Status signalling in the western Greenfinch, Carduelis chloris.
Greenfinch plumage varies both between and within age and sex
classes. This study looked at the possible causes and consequences
of this variation in a colour-ringed population. Plumage colour was
both repeatable and heritable.
The function of colourful plumage in the breeding season was
reviewed. Many aspects of the breeding biology of Greenfinches were
studied and the effects of plumage on breeding success investigated.
Brightly coloured birds seemed to have greater reproductive success
than dull ones. Brightly coloured males were also more likely to
ret urn to the study area in the following breeding season.
Greenfinches are usually regarded as monogamous, but I found that
over 25% of nests involved polygamy. Polygyny, polyandry and
possible cases of polygynandry were recorded, but polygyny was the
most common of the three. It was demonstrated that the experimental
provision of food influenced the occurrence of polygyny. The
literature generally considers polygyny to be bad for females,
however in Greenfinches polygynous pairs were as successful at
producing independent offspring as monogamous pairs. Polygynous
birds recruited more offspring into the local population than
monogamous birds, although this may reflect differences in dispersal.
Since polygynous males were bright and had better survival and since
colour was found to be heritable, females may have been choosing
males for their good genes. If colour is an honest signal, there
must be some cost preventing dull birds from becoming bright. Bright
Greenfinches were more likely to be killed by Sparrowhawks during the
summer than dull Greenfinches and they were also more likely to be
injured. In comparison, dull birds were more likely to be killed by
Tawny Owls in the winter. Whether Greenfinch plumage variation acted
as a "badge of status" over the winter was investigated. The
brighter a Greenfinch's plumage the more likely it was to win
confrontations at a bird table in the winter, regardless of food type
(contra Maynard Smith & Harper 1988).
So what influenced a Greenfinch's plumage? Birds with damaged
feathers only regrew bright plumage if they were in good body
condition. Birds with low fat stores regrew paler feathers after
damage, which is possibly related to the fact that the carotenoid
pigment is stored in fat. Therefore, it is possible that after the
breeding season good quality nales recovered faster, put on more fat
and acquired brighter plumage in the moult. Plumage variation in the
House Sparrow was also investigated. In hand estimates of bib size
were correlated with spectrophotometric estimates of melanin content.
Bib size was not related to organ size.
The results of this study are compared with the literature on
status signalling . It is argued that badges are handicaps i.e.
uncheatable signals of individual quality, rather than being
arbitrary signals or signalling Resource Holding Potential.