Lower reproductive output of Mountain Bluebirds (Sialia currucoides) in clearcut versus grassland habitat is consistent with a passive ecological trap

Abstract

Clearcutting of forests results in habitats that structurally resemble grasslands and so may act as ecological traps for grassland birds. Several studies have implicated predation as the factor that decreases the number of offspring, but few have examined performance at other breeding stages. Consistent with a passive ecological trap, Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)) that settled in clearcuts in central British Columbia did not differ in age or quality from adults in grasslands. Nest building and laying date of the first egg did not differ between habitats, suggesting an equal propensity for settling in each habitat. In clearcuts, however, the body condition of female parents was lower, and they abandoned their nests more often in harsh weather. This higher total clutch loss in clearcuts meant that seasonal production of fledglings per female was 13% less in clearcuts. Furthermore, fledglings in grasslands weighed 4% more and female fledglings had plumage with shorter (UV-shifted) wavelengths (hence greater ornamentation) than those in clearcuts, suggesting that they were also of better quality. Thus, predation rates were not the cause of reduced reproduction in clearcuts; rather, our results suggest that lower prey abundance was linked to nest abandonment in harsh weather and reduced both the number and quality of offspring in those habitats.