Generation lengths of the world's birds and their implications for extinction risk

Birds have been comprehensively assessed on the IUCN Red List more times than any other taxonomic group. However, to date, generation lengths have not been systematically estimated to scale population trends when undertaking assessments, as required by the Red List Criteria. We compiled information from major databases of published life history and trait data for all birds, and imputed missing life history data as a function of species traits using Generalized Linear Mixed Models. Generation lengths were derived for all species, based on our modelled values of age-at-first-breeding (F), maximum longevity (L) and annual adult survival (S). The resulting generation lengths vary from 1.42 to 27.87 years, with a median of 2.99 years. We found that most species (61%) have generation lengths shorter than 3.33 years, meaning that the period of three generations - over which population declines are assessed under Criterion A of the Red List - was shorter than 10 years, the value used for Red List assessments of species with short generation times. For these species, our trait-informed estimates of generation length suggest that 10 years is a robust precautionary value for threat assessment. In other cases, however, for whole families, genera or individual species, generation length has a substantial impact on their estimated extinction risk, resulting in higher extinction risk in longer-lived species. While our approach is an effective means of addressing data gaps, there is some evidence that generation lengths for some species may still have been underestimated, owing to a paucity of life history data. Overall, our results will strengthen future extinction risk assessments and augment key databases of avian life history and trait data. We hope the findings stimulate future research to fill the remaining data gaps. Article impact statement: Accurate generation lengths for all birds will improve assessment of extinction within the group. This article is protected by copyright. All rights reserved.