Within-clutch patterns of yolk testosterone vary with the onset of incubation in black-headed gulls

Hatching asynchrony in birds produces an age and size hierarchy among siblings. Later-hatching chicks have a competitive disadvantage, and brood reduction may occur when food availability is insufficient to raise all chicks. When early-hatched chicks fail to survive or if the circumstances allow raising all chicks, mothers should reverse the disadvantage to later-hatched chicks. Increasing deposition of maternal androgens with the laying sequence has been suggested to compensate for detrimental effects of hatching asynchrony, allowing a more precise adjustment of the survival probabilities of each chick. Here, we show for black-headed gulls that the increase in yolk testosterone with each successive egg is greater when the mother incubates longer before clutch completion, which is the major determinant of the degree of hatching asynchrony. This finding supports the idea that yolk testosterone has a compensatory function in the context of hatching asynchrony. Our data further show that if the time needed to complete a clutch is lengthened, the developmental differences due to incubation between the first- and the last-laid eggs increase. In addition, the onset of incubation before clutch completion occurs sooner as the breeding season progresses. Both long inter-egg intervals and the seasonal shift in incubation behavior enhance the necessity of compensation for later-hatching chicks. Indeed, yolk levels of testosterone increased more steeply over the laying order, if the duration of the egg-laying period was extended and in later-laid clutches. We suggest that prolactin plays a key role in the adjustment of testosterone allocation to the incubation pattern. Copyright 2004.