Jackdaws prefer safe cavities with small minimum nest-entrance dimensions and avoid those with a high risk of nest predation. In experimental (fed) colonies, however, there is a tendency to use all cavities, which results in an increased breeding density. No nests are preyed upon by Ravens Corvus corax in the experimental colonies because supplemental food favoured group defence by increasing colony size and by increasing the time the Jackdaws spent in the colony. Additional food advances laying date, increases clutch size independently of laying date and increases fledging success. Supplementary food significantly increases fledging success in less than half of all experimental studies on birds. It is suggested that the key to this problem is the species’ breeding strategy, and we it has been shown that supplementary food significantly increases fledging success in brood-reduction strategist species but not in species which directly adjusts their clutch size (Soler and Soler 1996).
Social dominance is intuitively assumed to be associated with higher fitness, because social dominance implies better access to resources. It has been found that, in a colony of jackdaws, the dominant males consistently produced fewer fledglings, which have lower chances of survival to 1 year of age. Laying date and clutch size are independent of dominance, but females that mated with dominant males are in poorer condition and laid smaller eggs. Parental survival is independent of social dominance, and the frequency of extrapair fertilizations in jackdaws is negligible. Dominance is a stable trait of individuals, and not a state that all individuals eventually attained. It has been concluded that, in this colony, dominant jackdaws have lower fitness. This is the first example of such a pattern in a free-living species. It is hypothesized that the high density of our colony resulted in high testosterone titres, which suppressed paternal care of mate and offspring to the extent that it outweighed the benefits of higher resource access (Verhulst and Salomons 2004).
In the jackdaw (Corus monedula), eggs hatch asynchronously with the youngest chicks in the brood often starving to death. So far, it is unknown whether there are sex differences in vulnerability to starvation. Adult females are smaller than males suggesting that daughters should be cheaper to produce than sons and so, less likely to starve when nest conditions are poor. Generalised linear models revealed that parents seemed to be investing differently in sons and daughters depending on their chances of success. Broods produced late in the season were significantly female biased, particularly those from small clutches. Females hatched towards the end of the season, when conditions were poor, were more likely to fledge than males. Nestlings that were relatively large at hatching were more likely to fledge. This effect was particularly important for last hatched individuals. Overall, males had a higher mortality rate than females. The most likely cause was starvation due to higher energetic requirements, because males were larger than females at fledging. It is suggested that in species with brood reduction, sex-biased mortality may be at least as important as primary sex ratio manipulation in determining avian sex ratios (Arnold and Griffiths 2003).
Reference List:
- Arnold.E.K., & Griffiths.R., (2003) Sex-specific hatching order, growth rates and fledging success in jackdaws Corus monedula. Journal of Avian Biology, 34, 275-281.
- Soler.M., & Soler.J.J., (1996) Effects of experimental food provisioning on reproduction in the Jackdaw Corvus monedula, a semi-colonial species. IBIS, 138, 377-383.
- Verhulst.S., & Salomons.M.H., (2004) Why fight? Socially dominant jackdaws, Corvus monedula, have low fitness. Animal Behaviour, 68, 777-783.
