Extreme Between-Year Variation in Productivity of a Bridled Titmouse (Baeolophus wollweberi) Population

Environmental variability affects life-history decisions such as allocation of resources to reproduction. However, in the case of small passerine birds, pairs that acquire a territory do not usually skip a breeding season. This study presents breeding-success data from two contrasting years for a population of Bridled Titmice (Baeolophus wollweberi). During an El Niño year (1998), 20 pairs attempted to breed, and 85% were successful. Survival of juveniles from 10 banded broods through nutritional independence was 100%, excluding one depredated nest. Breeding success was much lower in the following La Niña drought year. Fifty-seven pairs established territories in the study area. Twenty-seven never progressed to nest building. Thirteen pairs were successful; survival of young from banding to nutritional independence was 41.4%. An average of 0.47 fledglings per breeding pair was produced in 1999, compared to 4.40 the previous El Niño year. This study examines possible adaptive significance of forgoing breeding in a poor year.