Family living in a Neotropical bird: variation in timing of dispersal and higher survival for delayed dispersers

Prompt natal dispersal and reproduction of offspring is viewed as the strategy to maximize lifetime reproductive success. Yet, across many species, offspring remain with their parents after independence rather than disperse. Why some species live in family groups and the behavioural mechanisms influencing the extent of family living remain unresolved. Family living is predicted to arise through slow life history traits predisposing certain lineages to family living, and then through social and ecological factors resulting in greater benefits of remaining with parents compared to dispersing. We studied family living in the western slaty-antshrike, Thamnophilus atrinucha, a bird with a slow life history, in Panama. We quantified the extent of delayed dispersal, and examined both the behavioural mechanisms influencing dispersal as well as the survival consequences. We observed that delayed dispersal increased offspring survival. Parental aggression towards offspring increased with offspring age and, in particular, when parents renested. Thus, offspring dispersed earlier when their parents renested. Fledging date and mass of offspring also influenced timing of dispersal. Although we observed higher survival of delayed dispersers, only a small proportion of offspring delayed dispersal until the next year and all dispersed to wander within other breeding pairs' territories ( i.e. 'float'). Our results support theoretical predictions stating that benefits to parents from offspring retention decrease with offspring age and with renesting, leading to natal dispersal. Furthermore, we observed a reduced survival benefit of retention once offspring reached a certain age. Examining parental tolerance, the costs of floating, and how they vary with offspring age are essential to understand the costs/benefits of family living. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.