and less understory vegetation, especially in the second year of our study when trees were 4-6 m tall. Predation of field sparrow (Spizella pusilla) nests was lower in landscapes with higher agricultural landcover. Nest predation risk for shrubland birds appears to be greater near agricultural edges than mature forest edges, and natural forest succession may drive patterns of local extinction of shrubland birds in regenerating forest patches. Thus, we suggest that habitat patches managed for shrubland bird populations should be considerably large or wide (>250 m) when adjacent to crop fields and maintained in structurally-diverse early seral stages.
Date: 2009-12-07
Degree: MS
Discipline: Fisheries and Wildlife Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/2080


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