Why offspring delay dispersal: experimental evidence for a role of parental tolerance.

Approximately 3% of all bird species live in families based on a prolonged parent-offspring association. Formation of family groups often appears to be associated with various constraints on the possibilities of independent reproduction for the offspring. However, delayed dispersal is not the only alternative for offspring that defer reproduction. To account for the formation of a family group it is also necessary to explain why non-dispersing offspring forego the alternative options to join other groups as 'extra birds' or to become 'floaters' and roam between territories. We removed fathers from Siberian jay Perisoreus infaustus family groups to test the proposal that nepotistic parental tolerance gives the natal territory a special value for the offspring, which they cannot find elsewhere and that makes them stay. In this species, parents are more tolerant of their retained offspring than towards immigrant extra birds. In response to the removal of fathers, retained offspring dispersed, whereas there was no similar response among the unrelated extra birds. Retained offspring, however, left only after despotic immigrant alpha-males replaced removed fathers, indicating that the presence of fathers is an essential motive for offspring to delay their dispersal. By blocking immigrant and unrelated males from becoming alpha-males and by being tolerant themselves, fathers provide a 'safe haven' in the natal territory where retained offspring can avail themselves of available resources without any, or only mild, competitive interference.