Sex ratio and fledging success of supplementary-fed Tengmalm's owl broods

A nest box population of Tengmalm's owls (Aegolius funereus) in northern Sw eden was studied to investigate the effects of extra food on the sex ratio between hatching and fledging in this sexually size-dimorphic species. The brood size and brood sex ratio of supplementary-fed and control broods were compared. Newly hatched nestlings were blood sampled and sexed by polymera se chain reaction (PCR) amplification of the sex-linked CHD1Z and CHD1W gen es. The brood sex ratio at hatching was strongly male biased (65%); this wa s also the case in broods where all eggs hatched (72%). There was no relati onship between hatch order and sex ratio, and hatching sex ratio did not va ry significantly with laying date. Brood size decreased between hatching an d fledging, but did not differ between fed and control broods at either sta ge. Brood sex ratio did not differ between hatching and fledging, and fledg ing sex ratio did not differ between fed and control broods. It was conclud ed that, at least during the year in which the study was carried out, feedi ng had no effect on brood reduction, and that male and female nestlings did not show any differential mortality. The mechanisms behind the male-biased sex ratio at hatching, and any possible adaptive reasons for it, are not k nown.