Natural or sexual selection: which evolutionary mechanism determines tail length in Tyrannus savana?

The morphological evolution of forked tails in birds can be the result of natural selection (NS), maximizing a bird’s aerodynamic performance or via sexual selection (SS), increasing tail length due to the preference of the selective sex when choosing a partner. Migration can be an important factor that maximizes the aerodynamics of tails. In order to study which selection pressures (NS or SS) may act on the tail morphology of migratory birds, we compared tail length in two populations of Fork-tailed Flycatcher (Tyrannus savana) with different migratory behavior (resident and migratory). Additionally, we studied the relationship of tail length and reproductive success in the migratory population. If NS constrains SS in the migratory population, we expected shorter tails and lower variation in reproductive success among migratory males with different tail lengths. Tail length of the migratory males was shorter than that of resident males. We found no relationship between reproductive success and male tail length. A low frequency of extra-pair fertilizations was observed (5% -10%), which were not associated with male tail length. Long-tailed males mated with long-tailed females, which supports the existence of a SS mechanism. These results support a tradeoff between NS and SS in the evolution of tail length in Fork-tailed flycatchers.