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Delayed dispersal and sociality
Family living: genes or environment?
Carrion crows in Europe breed mostly in monogamous pairs, and young disperse as soon as they reach independence (4-5weeks after fledging). Conversely, in northern Spain offspring delay dispersal up to 4 years and stay on the natal territory with their parents, assisting them in rearing new broods. With a field experiment we demonstrated for the first time that the environment promotes the formation of such stable long-lasting families.
Chicks hatched from non-cooperative Swiss crows' eggs, that were transferred to nests of Spanish cooperative crows, delayed natal dispersal and stayed with their foster parent's, helping them to raise a new brood in the following year. In other words, young with socially monogamous ancestors (Swiss population) switched to family living when they were introduced in our kin structured Spanish population, showing that crows have the potential for shaping their social behaviour in response to current conditions.
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Geographic variability of social behaviour
What exactly in the environment determines the variability of social organization of crows? A well accepted theory predicts that offspring delay dispersal in response of a shortage of vacant breeding territories and that helping at the nest is "the best of a bad job" in circumstances where "living home" is a troublesome matter. To test this idea we compared juvenile dispersal, territory turnover, habitat saturation (number of competitors per breeding vacancy), variability of territory quality and territorial behaviour of two crow populations that differ in social organization: extensive kin sociality in Spain versus social monogamy in Italy . Surprisingly, we found that offspring delay dispersal and help at the nest in the less competitive environment, namely where young have in principle more opportunities to find a suitable territory to breed on their own.
We also found that in Spain adults live in their territories year-round and defend them constantly against intruders. In Italy , however, adults abandon their territories after breeding and often join big flocks that forage on temporally highly productive feeding grounds. From the point of view of the offspring, the natal territory has very different value in the two places. While in Spain it represents a "save haven" where resources are easily accessible and defended by the parents against intruders, in Italy it has nothing special to offer as soon as the parents leave and join the flock. We believe that this difference explains the different dispersal tactics of young crows, which stay "at home" in Spain to enjoy their privileges, but that leave and break the family bonds in Italy , because the natal territory does not provide any advantage. In summary, crows seem to show us that they can not form a family without a good home.
Dispersal and territory quality
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If offspring value conditions at home to decide whether to disperse or not, family's wealth should be an important factor in shaping their tactics. This is what the Evolutionary Theory of the Family (Emlen PNAS .) predicts for animals in general, and it indeed applies to the society of crows! In our Spanish cooperative population, delayed natal dispersal is widespread but still a certain proportion of young leave the natal place early in live.
In a field experiment we manipulated the relative quality of a number of territories by adding food every day throughout the year. |
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We found that young carrion crows from fed territories were more philopatric (i.e. more likely to stay at home with the parents) and more helpful at their family's nest than unfed ones. The results suggest that family wealth influence the dispersal decision of the offspring and eventually the cohesion of the family.
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