HATCHING ASYNCHRONY AND HATCHING SUCCESS IN THE HOUSE SPARROW - EVIDENCE FOR THE EGG VIABILITY HYPOTHESIS

We tested the ''egg viability'' hypothesis in the House Sparrow Passer domesticus by comparing the hatchability of clutches with different d egrees of hatching asynchrony using multiple linear regression analysi s. Hatching duration was directly and independently affected by clutch size, laying date and temperature. Taking these factors into account, nests with positive residuals (asynchronous nests) had a greater prob ability of hatching the early laid eggs successfully. This indicates t hat by starting incubation early and hatching asynchronously, birds ma y reduce embryo mortality. Asynchronous nests fledged a greater propor tion of early-hatched young, but total nestling mortality did not diff er between asynchronous and synchronous nests. Nestling mortality due to intrabrood competition did not constrain hatching asynchrony. The p resence of highly synchronous broods seemed to indicate that in some c ases hatching asynchrony was precluded by constraints acting on the la ying female.