Intraclutch Differences in Egg Characteristics Mitigate the Consequences of Age-Related Hierarchies in a Wild Passerine

Abstract

The relative age of an individual's siblings is a major cause of fitness variation in many species. In Blue tits (Cyanistes caeruleus), we show that age hierarchies are predominantly caused by incubation preclutch completion, such that last laid eggs hatch later than early laid eggs. However, after statistically controlling for incubation behavior late laid eggs are shown to hatch more quickly than early laid eggs reducing the amount of asynchrony. By experimentally switching early and late laid eggs between nests on the day they were laid, we controlled for the effect of differential incubation and found that the faster hatching times of late laid eggs remains. Chicks that hatched earlier were heavier and had higher probability of fledgling, and chicks that hatched from experimental eggs had patterns of growth and survival consistent with this. Egg mass explained a small part of this variation, but the remainder must be due to egg composition. These results are consistent with the idea that intrinsic differences between eggs across the laying sequence serve to mitigate the effects of age-related hierarchies. We also show that between-clutch variation in prenatal developmental rate exists and that it is mainly environmental in origin rather than genetic. K E Y W O R D S :Cyanistes caeruleus, development, eggs, maternal effects, quantitative genetics, selection.

Troubleshooting the potential pitfalls of cross‐fostering

“…; Hadfield et al . ). Since cross‐fostered broods are unlikely to be identical in age, size and mass, cross‐fostering will change a brood's composition, thereby affecting fitness.…”

“…The timing of cross‐fostering determines the accuracy of, for example, heritability estimates by dictating which early environment effects can be partitioned from genetic variation (Table ; Hadfield et al . ,b). Overall, cross‐fostering is a versatile tool for researchers addressing diverse research questions (Table ; Mateo & Holmes ).…”

Tetrodotoxin concentrations within a clutch and across embryonic development in eggs of the rough-skinned newts (Taricha granulosa)

“…For example, female canaries (Serinus canaria) allocate different quantities of testosterone in their eggs such that the concentration of hormones increases with the order of egg deposition (Schwabl, 1993). Similarly, birds may differentially allocate lipids (yolk) to their eggs such that early laid eggs are larger and have a competitive advantage, or late eggs are larger thereby equalizing the competitive advantage across the clutch (Hadfield et al, 2013;Howe, 1976). Alternatively, some eggs may simply have more time to absorb TTX if egg toxicity is a passive by-product of the physiology of newt reproductive cycles; this may explain why TTX toxicity is not correlated with the volume of the embryo (Hanifin et al, 2003).…”

Eggshell porosity covaries with egg size among female House Wrens (Troglodytes aedon), but is unrelated to incubation onset and egg-laying order within clutches

“…However, Wang and Beissinger (2009) found that variation in hatching spans was unrelated to the onset of incubation in five passerine species, despite considerable variation in the onset of incubation relative to clutch completion. Moreover, within clutches, the incubation periods of earlier-laid eggs were actually longer than those of later-laid eggs (Wang and Beissinger 2009; see also Viñuela 1997; Hadfield et al 2013), suggesting that mechanisms other than differences in the onset of incubation may contribute to variation in incubation periods, including properties of the eggs themselves. However, the properties of the egg that influence incubation periods have yet to be determined (see also Robinson et al 2008).…”