Local resource competition affects sex allocation in a bird: experimental evidence

Sex allocation theory as applied to local resource competition (LRC) predicts that parents should skew investment towards philopatric offspring when local resources are abundant. Alternatively, parents should allocate resources to the dispersing sex when local resources are deficient in order to limit competition among relatives. Nest sites have been shown to be the primary factor limiting populations of secondary cavity-nesting birds. In this study, we manipulated nestbox density to test its effect on the sex allocation patterns of the great tit, Parus major, a species in which female offspring are more likely to disperse than male offspring. We also investigated the relationship between the brood sex ratio and the time of breeding, which has been shown in many studies to influence sex allocation. Consistent with the LRC prediction, parents invested more in male offspring and produced a male-biased sex ratio in the area where nestboxes were abundant. In our study, the reproductive success of great tits declined as the season progressed; however, the time of breeding had no effect on the sex ratio of the offspring. Overall, the results of our study suggest that nestbox availability can influence sex allocation in great tits, and may also represent a relatively common phenomenon in other secondary cavity-nesting birds.