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.

Using Distance from Putative Source Woodlots to Predict Occurrence of Forest Birds in Putative Sinks

“…Results of these studies suggest that density and productivity-and hence the number of potential recruits-in large woodlots are much larger than those in small woodlots (e.g., at least four times greater for the Ovenbird; Burke & Nol 2000). By contrast, Redeyed Vireos (Vireo olivaceous) are not area sensitive in pairing success, nest densities (Dunford et al 2002), or productivity (Burke & Nol 2000).…”

“…Previous work on densities, pairing success, nest success, and productivity of the Ovenbird (Seuirus aurocapilla), Wood Thrush (Hylocichla mustelina), Veery (Catharus fuscescens), and Rose-breasted Grosbeak (Pheucticus ludovicianus) in southern Ontario indicates that some or all four of these attributes are higher in these species in large rather than small woodlots (Burke & Nol 1998. Results of these studies suggest that density and productivity-and hence the number of potential recruits-in large woodlots are much larger than those in small woodlots (e.g., at least four times greater for the Ovenbird; Burke & Nol 2000). By contrast, Redeyed Vireos (Vireo olivaceous) are not area sensitive in pairing success, nest densities (Dunford et al 2002), or productivity (Burke & Nol 2000).…”

“…The value of large forests to bird conservation is generally, although not consistently, accepted (Andrén 1994;Zuiduma et al 1996;Smith & Hellmann 2002). Large forests may have higher conservation value because these forests tend to be exempt from documented edge effects that reduce the productivity of forest-interior and areasensitive bird species, such as Brown-headed Cowbird (Molothrus ater) parasitism and nest predation Burke & Nol 2000). Edge effects, along with declines in food abundance in small forests (Burke & Nol 1998;Zanette et al 2000), can cumulatively influence all stages of the life cycle of birds, from settlement and pairing (Villard et al 1993;Rodewald & Yahner 2000) to reproduction and recruitment into the breeding population (Porneluzi et al 1993;Weinberg & Roth 1998;Porneluzi & Faaborg 1999;Bayne & Hobson 2002).…”

“…As a result, all large forests were chosen because of limited availability, whereas only some small forests were chosen. We use the criteria of Burke and Nol (2000) to categorize woodlots as putative sources or sinks: putative sources (hereafter large fragments) were those with >90 ha of core area and putative sinks (hereafter small fragments) were those with <90 ha. Core area was defined as forest >100 m from nearest edge (Austen et al 2001).…”

Putting Density Back into the Habitat‐Quality Equation: Case Study of an Open‐Nesting Forest Bird

“…Several authors use per capita reproductive success to compare habitat quality among sites or habitat types (e.g., King et al 1996;DeCecco et al 2000;Purcell 2006) or against a population growth rate (λ) calculated on the basis of published estimates of survival rates (e.g., Trine 1998; Burke & Nol 2000;Knutson et al 2006). In conservation planning the rationale for focusing on the reproductive performance of individuals is to ensure that priority is given to sites contributing positively to population growth so as to avoid population sinks or ecological traps.…”