<p>Males and females differ in morphology and behaviour, so that selection acts differently on the two sexes. This changes the relative reproductive success of males and females, and it is beneficial for parents to bias the sex ratio of their broods in favour of the sex with the best survival and breeding prospects. Differences between the sexes and brood sex ratio in Tengmalm’s owl (Aegolius funereus) in northern Sweden were investigated, using a molecular sexing technique based on PCRamplification of sex-linked CHD1 genes. Among owls caught during autumn migration, females were commoner than males, especially within juveniles. However, in contrast to earlier studies, it was shown that adult males sometimes undertake migratory movements indicatory of nomadism. Measurements of these owls revealed that sexual size dimorphism in Tengmalm’s owl is not as great as previously reported from studies carried out during the breeding season. Females were slightly larger (4% by mass) than males, probably owing to the different roles of males and females during breeding, when this dimorphism is greater. The size difference between male and female nestlings was found to be similar to that for adults in autumn, and to investigate whether this led to differential mortality, the effect of supplementary feeding on mortality of male and female nestlings was studied. Supplementary feeding reduced male mortality when vole abundance was low, and it was concluded that larger female nestlings out-competed their smaller brothers, who then suffered increased mortality when food was scarce. Recruitment of male nestlings into the breeding population declined with decreasing food supply at the time of fledging, a pattern not observed in females. Juvenile males were therefore more vulnerable to food shortage than females, both in the nest and after fledging. Mean brood sex ratio varied significantly among years characterized by different phases of the vole cycle and associated vole abundance. Broods were male-biased (63% males) in a year when the food supply was favourable during spring and summer, neutral (50%) in a year with an intermediate food supply, and female-biased (35% males) in a year when food was in short supply. Parents appeared to adaptively adjust the sex ratio of their broods according to the relative mortality risk and reproductive potential of sons and daughters.</p>

Males and females differ in morphology and behaviour, so that selection acts differently on the two sexes. This changes the relative reproductive success of males and females, and it is beneficial for parents to bias the sex ratio of their broods in favour of the sex with the best survival and breeding prospects. Differences between the sexes and brood sex ratio in Tengmalm’s owl (Aegolius funereus) in northern Sweden were investigated, using a molecular sexing technique based on PCRamplification of sex-linked CHD1 genes. Among owls caught during autumn migration, females were commoner than males, especially within juveniles. However, in contrast to earlier studies, it was shown that adult males sometimes undertake migratory movements indicatory of nomadism. Measurements of these owls revealed that sexual size dimorphism in Tengmalm’s owl is not as great as previously reported from studies carried out during the breeding season. Females were slightly larger (4% by mass) than males, probably owing to the different roles of males and females during breeding, when this dimorphism is greater. The size difference between male and female nestlings was found to be similar to that for adults in autumn, and to investigate whether this led to differential mortality, the effect of supplementary feeding on mortality of male and female nestlings was studied. Supplementary feeding reduced male mortality when vole abundance was low, and it was concluded that larger female nestlings out-competed their smaller brothers, who then suffered increased mortality when food was scarce. Recruitment of male nestlings into the breeding population declined with decreasing food supply at the time of fledging, a pattern not observed in females. Juvenile males were therefore more vulnerable to food shortage than females, both in the nest and after fledging. Mean brood sex ratio varied significantly among years characterized by different phases of the vole cycle and associated vole abundance. Broods were male-biased (63% males) in a year when the food supply was favourable during spring and summer, neutral (50%) in a year with an intermediate food supply, and female-biased (35% males) in a year when food was in short supply. Parents appeared to adaptively adjust the sex ratio of their broods according to the relative mortality risk and reproductive potential of sons and daughters.