Effects of forest fragmentation on the demography of ovenbirds (Seiurus aurocapillus) in the boreal forest

Abstract

The demography of ovenbirds (Seiurus aurocapillus) was compared in landscapes highly fragmented by agriculture, moderately fragmented by commercial harvesting, and in contiguous boreal forest of Saskatchewan. Various survey techniques demonstrated that ovenbirds were less common in small, isolated forest patches in fragmented landscapes relative to contiguous forest. Male ovenbirds in fragmented landscapes (84 to 87% paired) had a lower probability of attracting females relative to males in contiguous forest (97%). Competition for territories was intense, as removal experiments demonstrated the presence of non-territorial male floaters. However, floaters were more common in contiguous forest, indicating this habitat was most preferred. Nesting success was lower in small farm fragments (29%) than the other landscapes (43 to 58%), due to high nest predation. Apparent annual survival of males was lower in small farm fragments (30%) relative to the other landscapes (57 to 60%). The difference in adult survival among landscapes was not caused by increased mortality in small farm fragments, but likely occurred because failed breeders (20% annual return rate) dispersed more than successful breeders (50%). Overall, small farm fragments were population sinks, where the number of adult birds lost to mortality and emigration exceeded the number of young produced. However, population size was constant over the length of the study indicating that populations in small farm fragments were rescued from extinction by immigration of birds from contiguous forest, where excess juveniles were produced. Populations in farm fragments were more likely to be rescued from local extinction by first time breeders (95% first time breeders) than the other landscapes (65 to 75%). Increased turnover in small farm fragments, also resulted in a greater proportion of first time breeders (60%) in small farm fragments than the other landscapes (44 to 46%). These results support the idea that forest songbirds select habitat in a ideal preemptive manner. Younger individuals seem to be forced to settle in low-quality farm fragments, due to competition from older birds in the more optimal contiguous forest. Forest fragmentation bad a negative impact on ovenbirds and likely is an important factor influencing the decline of this and other Neotropical migrant species.