Sex-specific effects of altered competition on nestling growth and survival: an experimental manipulation of brood size and sex ratio
Article first published online: 2 DEC 2008
© 2008 The Authors. Journal compilation © 2008 British Ecological Society
Journal of Animal Ecology
Volume 78, Issue 2, pages 414–426, March 2009
How to Cite
Nicolaus, M., Michler, S. P. M., Ubels, R., Van Der Velde, M., Komdeur, J., Both, C. and Tinbergen, J. M. (2009), Sex-specific effects of altered competition on nestling growth and survival: an experimental manipulation of brood size and sex ratio. Journal of Animal Ecology, 78: 414–426. doi: 10.1111/j.1365-2656.2008.01505.x
- Issue published online: 9 FEB 2009
- Article first published online: 2 DEC 2008
- Received 11 July 2008; accepted 24 October 2008; Handling Editor: Jonathan Wright
- intraspecific competition;
- optimal clutch size;
- Parus major;
- sex allocation;
- sexual size dimorphism
- 1An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions.
- 2In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions (‘costly sex hypothesis’); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions (‘competitive advantage hypothesis’).
- 3In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females.
- 4We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found.
- 5Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally.
- 6Our results did not support the ‘costly sex hypothesis’ because males did not suffer from higher mortality under harsh conditions. The ‘competitive advantage hypothesis’ was also not fully supported because females did not suffer more in male-biased broods.
- 7We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.