Social and Environmental Factors Affect Natal Dispersal and Philopatry of Male Red-Cockaded Woodpeckers

Abstract

Natal dispersal behavior can vary considerably among individuals, but the causes of intraspecific plasticity in dispersal are poorly understood. We tested six hypotheses about social and environmental conditions that might influence natal dispersal of males in the cooperatively breeding Red-cockaded Woodpecker (Picoides borealis). Further, we examined whether variation in dispersal behavior is heritable. Dispersing from the natal territory during the first year rather than remaining as a helper was associated with four factors. First, dispersing male fledglings were, on average, significantly lower in body mass than their philopatric siblings, indicating an influence of social dominance on dispersal. Second, individuals were more likely to disperse from territories with many male fledglings, independent of the number of adult male helpers per territory, suggesting that sibling (rather than helper-offspring) competition for future reproduction may be the underlying mechanism. Third, the probability of remaining as a helper rather than dispersing was positively associated with quality of the natal territory and with the number of high-quality territories close to the natal site. This suggests an influence of the benefits of philopatry, because many males that initially remain as helpers eventually become breeders on the natal territory or a neighboring territory. Finally, we found evidence that ecological constraints influence dispersal: the probability of dispersing was positively related to the availability of vacant territories in the wider neighborhood of the natal site. Natal dispersal behavior was not influenced by resource competition, measured as group size on the natal territory, or by local density, estimated as the number of active territories in the vicinity of the natal site. Based on comparisons of father-son and brother-brother dispersal behavior, we found no evidence for heritability of philopatric behavior. Dispersal of male fledgling Red-cockaded Woodpeckers can be viewed as conditional on social and ecological factors in the natal territory and in the immediate neighborhood. These factors seem to serve as proximate cues that influence young birds to either disperse or remain as philopatric helpers.

Sex-bias and timing of natal dispersal in cooperatively breeding Puff-throated Bulbuls Alophoixus pallidus

“…Perhaps this experience results in increased reproductive success later in life, although only 10% of second-year females engaged in such behaviour. In contrast, male PBUs are generally philopatric, as has been reported in other cooperatively breeding birds (Miller and Smallwood 1997;Pasinelli and Walters 2002;Cockburn et al 2008). It has also been shown that survivorship of philopatric males is higher than those who disperse (Wiley and Rabenold 1984;Rowley and Russell 1997), and Ward and Weatherhead (2005) report that dispersing males experience lower reproductive success.…”

“…Hamilton 1964;Cockburn 1998;Dickinson and Hatchwell 2004) which confers an evolutionary advantage. In stable social groups, older birds or older helpers are competitively superior over subordinates (Cockburn et al 2008) and they may have an increased probability of 'winning' sites as vacancies occur (Pasinelli and Walters 2002;Cockburn et al 2003Cockburn et al , 2008). In our system, breeding vacancies appear to be limiting, and thus staying home as a helper may be important.…”

“…This process involves a complex combination of individual decisions on when, where, how far, or whether to disperse. The transition period is particularly important in the complex dynamics of cooperative breeding species, where some individuals may remain in their natal territory for several months post-fledging, some remain with parents indefinitely (Pasinelli and Walters 2002;Cockburn et al 2008;Woxvold and Mulder 2008), and others disperse shorter or longer distances (Eikenaar et al 2008).…”

“…In most cases, juveniles remain with their parents as helpers (e.g. Curry and Grant 1990;Cockburn 1998;Pasinelli and Walters 2002), although delayed dispersal is not always associated with helping (e.g. Gayou 1986;Ekman et al 1994).…”

Do sibling tits (Parus major, P. caeruleus) disperse over similar distances and in similar directions?

“…We chose this approach rather than randomly selecting a single record for each sibling group (cf. Pasinelli and Walters 2002) because in this way we maximised the amount of information without inflating the degrees of freedom. We verified that the (untransformed) relative differences and distances were normally distributed, even though this did not necessarily apply to the basic data (distances and angles).…”