Adult sex ratios in wild bird populations

Abstract

Offspring sex ratios in wild bird populations, and the extent to which they vary from the equality expected by random genotypic sex determination, have received much recent attention. Adult sex ratios (ASRs) in wild birds, on the other hand, remain very poorly described, and many of the questions about them posed by Ernst Mayr in 1939 remain unanswered. This review assesses population-level sex ratio patterns in wild bird populations, with an emphasis on the ASR. A quantitative assessment of over 200 published estimates of ASR, covering species from a wide range of taxa, regions and habitats, supported Mayr's assertion that skewed ASRs are common in wild bird populations. On average, males outnumbered females by around 33%, and 65% of published estimates differed significantly from equality. In contrast, populationlevel estimates of offspring sex ratio in birds did not generally differ from equality, and mean ASR across a range of wild mammal species was strongly female-skewed. ASR distortion in birds was significantly more severe in populations of globally threatened species than in nonthreatened species, a previously undescribed pattern that has profound implications for their monitoring and conservation. Higher female mortality, rather than skewed offspring sex ratio, is the main driver of male-skewed ASRs in birds, and the causes and implications of this are reviewed. While estimates of ASR in wild bird populations may be subject to a number of biases, which are discussed, there is currently no quantitative evidence that an ASR of one male to one female represents the norm in birds. A better understanding and reporting of ASRs in wild bird populations could contribute greatly to our understanding of population processes and could contribute much to theoretical and applied research and conservation.

The evolution of sex roles in birds is related to adult sex ratio

“…Furthermore, different detectability of the sexes, a potential confound of field estimates of ASR 19 , is not likely to bias our results: the more polygamous sex is expected to be more conspicuous because of elaborate plumage, displays and general activity 2 , which would potentially bias ASR estimates towards the direction opposite to our findings (that is, biasing ASR estimates towards the polygamous sex).…”

“…We tested whether ASR predicts mating systems and parental care using Phylogenetic Generalized Least Squares 17,18 . Although data on ASR from wild populations are difficult to obtain 19 , the information now available for shorebirds permits tests of the theoretical predictions using statistically robust sample sizes.…”

A large population parental care game: Polymorphisms and feedback between patterns of care and the operational sex ratio

“…Clutton-Brock et al (1985) and Clutton-Brock and Isvaran (2007) note that in dimorphic mammalian species there is a very female biased ASR, due to the fact that males take longer to mature and male-male competition leads to a higher rate of mortality among adult males than among adult females. Liker and Székely (2005) and Donald (2007) observe that in most bird species there tends to a more balanced ASR than in mammalian species. Also, in avian species where just female care is observed the ASR tends to be female biased.…”

The consequences of winter flock demography for genetic structure and inbreeding risk in vinous-throated parrotbills, Paradoxornis webbianus

“…Demography and genetic structure in parrotbills Donald, 2007). Firstly, this study has shown that adult survival rate is low, but also extremely variable among years (Table 1).…”