The relative number of adult males to females in a population, the adult sex ratio (ASR), is an important demographic variable in populations, which plays a role in, for instance, mating systems and viability of populations. The origins of variation in ASR between populations are often unclear. A skewed ASR in a local population may result from a skewed sex ratio among fledglings, sex differences in survival or dispersal or a combination of these. Meta-analyses show that ASRs across bird species are mostly male-biased and that male-biased survival is the main contributor to this bias. Here, we examine the origins of a female-biased ASR in a small population (3–24 breeding females in 2007–2017) of a passerine, the Northern Wheatear Oenanthe oenanthe. Because 95% of all fledglings were sexed, 88% of the individuals were colour-banded and because this species is highly philopatric, we could accurately examine: (1) fledgling sex ratio, (2) sex-specific first year and adult apparent survival and (3) sex-specific dispersal. It appeared that survival was not sex-biased for either age cohort but that fledgling sex ratio was male-biased, especially so among late broods. In addition, natal dispersal out of the breeding population was likely male-biased, in contrast to the general pattern of female-bias in avian dispersal. So, perhaps females are more common in our relict population solely because more males disperse from the population than females, without being compensated by male-biased fledgling sex ratio or survival and without compensation from immigrating males. Likely, the population sex ratio (including both adults and young) at the end of the breeding season is male-biased, because of a pronounced male-biased sex ratio among fledglings from late nests. Perhaps this male-bias induces prospecting young males to settle elsewhere to avoid competition next year, but at the same time convinces prospecting females to settle particularly in this area.
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Vol. 109 • No. 1