EXTENT OF DOUBLE-BROODING AND SEASONAL MOVEMENT OF NESTING FEMALES IN A NORTHERN POPULATION OF WOOD THRUSHES

Abstract

Accurate estimates of the amount of double-brooding within a population are an important parameter for assessing the population sustainability of forest birds. In 1998 and 1999, we color-banded adult female Wood Thrushes (Hylocichla mustelina) at 70 nests early in the breeding season to assess the frequency of double-brooding in a northern population. We found that double-brooding was a common breeding strategy among Wood Thrushes in southern Ontario. Forty-seven females fledged first broods and 74% (n = 35) of them initiated egg-laying in a subsequent nest. It is possible that at least 87% (n = 41) of the females were double-brooded, based on the evidence of 6 later nests built within 50 m of successful first nests that were depredated or fledged young before their owners could be identified. Other second-brood nests were probably missed because they were overlooked and because some females moved considerable distances (100–400 m) between nestings. Most birds that failed in their early nesting attempt were not found again on the site, precluding verification of their renesting efforts.