DELAYED MATURATION IN PLUMAGE COLOR - EVIDENCE FOR THE FEMALE-MIMICRYHYPOTHESIS IN THE KESTREL

In many sexually dichromatic species, young males have female-like plu mage during their first potential breeding year. The female-mimicry hy pothesis (FMH) supposes that by possessing female-like plumage young m ales deceive older conspicuous males into believing that they are fema les, thus reducing competition from adult males. The status-signalling hypothesis (SSH) supposes that adult males can distinguish sex, but p ostulates that young males reduce competition from adult males by reli ably signaling low status with their dull plumage. We tested these hyp otheses in the European kestrel (Falco tinnunculus). Female-like young males settled to breed closer to adult males than did other adult mal es (Figs. 1a, b). By settling near adult males, young males seemed to increase their chance of mating with adult females. Adult female-young male pairs had better reproductive success than yearling-yearling pai rs. These results suggest that there is an adaptive value in possessin g a female-like plumage colour in the breeding season. To test the FMH , we measured sexual preference of adult males when adult females and young males were simultaneously shown in an aviary. Adult males were u nable to recognize sex, because in half the cases they preferred young males (Fig. 3). However, when adult males and females were shown simu ltaneously, males preferred females (Fig. 2). Our results support the FMH rather than the SSH, because young males successfully deceived old er males by their plumage.