Male-male pair bonding, nesting and egg incubation in a wild passerine

Examples of nesting behavior between pair-bonded males are exceedingly rare among songbirds. We observed a male same-sex pair bond that resulted in the partial development of one egg while monitoring the breeding activity of a population of wild Crimson Finches (Neochima phaeton) over four breeding seasons. This male–male pair built four nests and incubated two clutches, each with a single egg of unknown origin, with at least one developing partially before failing. This same-sex behavior is unusual among passerines and its origin and evolutionary significance remain poorly understood.