Season-long fecundity, survival, and viability of ovenbirds in fragmented and unfragmented landscapes

From 1992-1995, we compared the season-long fecundity of marked Ovenbirds (Seiurus aurocapillus) on three sites in large (>2000-ha) forest patches in a fragmented landscape in central Missouri (U.S.A.) with four sites in a contiguously forested landscape of almost 2 million ha in the Missouri Ozarks. Our purpose was to determine to what extent the negative effects of fragmentation on nests affect individual bird's annual productivity. To measure annual productivity, we determined the proportion of territorial males that ultimately succeeded in raising young in a season and multiplied this proportion by the average brood size obtained from nest observations. Fewer pairs successfully raised young in the fragmented landscape (50 ± 11%) than in the unfragmented landscape (70 ± 7%). In the fragmented landscape, 25 ± 6% of pairs raised at least one Brown-headed Cowbird (Molothrus ater), whereas only 1 ± 1% of pairs raised cowbirds in the unfragmented landscape. Lower season-long success and increased brood parasitism led to annual productivity of 0.70 and 1.47 juvenile females per female in the fragmented and unfragmented landscapes, respectively. Territory size (mean = 2.76 ha) was not affected by landscape, but density was lower in the fragmented landscape (1.6 ± 0.41 males per 10 ha) than in the unfragmented landscape (2.2 ± 0.32). The ratio of second-year to after-second-year males did not vary with landscape. Male pairing success (67 ± 6%) and survival (0.621 ± 0.21) was not affected by landscape. We concluded that it was unlikely that sites in the fragmented landscape contained viable Ovenbird source populations, whereas populations in the unfragmented Ozark region were likely to be sources in most years. Nevertheless, any changes in the Ozarks that increase nest predation or parasitism may have consequences for the regional population of Ovenbirds and other forest-breeding Neotropical migrants in the midwest.