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

Abstract

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 α-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 α-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.

Nepotism and subordinate tenure in a cooperative breeder

“…We expected an effect of decreased nepotistic tolerance on both sexes of subordinate, similar to other nepotistic species [4,6,7]. However, subordinate female tenures are unresponsive to changes in relatedness to the dominant pair ( figure 2b and table 1).…”

“…western bluebirds, Sialia mexicana) or both (e.g. Florida scrub jays, Aphelocoma coerulescens), subordinates lose nepotistic tolerance and are more likely to disperse [4,6,7]. On the other hand, when unrelated dominants provide adult subordinates with within-group breeding opportunities, sexual competition may result [1,8,9], increasing the likelihood of delayed dispersal (e.g.…”

“…Subordinates may delay dispersal owing to ecological constraints on independent breeding and/or because delaying independent breeding until older is itself adaptive (reviewed by Covas & Griesser [2]). The presence of closely related dominants has been found to affect the likelihood of delayed dispersal of subordinates via changes in nepotistic tolerance or sexual competition [3][4][5][6]. For example, in the presence of unrelated dominant males (e.g.…”

Winter resource wealth drives delayed dispersal and family-group living in western bluebirds

“…Prolonged brood care can confer advantages to delayed dispersers regardless of variation in resource wealth, but is tied to resource wealth in that wealthier parents can better afford to share resources (Ekman & Rosander 1992). Similarly, if variation in resource abundance accentuates the relative benefits of staying on high-quality territories then delayed dispersers can increase their fitness by staying, whether or not parents exhibit nepotism (Ekman & Griesser 2002). In this sense, the two hypotheses are not strict alternatives, nor are they necessarily inter-dependent.…”

“…Hypotheses for the evolution of delayed dispersal focus on two key benefits for offspring that stay home: (i) prolonged brood care (nepotism, parental facilitation) (Brown 1987;Ekman & Griesser 2002) and (ii) access to resources (wealth, quality territories) (Hannon et al 1987;Stacey & Ligon 1987, 1991. Prolonged brood care can confer advantages to delayed dispersers regardless of variation in resource wealth, but is tied to resource wealth in that wealthier parents can better afford to share resources (Ekman & Rosander 1992).…”