The consequences of winter flock demography for genetic structure and inbreeding risk in vinous-throated parrotbills, Paradoxornis webbianus.

The adaptive significance of flocking behaviour has been intensively studied, especially among birds, but the demography and genetic structure of winter flocks is poorly understood, despite their importance for many biological processes. For three successive winters, we studied the demography and genetic structure of winter flocks in a small passerine, the vinous-throated parrotbill Paradoxornis webbianus. Our objectives were to determine the match between observed demography and the genetic structure of winter flocks, and to investigate the consequences of kin structure for the risk of inbreeding during the breeding season. The size of five main study flocks ranged from 60 to 120 individuals. The sex ratio of each flock did not deviate from parity and survival rates of adults were moderately low (36 and 17% in 2 years). Adults showed strong fidelity to a winter flock not only within a season but also between years. As expected from observational data, we found significant genetic differentiation and isolation by distance among males and females in winter flocks at a fine spatial scale. Likewise, relatedness among individuals within a winter flock was significantly greater than between flocks. Despite these demographic and genetic circumstances, the inbreeding rate was relatively low (4%). We conclude that fine-scale genetic structure occurs in the winter flocks of birds as a result of demographic processes without physical barriers. This genetic structure introduced a risk of inbreeding, but our evidence on whether individuals avoided incest was equivocal.