The Environment Affects The Size Of These Bird Eggs

Female birds can change egg contents based on social status, the qualities of dad, and in response to environmental factors.

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For example, when population density is high and a female’s chicks are likely to experience a high level of competition, moms often provision their eggs with extra androgens or testosterone to ensure that their offspring will be able to hold their own. When predator levels are predictably high, moms can provision their eggs with components for increased growth rates or faster wing development – all in the name of ultimately maximizing her own genetic representation in future generations.

Cooperatively Breeding Moms

Cooperatively breeding bird moms consider the current and near-future availability of food. The primary question for any cooperatively breeding mom to consider is this: if environmental conditions are challenging, is it worth the risk of providing very little to eggs? How much can or should a mom rely on helpers to come through for the sake of a brood that isn’t their own?

The Effect Of Environmental Conditions

When environmental conditions are particularly favourable, such as when the surrounding area is cool and lush with plenty of available food sources, dominant breeding fairy-wren moms lay much smaller eggs and realize a benefit to their overall biological fitness for a comparatively small investment.

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On the other hand, when the environmental conditions are unpredictable or unfavourable, such as when climate is hot and dry with a concurrent decrease in available food sources, dominant breeding moms maintain a high level of investment, producing clutches with large eggs.

This indicates that fairy-wren females are aware of the increased risk of relying on helpers when conditions are rough, and they act accordingly.

This is evidence for the bigger-is-better hypothesis, which predicts that mothers can benefit by producing offspring (eggs) that are larger when conditions are potentially more threatening. Larger chicks are generally less vulnerable to external conditions like heat stress or evaporative weight loss, and so the chances of their survival in unfavourably hot and dry conditions is much higher than small chicks.

Moms Who Aren’t In Cooperatively Breeding Societies

Compared to the dominant mothers with helpers, mothers without helpers do not appear able to make such fine-tuned changes to their eggs.

Since these moms are doing all the foraging and chick provisioning, on their own, they do not have the energy to be able to produce larger eggs when environmental conditions become less favourable.

And even during favourable environmental conditions, creating smaller, less-fit eggs isn’t a great idea, as these moms don’t have any extra help to care for their future offspring. Overall, dominant moms in cooperatively breeding scenarios seem to have the upper hand when it comes to biological fitness. The conundrum of this situation for individuals that do not happen to find themselves in a dominant position is very real.


This post is an adapted excerpt from my book: “Wild Moms: Motherhood in the Animal Kingdom

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