Delayed plumage maturation in Eurasian kestrels: female mimicry, subordination signalling or both?

Delayed plumage maturation (DPM) of young males in sexually dichromatic birds has been explained under the female mimicry hypothesis (FMH) or the subordination signalling hypothesis (SbSH), among other adaptive ideas proposed. In this study, we tested in the Eurasian kestrel, Falco tinnunculus, the predicted sexual deception and aggressiveness under the FMH and SbSH to explain DPM in birds. We analysed aggressive and courtship behaviour of breeding males and females in the presence of adult male, adult female and 1-year-old male natural decoys. We designed two different treatments: one presenting un-moulted 1-year-old males and a second presenting partly moulted 1-year-old males, as observed in nature. Males attacked adult male decoys more frequently than 1-year-old male and female decoys in both moulted and un-moulted treatments and no sexual displays were observed. Females were more aggressive towards female and 1-year-old male decoys as compared to male decoys in the un-moulted treatment. In the moulted treatment, females were more aggressive towards female decoys as compared to both male decoys. In addition, females solicited copulas from moulted, but not from un-moulted, 1-year-old male decoys. Our results suggest that FMH can be a mechanism which explains DPM in un-moulted 1-year-old males. However, when they show moulted plumage (more than 80% in the studied population during the spring), the SbSH seems to be the mechanism that better explains sexual and agonistic behaviour from adults towards 1-year-old males.