Sexual selection and blue tit (Parus caeruleus) crown coloration

Conspicuous, sexually dimorphic plumage in birds is most likely a
consequence of sexual selection favouring more ornamented males at
obtaining a territory and/or a mate. Recent comparative analyses
suggest that, among socially monogamous species, extra-pair
paternity has also contributed to the elaboration of male
ornaments. If females prefer more ornamented males as social or
extra-pair mates this could translate into strong directional
selection for ornament elaboration, since these males might sire
more offspring in their own brood (within-pair success) or in
broods of other males (extra-pair success). In this thesis I study
the expression of the UV/blue crown coloration in the blue tit
(Parus caeruleus) and investigate whether this trait could be
selected through increased male within- or extra-pair success. Blue
tits are socially monogamous passerines with relatively high levels
of extra-pair paternity, and males in this species display more
ultraviolet(UV)/blue reflectant crown feathers than females. Based
on three years of data I found that crown coloration could be a cue
used by females to assess male age since blue tits became more
UV-ornamented as they aged. Crown coloration, however, did not
correlate with survival to the next breeding season, suggesting
that more UV-ornamented males are not necessarily of higher
quality. While crown UV-ornamentation increased between years, it
declined in the course of a year due to feather wear and dirt
accumulation and this could affect female perception of male
attractiveness. However, although the decline in UV ornamentation
between winter and spring was large, it had no effect on male
reproductive success, and winter and spring colour were still
positively correlated. Using genetic paternity analysis I could
show that more UV-ornamented males do not benefit through increased
within-pair or extra-pair success. On the contrary, less
UV-ornamented, adult males sired most of the extra-pair offspring.
Hence the most successful males of the population were adult males
that resembled juveniles in their crown colour. Accordingly,
females seem to recognise less UV-ornamented males as highly
successful, since they biased brood sex ratio towards male
offspring if paired to these males. The causality of these patterns
was tested in a colour manipulation experiment, where I treated
males to become more (UV+) or less (UV-) ornamented within the
natural range of variation. Against expectations UV(+) males sired
significantly more extra-pair offspring than UV(-) males while the
proportion of within-pair offspring was unaffected by the
manipulation. Brood sex ratios did not differ between treatments
but depended on male colour before manipulation. While these
results do suggest that crown colour plays a role in paternity and
brood sex allocation, they do not provide experimental support for
the observed correlational patterns. I discuss the discrepancy
between observational and experimental data, emphasising potential
problems with the experimental manipulation of structural plumage
colour. Finally, given that more UV-ornamented males did not sire
more offspring, I explore the possibility that they would benefit
by pairing with high quality females. High quality females in other
species are often more ornamented, and birds of high quality pair
assortatively based on ornament expression. This was not the case
in this blue tit population, since female colour did not appear to
indicate relevant female qualities (fecundity, seasonal
reproductive success) and blue tits did not mate assortatively by
crown colour in any of the three study years. To conclude,
selection seems to favour older, less UV-ornamented males in this
population. Whether this is due to female preference is unclear.
Alternatively I hypothesise that being less ornamented may enable
males searching for extra pair copulations to intrude into other
territories without eliciting aggression by territory owners,
perhaps by mimicking juveniles. Detailed behavioural observations
in the wild coupled with choice chamber experiments in captivity
are necessary to test this idea.