Hatching asynchrony in the mountain white-crowned sparrow (Zonotrichia leucophrys oriantha): a selected or incidental trait?

AI3STRACT.--FemaleMountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) begin diurnal attentivehess with the laying of the first egg but withhold nocturnal incubation until the penultimate egg is laid.To better understand this behavior, we measured hatching times, egg volumes, and patterns of nestling growth and mortality over a 4-yr period in Z. l. oriantha breeding at Tioga Pass, California.Additional data from 11 yr of study on nestling mortality and nest predation also were examined.Hatching patterns matched the attentiveness patterns observed during the egg-laying period.Eggs tended to hatch in the order of laying, and the time period between hatching of the last two eggs was greater than between other successive eggs.The lag between the hatching of the last two eggs showed a slight increase over the course of the breeding season.Although chicks from last-laid eggs came from generally larger eggs, they tended to be smaller than their siblings because of the hatching asynchrony.Smaller chicks died or disappeared with greater frequency than their larger siblings from nests when the effective food supply had been reduced experimentally.Such selective starvation or "brood reduction" may occur naturally in as many as 14% of all successful nests.The phenomenon of brood reduction is discussed in relation to four current hypotheses, while also considering the implications of intraclutch variations in egg volume.Although hatching asynchrony may provide the advantages proposed in these hypotheses, there is reason to believe that it originally evolved for other reasons.We propose that hatching asynchrony is an epiphenomenon of the hormonal mechanism governing egg-laying and incubation.This hypothesis provides a testable alternative to existing "adaptive" hypotheses.