Lesser double-collared sunbirds Nectarinia chalybea do not compensate for hatching asynchrony by adjusting egg mass or yolk androgens

Substantial amounts of maternal androgens are found in birds' eggs and have been shown to benefit offspring development. Within-clutch patterns of increasing androgen concentrations over the laying sequence are often hypothesized to compensate for the negative effects of hatching asynchrony. However, detrimental effects to offspring fitness of exposure to high yolk androgen levels have also been demonstrated. This suggests that mothers should forego these costs to their offspring when the need for compensation for hatching asynchrony is low or when alternative compensatory strategies, e.g. in terms of increasing egg mass, are available. Here we show that in the south-temperate lesser double-collared sunbird Nectarinia chalybea, a species with hatching asynchrony but also with high survival of last-hatched chicks, mothers do not deposit resources differentially in terms of either yolk androgen concentration or egg mass across the laying sequence. We discuss to what extend this challenges the original explanation of within-clutch variation in these egg parameters and offer some explanation for their between-clutch variation which was related to female body mass.