| Abstract: | Family cohesion in birds is often explained as a product of limitations on breeding opportunities leading to delayed dispersal among the offspring. Yet, it is not independent reproduction but floating (queuing outside the natal territory for a breeding opportunity) that is the alternative to delaying dispersal. In contrast to a strong phylogenetic bias in cooperative breeding that has been taken to indicate a hard-wired behaviour maintained by evolutionary inertia, offspring behaviour is plastic and facultative in group living in the short term, indicating that family cohesion is under selection. Non-breeding offspring could gain inclusive fitness that would promote family cohesion from delaying dispersal and providing alloparental care to subsequent broods to boost group productivity. This holds in particular for offspring hatched from early broods in multi-brooded species that have the opportunity to gain inclusive fitness from help in rearing siblings hatched from later broods in the same season. Yet, seasonality will circumscribe the potential for alloparenting to be the immediate factor selecting for family cohesion. The option of gaining inclusive fitness from providing alloparental care is not open to the offspring among single-brooded species until after they have survived a non-breeding season, rather indicating kin cooperation returning enhanced survival prospects from general group living effects as a more immediate factor selecting for family cohesion. Indeed, a variety of family groups maintained in the absence of alloparental care underlines the capacity of general group living enhancing survival as a primary agent selecting for family cohesion. These seasonal constraints on fitness components selecting for family cohesion may contribute to the large scale geographical pattern with a relative paucity of family cohesions among bird species in the northern hemisphere. |
