Why fight? Socially dominant jackdaws, Corvus monedula, have low fitness

Social dominance is intuitively assumed to be associated with higher fitness, because social dominance implies better access to resources. We found that, in a colony of jackdaws, the dominant males consistently produced fewer fledglings, which had lower chances of survival to 1 year of age. Laying date and clutch size were independent of dominance, but females that mated with dominant males were in poorer condition and laid smaller eggs. Parental survival was independent of social dominance, and the frequency of extrapair fertilizations in jackdaws is negligible. Dominance was a stable trait of individuals, and not a state that all individuals eventually attained. We conclude that, in this colony, dominant jackdaws had lower fitness. To our knowledge, this is the first example of such a pattern in a free-living species. We hypothesize 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.