Shedding light on gulls’ eggs: how light exposure triggers different development in the last egg of a clutch

In one of those quirks of circumstance a photograph of a much smaller than usual Black-headed Gull appeared on the Ayrshire Nature Facebook page. A short time later, news of a new paper offering a possible explanation arrived in my inbox.

In gulls the young hatch in the order in which eggs are laid. Small young gulls are those that hatched last and are at the receiving end of intense competition for resources from their earlier hatched siblings. These last hatched chicks are known to exhibit a resilient “junior phenotype” ‘characterized by accelerated hatching, increased begging behavior and a slower growth rate’. Is there some cue from the outside world that causes them to develop these physiological and behavioural features?

Francisco Ruiz-Raya, Jose Noguera and Alberto Velando of the University of Vigo in Spain considered that light might be the trigger for changing the pre-hatching development to the junior phenotype. As senior birds hatch, the parents spend less time incubating and more time feeding them. The last egg is therefore exposed to light for longer. To test the hypothesis they studied Yellow-legged Gulls (Larus michahellis), which lay three eggs per clutch at intervals of 1-3 days with incubation beginning after the second egg is laid. Junior birds typically hatch 1-3 days after their siblings. The difference of exposure of the eggs to light was marked. Third-laid eggs spent 82% of their time exposed to high light intensities during a six-hour period two days before hatching. By contrast senior eggs spent only 0.05% of their time. If light is the trigger, then the signal is loud (well, bright) and clear.

Yellow-legged Gull (Larus michahellis) in the Gulf of Olbia (Sardinia, Italy)
Gzzz, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

By manipulating the exposure of eggs to light it became clear that the junior phenotype of the can indeed be triggered by their exposure to light while still in the egg. The authors also found corresponding physiological changes associated with different patterns of gene expression. Somehow light exposure at perhaps a critical time before hatching results in the production of chicks with different physiological and behavioural characteristics from the same clutch of eggs.

Ruiz-Raya F, Noguera JC, Velando A. 2022. Light received by embryos promotes postnatal junior phenotypes in a seabird. Behavioral Ecology doi.org/10.1093/beheco/arac079