Pair formation and copulation behavior in least flycatcher clusters

Least Flycatcher Empidonax minimus pairs form dense clusters of territories on the breeding grounds. We describe pair formation and copulation behavior (both within- and extra-pair) in Least Flycatcher clusters. Pair formation involved a complex behavioral sequence of trill vocalizations and visual display. Within-pair copulations were five times more likely to achieve cloacal contact than were extra-pair copulations. Least Flycatchers exhibited an overall within-pair mating effort of 2.0 ± 0.5 events pair⁻¹ hr⁻¹ compared to an extra-pair mating effort of 1.8 ± 0.3 events pair⁻¹ hr⁻¹. Within- and extra-pair mating behavior by focal birds were distinctly different with respect to rate, conspicuousness, duration, aggression intensity, and pre-copulatory display. The rate of territory incursions for extra-pair copulations was high. We discuss copulation behavior in the context of male and female mating tactics, and highlight the disparity in our knowledge of mating behavior in other members of this genus.