Female-biased sex ratios and the proportion of cryptic male morphs of migrant juvenile Ruffs (Philomachus pugnax) in Finland

Biases in sex ratio may affect the viability of populations, and may arise for different rea-sons, such as biased primary ratio and differential juvenile or adult mortality of sexes. Global populations of Ruffs are thought to be strongly female biased. To determine the demographic origin of this sex bias, we report the sex ratios among juvenile Ruffs on their southward migration in Finland during 1985–2006. We also quantify the proportion of cryptic, female-like 'faeder'males at this demographic stage, and examine migration tim-ing by sex. We found a strong female bias in juvenile populations; across the study years, 34% of individuals were males. Female juveniles migrated earlier than male juveniles. Faeder males made up ca. 1% of juvenile populations, similar to estimates from mixed-age populations elsewhere. These results, combined with previous studies, confirm the strong female bias and the low frequency of 'faeders'at the juvenile stage of the Ruff. The sex bias might thus demographically originate from sex-allocation strategies by females at the egg stage. Given the genetic determination of the status of faeders, their proportion among juveniles provides the first estimate of the morphs' proportional reproductive suc-cess.