When the reproductive value of sons vs. daughters differs, sex allocation theory predicts females should bias the sex ratio of their broods towards the higher-value sex. Females in numerous bird species appear to bias offspring sex in response to self and mate condition, and breeding habitat quality. Over three breeding seasons, we monitored mountain chickadees breeding along a rural to urban habitat gradient. We did not find female condition or the condition of the putative father or true genetic father to influence offspring sex. We found marginal evidence for sex allocation in relation to habitat urbanization, though opposite to our predictions. In urban habitat, offspring were more likely to be female as the degree of habitat urbanization increased. We suggest habitat quality may be influential in mountain chickadee reproductive decisions; however, the ecology of mountain chickadees may not fulfill the assumptions of sex allocation theory.
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