On the social and genetic composition of rook Corvus frugilegus and jackdaw C. monedula flocks
From December 1985 to December 1988, an study was made of the functional significance of rook and jackdaw flocks with a view to identifying reasons for intra and interspecific flock cohesiveness. Flocks of rooks were investigated by way of behavioural observation for competitive heterogeneity and the impact of such on the dispersion of individual members. Thus, immature and adult birds were compared for their respective abilities to compete within foraging flocks. Evidence for kin selection in foraging flocks was investigated in jackdaws by way of behavioural observation of individually marked (wing tagged) birds and using DNA fingerprinting to ascertain the degree of relatedness between associating individuals. Interspecific associations were studied a) to identify the similarities and differences between foraging rooks and jackdaws and b) to provide hypotheses for functional flocking without cause for genetically related explanations. Immature rooks did not forage as efficiently as adults in larger flocks, and thus selected smaller flocks, with higher immature: adult ratios. Adult interference was a possible cause of immature dispersion. Kin selection was not found to be a significant component of co-foraging groups, though female jackdaws had higher than average levels of relatedness, possibly due to lower female than male dispersal. The daily behaviour of rooks and jackdaws was remarkably similar and 80% of foraging flocks contained both species. Commonly co-occuring food resources, low direct resource competition and subsequent low levels of interspecific aggression were thought responsible for this association. In conclusion, competitive heterogeneity was found in rooks that effected individual dispersion, discouraging young birds rather than encouraging them, to remain within the colony. Likewise little evidence of kin selection was found in foraging jackdaws. Rooks and jackdaws associated together frequently, on the basis, it was suggested, of where one species was feeding, the other was also likely to forage successfully.