How do studies on avian survival treat sex? A systematic review

Abstract

Ornithology increasingly recognizes the importance of documenting variation below the species level. One common source of intraspecific variation is the variation that exists between sexes. Across many species of birds, adult male birds have higher survival rates than females. Thus, studies on survival should either treat sexes separately or justify why not—but it is unknown the extent to which this is practiced in peer-reviewed literature. We conducted a systematic review of studies on survivorship in wild birds via 1,311 studies from 1966 to 2022. We found that 36% of all avian survival studies (27% of studies on adults and 74% of studies on juveniles <1 year old), do not mention sex at all. Of 763 survival studies on adult birds that mentioned sex, authors considered sex to varying degrees, including choosing not to sex individuals (4%), pooling data for both sexes (25%), restricting analyses to only one sex (28%), or reporting survival separately for each sex (43%). We found that multiple factors may explain how authors chose their treatment of sex. Specifically, studies were more likely to treat sex more independently when species exhibited greater visual dichromatism, when studies focused on just one or a few species, and when studies had larger sample sizes. Our finding that a quarter of the avian survival literature omits sex as a variable suggests there remains an opportunity for ornithologists to further consider and understand whether there are survival differences between intraspecific groups such as, but not limited to, sex.