Size matters: competition between male and female great tit offspring.

1. Studies of sex allocation in birds have traditionally centred on Fisher's (1930) theory of equal parental investment in male and female offspring. They concentrate particularly on sexually dimorphic species, where costs of rearing offspring are assumed to vary between male and female young because of body size differences. 2. Higher mortality of the larger sex (males), particularly in poorer conditions, is expected to result in female-biased sex ratios in the great tit, Parus major. Several studies have found the contrary, reporting male-biased fledging and recruiting sex ratios when conditions are poor. One reason why this may be the case is that males can gain more food resources than their sisters because of the competitive advantage afforded by their larger size. They may thus suffer less mortality in the nest or fledge in better condition, thereby enhancing their survival prospects. 3. This study investigates the importance of size in competitive interactions between nestlings of different sexes. A cross-fostering design was employed to create broods of mixed size through swapping half a brood of 2-day-old 'small' and half a brood of 4-day-old 'large' nestlings. Nestling morphometrics and mortality were measured during the nestling period. Nestlings were sexed by PCR amplification of sex-linked genes. To test for a male advantage in competitive environments, size and mortality measures were compared between 'small' males and females, and 'large' males and females (i.e. the interaction term 'size treatment' and 'sex'). 4. There was greater sexual dimorphism between small nestlings than large nestlings at fledging. This is interpreted as revealing enhanced competitive ability of male offspring under stressful conditions. Offspring from the 'large' group suffered lower mortality, but there was no difference in mortality according to sex, and no significant size*sex interaction. Similarly, no difference in recruitment was found, although this may be due to the small overall proportion of birds recruiting (3·1%). 5. The study suggests that male biased fledging/recruitment sex ratios in great tits may be explained by enhanced competitive ability of male nestlings in poor rearing environments.