Population bottlenecks and increased hatching failure in endangered birds.

Severe population bottlenecks are expected to lead to increases in inbreeding depression and to reduce the long-term viability of populations. We compared hatching failure across 51 threatened bird species to test the relation between the size of population bottleneck and population viability. Bottleneck size was defined as the lowest population size recorded in a species. Hatching failure was estimated as the proportion of eggs that failed to hatch due to infertility and embryonic death, both of which increase with inbreeding. The size of the bottleneck varied from 4 to 20,000 individuals across species and had a significant negative effect on hatching failure, a pattern that was consistent when we controlled for the confounding effects of phylogeny, body size, clutch size, time since the bottleneck occurred, and latitude. Hatching failure varied from 3 to 64% across species and was more than 10% in all populations passing through bottlenecks below 100–150 individuals. Our results show that the negative consequences of bottlenecks on hatching success are widespread in the populations of species we examined, and emphasize the conservation benefit of preventing bottlenecks below 150 individuals.