Blue tit hatching time is unaffected by environmental variation experienced by the mother

Caroline E. Thomson, Jarrod D. Hadfield

In birds, eggs within a clutch often hatch at different times, rather than all at once, due to the mother starting incubation before all the eggs in the nest have been laid. Chicks that hatch last in the nest are often at a competitive disadvantage and therefore often attain a lower weight and have a higher chance of dying before leaving the nest.

Previously, in a population of blue tits (Cyanistes caeruleus), we cross-fostered eggs between nests to manipulate their relative position within the egg laying order. This showed that eggs that are laid last in a nest hatch more quickly than eggs laid first for a given amount of incubation. This implied that there were differences within the eggs that were causing hatching times to differ between individuals.

As the eggs are formed within the mother, any differences in the eggs are due to changes she causes during the formation of the egg, whether she does this actively or passively. We predicted that the amount of food available to the mother might affect the contents of the eggs she produces, and thereby hatching time.

Using the same population of blue tits, we tested this by providing extra food to certain birds, and looked for any effects on the hatching time of eggs. We expected that increased food to mothers would accelerate hatching times of certain eggs in the nest but found no such effects. This suggests that the changes in hatching time of eggs is not due to changes in the food available to females when forming the eggs. The supplementary food did not change the mass or hatching probability of the eggs either. We also tested whether the weather experienced by females when forming eggs affected hatching time, as this could also affect resource availability, but found that there were no effects.

Read the article in full here.

Image: blue tit photo provided by authors.

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