Offspring sex ratio allocation in the parasitic jaeger: selection for pale females and melanic males?
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The maintenance of plumage color polymorphism in the parasitic jaeger (Stercorarius parasiticus) is still not well understood. Earlier studies indicated that selection may favor pale females and melanic males. If so, females would maximize their fitness, producing pale female and melanic male offspring. We therefore predicted that females might bias their offspring sex ratio toward daughters in pale pairs and toward sons in melanic pairs. Females might also choose to mate assortatively in relation to plumage color, thereby maximizing the probability of producing either pale or melanic offspring. Because females are larger than males, differential rearing costs may affect the offspring sex ratio independent of parental plumage color. We examined offspring sex ratio allocation, breeding variables indicative of parental quality, and mating pattern in relation to plumage color in a colony of parasitic jaegers in northern Norway. Jaegers tended to mate assortatively in relation to plumage color. The reproductive performance declined with season, and matched pairs appeared to be of lower quality than mixed pairs. The proportion of male offspring increased with hatching date in matched pale and mixed pairs, whereas the situation was reversed in matched melanic pairs. Matched pale pairs produced an overall surplus of favorable pale but costly daughters despite their lower quality, while melanic pairs produced a surplus of favorable melanic sons. However, differential offspring rearing costs and parental rearing capacity may have additionally affected the realized offspring sex ratio. Mixed pairs producing an overall surplus of pale and melanic daughters allocated their resources according to differential rearing costs and parental quality only. We suggest that both strategies of sex ratio allocation together with differences in reproductive success in matched versus mixed pairs may have a balancing effect on the mating pattern between plumage morphs and may contribute to the maintenance of the color polymorphism in this species.