Forest Fragmentation Effects on Ovenbird Populations in the Urban Region of Eastern Massachusetts, USA

by David C. Morimoto¹, Michelle A. Frankel², Marta Hersek³, Fred E. Wasserman³

¹ Natural Science and Mathematics Division, Lesley University, Cambridge, MA 02138

² Audubon Connecticut, 613 Riversville Road, Greenwich, CT 06831

³ Department of Biology, Boston University, 5 Cummington St., Boston, MA 02215

Abstract

We compared pairing and reproductive success of ovenbirds (Seiurus aurocapilla) in three large (120‒312 hectare) and nine small (10‒60 hectare) forest reserves in a suburban landscape over six years and related ovenbird success to patch-scale and landscape-scale features. We applied estimates of ovenbird reproductive success to population viability models and compared results with those of ovenbird studies in other regions and landscapes. Pairing success was high at all sites but not significantly higher in large (98%) vs. small (88%) reserves. The probability of nest survival was significantly higher in large (42%) vs. small (17.5%) reserves, as were reproductive success (61.3% vs. 46.8%) and fledging success (70% vs. 50%). Density was significantly higher and territories were significantly smaller in large reserves. The amount of forested area within 2 kilometers of the forest center was somewhat positively related to the proportion of successful nesting attempts and fledging success (p < 0.10). There was no significant difference in predation or parasitism rates by brown-headed cowbirds (Molothrus ater) between large and small forests, but parasitized nests in small reserves fledged significantly more cowbird nestlings. Source-sink results varied with estimates of adult survivorship and annual productivity, but most models found that large reserves were above the source-sink threshold and small sites were population sinks or very near the source-sink threshold. Suburban landscapes in heavily urbanized regions of the northeastern U.S. can likely support viable populations of ovenbirds with forest cover > 40%, with the maintenance of reserves ≥ 120 hectares, and with the preservation of small woodlots close to larger tracts of forest. The rates of pairing and breeding success in our irregularly patterned suburban landscape were higher than those found in other regions and landscapes, supporting the conclusion that regional and landscape context are important considerations in the conservation and management of ovenbirds, and pointing to the importance of local and regional studies for determining minimum area requirements for ovenbirds and for informing municipal planning and conservation efforts.

Keywords: forest fragmentation, ovenbirds, conservation, minimum-area breeding requirements, source-sink, urban