Fiery red heads: female dominance among head color morphs in the Gouldian finch

Although the evolution of genetic color polymorphisms has receivedmuch theoretical interest, few empirical studies have investigatedthe adaptive function of alternative color morphs. Furthermore,most studies have focused almost exclusively on the evolutionand adaptive expression of male coloration, leaving the roleof conspicuous female coloration largely unknown. Using thecolor polymorphic Gouldian finch (Erythrura gouldiae), thisstudy experimentally tests the status signaling function ofhead color (red, yellow, and black) among the 3 female colormorphs. In standardized dominance contests between unfamiliarfemales of different head colors, red-headed females dominatedboth black- and yellow-headed females. During contests betweenthe morphs, red-headed females passively displaced black- andyellow-headed opponents, whereas interactions between red-headeddyads were particularly aggressive and more frequent than interactionswithin dyads of the other color morphs. This effect of red dominancefurther persisted when head color was experimentally altered;red-manipulated females (of the other morphs) dominated bothblack- and yellow-headed females, whereas blackened red-headedfemales were dominated by naturally red-headed birds. Togetherwith similar dominance-related differences among male morphs,these results suggest that the 3 color morphs may display alternativestrategies in dominance behavior.     

Offspring sex ratios correlate with pair-male condition in a cooperatively breeding fairy-wren

We examined sex allocation patterns in island and mainland populationsof cooperatively breeding white-winged fairy-wrens. The markeddifferences in social structure between island and mainlandpopulations, in addition to dramatic plumage variation amongmales both within and between populations, provided a uniquesituation in which we could investigate different predictionsfrom sex allocation theory in a single species. First, we testthe repayment (local resource enhancement) hypothesis by askingwhether females biased offspring sex ratios in relation to theassistance they derived from helpers. Second, we test the malequality (attractiveness) hypothesis, which suggests that femalesmated to attractive high-quality males should bias offspringsex ratios in favor of males. Finally, we test the idea thatfemales in good condition should bias offspring sex ratios towardmales because they are able to allocate more resources to offspring,whereas females in poor condition should have increased benefitsfrom producing more female offspring (Trivers-Willard hypothesis).We used molecular sexing techniques to assess total offspringsex ratios of 86 breeding pairs over 2 years. Both offspringand first brood sex ratios were correlated with the pair-male'sbody condition such that females increased the proportion ofmales in their brood in relation to the body condition (masscorrected for body size) of their social partner. This relationwas both significant and remarkably similar in both years ofour study and in both island and mainland populations. Althoughconfidence of paternity can be low in this and other fairy-wrenspecies, we show how this finding might be consistent with themale quality (attractiveness) hypothesis with respect to malecondition. There was no support for the repayment hypothesis;the presence of helpers had no effect on offspring sex ratios.There was weak support for both the male quality (attractiveness)hypothesis with respect to plumage color and the maternal conditionhypothesis, but their influence on offspring sex ratios wasnegligible after controlling for the effects of pair-male condition.     

Host immune function and sexual selection in birds

Parasites have been hypothesized to affect sexual selection of their hosts, if secondary sexual characters reliably signal absence of infectious parasites, superior parenting ability caused by the absence of parasites, or heritable resistance to parasites, for which there is some intraspecific and interspecific evidence. Measures of immune defence of hosts provide reliable information on the current infection status of individuals of the chosen sex, usually males, and correlations between immune defence and development of secondary sexual characters thus provide a novel critical test of parasite-mediated sexual selection. In a comparative study of birds, sexually dichromatic species had higher immune defences, measured in terms of leukocyte concentration and the size of spleen and bursa of Fabricius, respectively, than closely related, monochromatic species. Male plumage brightness was consistently negatively related to the size of the spleen in males of sexually dichromatic species, but not in males of monochromatic species. Hence, the brightest males, which frequently are preferred as mates by choosy females, had low levels of immune defence, suggesting that such males were healthy. This provides evidence for a general role of parasites in sexual selection among their bird hosts.