Clock gene polymorphism and scheduling of migration: a geolocator study of the barn swallow Hirundo rustica

Circannual rhythms often rely on endogenous seasonal photoperiodic timers involving ‘clock’ genes, and Clock gene polymorphism has been associated to variation in phenology in some bird species. In the long-distance migratory barn swallow Hirundo rustica, individuals bearing the rare Clock allele with the largest number of C-terminal polyglutamine repeats found in this species (Q(8)) show a delayed reproduction and moult later. We explored the association between Clock polymorphism and migration scheduling, as gauged by light-level geolocators, in two barn swallow populations (Switzerland; Po Plain, Italy). Genetic polymorphism was low: 91% of the 64 individuals tracked year-round were Q(7)/Q(7) homozygotes. We compared the phenology of the rare genotypes with the phenotypic distribution of Q(7)/Q(7) homozygotes within each population. In Switzerland, compared to Q(7)/Q(7), two Q(6)/Q(7) males departed earlier from the wintering grounds and arrived earlier to their colony in spring, while a single Q(7)/Q(8) female was delayed for both phenophases. On the other hand, in the Po Plain, three Q(6)/Q(7) individuals had a similar phenology compared to Q(7)/Q(7). The Swiss data are suggestive for a role of genetic polymorphism at a candidate phenological gene in shaping migration traits, and support the idea that Clock polymorphism underlies phenological variation in birds.