Effects of Plot Size and Habitat Characteristics on Breeding Success of Scarlet Tanagers

-We studied the effects of forest patch size and habitat characteristics on breeding success of Scarlet Tanagers (Piranga olivacea) in western New York in 1995 and 1996. Twenty forest stands were grouped into four size classes: Group I (<10 ha, n = 6), Group 11 (10 to 50 ha, n = 7), Group III (>50 to 150 ha, n = 5), and Group IV (>1,000 ha, n = 2). Group I, II, and III sites were habitat patches located in fragmented landscapes, whereas Group IV sites were located in continuous forests. Although densities of male tanagers were similar in Group II, III, and IV sites, tanagers were absent from all forest patches smaller than 10 ha. Territory size did not differ among males in Group II, III, and IV sites. Pairing success exceeded 75% in all forest size classes in 1995 and 1996, and 100% of the observed males were paired in continuous forest sites. Pairing success differed significantly among forest size classes in 1995 and approached significance in 1996. Fledging success increased significantly with area and was highest (64%) in continuous forest sites. Stepwise multiple regression and principal components analysis indicated that male tanagers breeding in forest patches with higher canopy cover and lower density of oaks had higher pairing success than males in patches with lower canopy cover and higher density of oaks, and that males breeding in larger forest patches with more surrounding forest cover had higher fledging success than males in small patches with less surrounding forest cover. Our results indicate that: (1) breeding density is not a good indicator of habitat quality for forest-interior Neotropical migrants, and (2) large tracts of continuous forest are important for maintaining populations of these species. Received 19 March 1997, accepted 5 May 1998. MANY POPULATIONS of forest-interior Neotropical migrants are considered to have declined since the 1960s (Robbins et al. 1989b, Sauer and Droege 1992, but see Thomas and Martin 1996). Between 1978 and 1987, 71% of Neotropical migrant species in eastern North America displayed negative population trends, with forest-breeding species registering some of the most significant declines (Robbins et al. 1989b). Possible explanations for these population declines include the fragmentation of breeding habitat throughout North America and the loss of wintering habitat from increased tropical deforestation in South America (Robbins et al. 1989b, Rappole and McDonald 1994). Forest fragmentation, defined as the breaking up of habitat into smaller parcels, leads to a decrease in average forest patch size and an increase in the isolation of such patches (Forman 1995). During the breeding season, 1 Present address: The Nature Conservancy of Tennessee, 50 Vantage Way, Suite 250, Nashville, Tennessee 37228, USA. E-mail: [email protected] 2 Address correspondence to this author. E-mail: [email protected] these fragmentation effects may reduce habitat quality and availability for migratory birds that require large areas of forest. Forest fragmentation may adversely affect the breeding success of area-sensitive songbirds through increased nest predation and brood parasitism and/or reduced pairing success (Brittingham and Temple 1983, Wilcove 1985, Temple and Cary 1988, Gibbs and Faaborg 1990, Villard et al. 1993). Most analyses of forest fragmentation and breeding success of area-sensitive songbirds have focused on species that nest on or near the ground, such as Ovenbirds (Seiurus aurocapillus; Gibbs and Faaborg 1990, Villard et al. 1993, Porneluzi et al. 1993, Van Horn et al. 1995, Burke and Noll 1998), Kentucky Warblers (Oporornis formosus; Gibbs and Faaborg 1990), and Black-and-white Warblers (Mniotilta varia; Sodhi and Paskowski 1997). However, few data exist on how fragmentation and patch size affect breeding success of canopy-nesting species such as the Scarlet Tanager (Piranga olivacea). Scarlet Tanager numbers are estimated to have declined at an average rate of 1.2% per year from 1978 to 1987 in eastern North America (Robbins et al. 1989b)