Sex Difference in Condition Dependence of Carotenoid Gapes in the Eurasian Roller (Coracias garrulus).

In altricial birds, sex differentiation can start early in the ontogeny in the form of color, physiology, and/or growth and may potentially result in sex-specific condition dependence of traits mediating parent-offspring communication. Carotenoids have long been hypothesized to modulate the expression of gape coloration, but their sex-specific role enforcing honesty of gape coloration remains poorly studied. In a within-nest design, we provided carotenoid supplementation to nestlings of the Eurasian roller (Coracias garrulus) and measured the response in circulating carotenoids, coloration of the gape, cutaneous immune responsiveness to phytohemagglutinin, and growth while accounting for the sex of nestlings. Male nestlings supplemented with carotenoids displayed enhanced pigmentation of their gapes and grew faster than control nestlings, but there was no within-individual correlation between gape color and growth in either carotenoid-supplemented or control males. Female nestlings, however, diverted most supplemented carotenoids into growing fast at the expense of reducing their level of circulating carotenoids and displaying less-pigmented gapes. Nestling cutaneous immune response was not affected by carotenoid supplementation in either sex. Our results provide only weak support for the hypothesis that carotenoids enforce the honesty of gape color signals in nestling rollers and demonstrate sex specificity in how nestlings divert a surplus of carotenoids into different physiological functions.

Methionine supplementation influences melanin-based plumage colouration in Eurasian kestrel, Falco tinnunculus, nestlings.

The extent to which the expression of melanin-based plumage colouration in birds is genetically or environmentally determined is controversial. Here, we performed a between-nest design supplementation with either the sulphur amino acid dl-methionine or with water to investigate the importance of the non-genetic component of melanin-based plumage colouration in the Eurasian kestrel, Falco tinnunculus. Methionine affects growth and immunity, thus we aimed to modify nestling growth and immunity before feather development. Then, we measured the effect of the experiment on colouration of two melanin-based plumage patches of nestling kestrels. We found that methionine slowed down nestling growth through treatment administration and that nestlings compensated by speeding up their growth later. We did not find any effects of methionine on nestling immunity (i.e. lymphocyte counts, natural antibody levels or complement-mediated immunity). Effects on growth seemed to be mirrored by changes in nestling colouration in the two sexes: methionine-nestlings showed less intense brown plumage on their backs compared with control nestlings. These results provide support for a non-genetic determination of a melanin-based plumage patch in the two sexes of nestling kestrels.