Ovulation order mediates a trade-off between pre-hatching and post-hatching viability in an altricial bird

Abstract

Simultaneously dependent siblings often compete for parentally provided resources. This competition may lead to mortality, the probability of which may be a function, in part, of the individual offspring's production order. In birds, serial ovulation followed by hatching asynchrony of simultaneous dependents leads to differences in-post-hatching survival that largely depend on ovulation (laying) order. This has led to the widespread assumption that early laid eggs are of greater value and therefore should possess different maternally manipulated characteristics than later-laid eggs. However, this perspective ignores the potential effect of laying order on pre-hatching viability, an effect which some studies suggest should offset the effect of laying order on post-hatching viability, I examined the relationship between laying order and hatching and fledging probability in wild, free-living Lincoln's sparrows (Melospiza lincolnii). In broods with complete hatching success, first-laid-and therefore first-hatched offspring had the highest probability of fledging, and fledging probability declined with increasing laying order. Horwever, first-laid eggs were less likely than later-laid eggs to hatch. This effect of laying order on pre-hatching viability seemed to offset that on post-hatching viability, and consistently, maternal investment in egg size varied little if at all with respect to laying order. These results suggest that ovulation order mediates a trade-off between pre-hatching and post-hatching viability and should encourage a re-evaluation of the solitary role post-embryonic survival often plays when researchers make assumptions about the value of propagules based on the order in which they are produced. © 2008 Keith Sockman.