Heritable choice of colony size in cliff swallows: does experience trump genetics in older birds?

The variation in breeding-colony size seen in populations of most colonial birds may reflect heritable choices made by individuals who are phenotypically specialized for particular social environments. Although a few studies have reported evidence for genetically based choice of group sizes in birds, we know relatively little about the extent to which animals potentially rely on experience versus innate preferences in deciding with how many conspecifics to settle at different times of their lives. We conducted a cross-fostering experiment in 1997-1998 on cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in which some individuals were reared in colonies different in size from those in which they were born. Breeding-colony sizes chosen by this cohort of birds were monitored by mark-recapture throughout their lives. A multistate mark-recapture analysis revealed that birds in their first breeding year chose colony sizes similar to those of their birth, regardless of their rearing environment, confirming a previous analysis. Beyond the first breeding year, however, cliff swallows' colony choice was less dependent on where they were born. Birds born in small colonies and reared in large colonies showed evidence of a delayed rearing effect, with these birds overwhelmingly choosing large colonies in later years. Heritabilities suggested strong genetic effects on first-year colony choice but not in later years. Cliff swallows' genetically based colony-size preferences their first year could be a way to ensure matching of their phenotype to an appropriate social environment as yearlings. In later years, familiarity with particular colony sites and available information on site quality may override innate group-size preferences when birds choose colonies.