Biology Reference
In-Depth Information
A male can care simultaneously for the clutches of several females. Early in the season,
when the sex ratio is 1:1, males compete for females and actively court females at their
nest sites. However, during the summer the sex ratio becomes female-biased because of
higher male mortality, likely due to the costs of parental care and mating competition.
This leads to sex role reversal, with females now competing for access to males and
playing the most active role in courtship (Forsgren et al ., 2004).
Sex differences in competition
In some cases the two sexes may have different ornamentation which is related to sex
differences in competition. The remarkable eclectus parrot, Eclectus roratus , of north-
eastern Australia has pronounced sexual dimorphism (Fig. 7.16). The bright red and blue
females compete for scarce nest hollows, particularly ones less susceptible to flooding, where
fledging success is higher. Females are hidden inside these nest hollows during incubation
and nestling care, so they do not need to be cryptic. They display below the canopy, where
their bright colours contrast with the dark limbs and trunks of the trees. The bright green
males compete for access to females with the best nest sites and their colouration reflects a
compromise between camouflage from predators and the advantage of conspicuousness
(scarlet underwing coverts) for displays (Heinsohn et al ., 2005; Heinsohn, 2008).
The eclectus parrots remind us that both sexes are likely to have to compete with
members of their own sex. For females, whose reproductive success is most often limited
by resources, the competition will most often be for access to resources necessary for
Eclectus parrots:
sex differences in
colours and in
competition
(a)
(b)
Fig. 7.16 Sexual dimorphism in the eclectus parrot. The male is bright green, with red
underwing coverts. The female is bright red and blue. Photos © (a) Lochman
Transparencies, (b) © Michael Cermak.
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