Landscape and Fragment Size Effects on Reproductive Success of Forest-Breeding Birds in Ontario

Abstract

To determine the minimum size requirements and influence of landscape context on reproductive success of forest-breeding songbirds, we monitored nesting success of five species of songbirds on 40 fragments (12–2350 ha in total woodlot size) and two continuous forest sites in south-central Ontario from 1994 through 1997. Woodlot size was the most important variable contributing to differences in reproductive success, with local forest cover (within a 10-km radius) having no significant additional effect on productivity for any species. For all species, except Red-eyed Vireo (Vireo olivaceous), adult female reproductive success was at or above replacement levels in large fragments (mean of 121 ha core area, 849 ha woodlot area) and continuous forest, and below replacement levels in small fragments (mean of 7.8 ha core area, 93 ha woodlot area). Red-eyed Vireo productivity was particularly low, with single-brooded females unable to maintain populations in any woodlots monitored, although populations were close to replacement levels in continuous forest. Only the very largest fragments acted as sources for Ovenbird (Seiurus aurocapillus) and Wood Thrush (Hylocichla mustelina) (i.e., >23 ha in core area, 225 ha in total woodlot area). Ovenbird and Wood Thrush were the most area-sensitive, and Rose-breasted Grosbeak (Pheucticus ludovicianus) and Veery (Catharus fuscens) the least, although all four species had significantly lower reproductive success in small as compared to large forest fragments. Reproductive success of Rose-breasted Grosbeaks and Veery were similar in large forest fragments and continuous forest and were close to maintenance levels in small fragments, despite published reports of significant declines that might be attributable to poor productivity. Nest predation was the major cause of nest failure, with brood parasitism by Brown-headed Cowbirds (Molothrus ater) further reducing the number of host young fledged from Ovenbird and Red-eyed Vireo nests. Parasitism rates on the remaining species were very low and not analyzed statistically. Local forest cover had little effect on predation rate or rate of parasitism by Brown-headed Cowbirds. Although local forest cover had little effect on reproductive success within fragmented landscapes, nest success was significantly higher in continuous forest than in all fragments combined for both Ovenbird and Red-eyed Vireo. Our data show a poor agreement with Breeding Bird Survey population trend estimates for Ontario between 1986 and 1996. As only 1% of the forest fragments locally available are large enough to function as source habitats, we strongly advocate their preservation. We recommend preservation of forest tracts at least 500 ha in size, particularly woodlots with >90 ha in core area, to function as sources that will help guard against population declines on a local scale.