We observed three polygynous trios (one male and two females) in a colony of lesser kestrels in southern Spain. All these trios broke up before egg-laying because the secondary female deserted and paired with another male. The time spent by the bigamous males with the female at the colony, the copulation rates, and the mate-feedings, were always lower for the secondary females. Primary females received levels of investment similar to that obtained by females paired with monogamous males. Aggressions between the two females paired with the same male were frequent. Polygyny in the lesser kestrel might be favoured by a female biased sex-ratio, and maintained because bigamous males and secondary females can get some benefits, independently of the final success of the breeding attempt.