Deviation from an equal sex ratio has been observed in several avian species and there is now descriptive as well as experimental evidence that females are able to modify the sex of their offspring within a brood. Less attention has been paid to consequences of sex ratio manipulation and sex specific differences e.g. in competitive disparities of male and female siblings and parent tactics to compensate for such competitive differences. In this study we examine differences in chick development in relation to sex and hatching order in the socially monogamous bearded tit Panurus biarmicus. Our results revealed that significantly more female than male nestlings were the biggest in the brood. Allometric measurements seem to be a good predictor of age (hatching order). This consequently means that female nestlings frequently hatch earlier and have a higher initial body mass. In contrast male nestlings seem to develop much faster than female nestlings. By modifying the hatching order in favour of female nestlings, mothers seem to promote daughters to compensate for their slower growth.
Breeding density and breeding synchrony have been reported to influence the intensity of sperm competition although results obtained so far are not conclusive. In this study we examine the investment of male lesser grey shrike (Lanius minor) into paternity guards in relation to breeding synchrony and breeding density. Male shrikes perform a double strategy to ensure paternity. They copulate frequently, mainly after territorial intrusions by other males, and guard their mates throughout the whole fertile phase. Our results indicate that, the two socio- ecological factors investigated, indirectly affect extra-pair behaviour in LGS’s. Males seem to be constrained by the frequency of intrusions by neighbouring males and this risk is associated with laying synchrony. The risk of intrusions depends on the timing and overlap of breeding attempts and males adjust their investment to paternity assurance accordingly. Furthermore, females seem to alter their egg laying patterns to minimise synchrony in situations where they find themselves in dense breeding situations. This is indicated by the negative correlation between breeding density and breeding synchronization. Extra-pair paternity, however, was not detected in our population. Thus the rate of extra-pair paternity is not necessarily an adequate measure to identify the influence of socio-ecological factors on male and female mating strategies.
Several comparative studies have previously identified breeding density and synchrony as potential determinants of reproductive success and extra-pair mating. However, the mechanisms and interaction of these two factors are poorly known. Here, we examined the effects of breeding density and synchrony on the behaviour, reproductive success and paternity losses in house sparrows. In order to test the effects of colony size, we created nest sites with varying numbers of nest-boxes. Our results show that there is an interaction between breeding synchrony and density, namely that breeding synchrony decreased with colony size. Neither colony size nor breeding synchrony seemed to influence brood size at fledging, although birds in larger colonies laid larger clutches. Moreover mate guarding behaviour was not influenced substantially by these two factors. Only nest guarding was significantly related to colony size and breeding synchrony. Paternity losses were not significantly related to colony size but they appeared to decrease with increasing synchrony. This finding supports the idea that extra-pair fertilisations are under male rather than female control.
Recently, several studies suggested that in species with biparental care, male parental effort (for instance in terms of nest building) serves as a sexual signal to females. In this study on free-living house sparrows Passer domesticus, we investigated how male contribution to parental care varies between breeding stages and whether early parental care of a male reflects his paternal effort in later stages. We found that nest building was performed predominantly by males. However, hatching success was not related to male participation in nest building or early nest guarding. The contribution of males to incubation and chick brooding was lower than in females. Investment in chick feeding did not differ between partners, but varied considerably between males. Only the male effort in chick feeding was related to the number of young at fledging age, suggesting that male assistance is essential to maximise reproductive success in house sparrows. Except for the positive correlation between male nest building and male incubation during egg laying, we found no relationship between early (nest building and nest guarding) and later paternal effort (late incubation, brooding and provisioning rates). Consequently, intensity of male nest building and early nest guarding do not seem to be honest indicators of later paternal effort in the house sparrow. Instead, we speculate that high early paternal effort might be a strategy of some males to manipulate the reproductive effort of their partners.