Dispersal, migration, and offspring retention in saturated habitats

We examine the evolutionary stability of year-round residency in territoria l populations, where breeding sites are a limiting resource. The model link s individual life histories to the population-wide competition for territor ies and includes spatial variation in habitat quality as well as a potentia l parent-offspring conflict over territory ownership. The general form of t he model makes it applicable to the evolution of dispersal, migration, part ial migration, and delayed dispersal (offspring retention). We show that mi gration can be evolutionarily stable only if year-round residency in a give n area would produce a sink population, where mortality exceeds reproductio n. If this applies to a fraction of the breeding habitat only, partial migr ation is expected to evolve. In the context of delayed dispersal, habitat s aturation has been argued to form an ecological constraint on independent b reeding, which favors offspring retention and cooperative breeding. We show that habitat saturation must be considered as a dynamic outcome of birth, death, and dispersal rates in the population, rather than an externally det ermined constraint. Although delayed dispersal often associates with intens e competition for territories, life-history traits have direct effects on s table dispersal strategies, which can often override the effect of habitat saturation. As an example, high survival of floaters selects against delaye d dispersal, even though it increases the number of competitors for each br eeding vacancy (the "habitat saturation factor"). High survival of territor y owners, by contrast, generally favors natal philopatry. We also conclude that spatial variation in habitat quality only rarely selects for delayed d ispersal. Within a population, however, offspring retention is more likely in high-quality territories.