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Want to measure the health of a bird population without having to catch birds and run tests on them? Just count the spots on their eggs, say Santiago Merino and colleagues at the University of Alcalá in Spain. The spottier the egg, the more stressed the parent, the team has found.
The team photographed 112 blue tit (Cyanistes caeruleus) egg clutches in central Spain, weighed the parents and ran blood tests. They found that females that laid eggs with more spots weighed less and had higher cellular concentrations of a stress-related protein called HSP70 than females that laid less spotty eggs (Journal of Avian Biology, vol 38, p 377).
The team suggests that the eggshell pigment protoporphyrin is responsible for the correlation. Levels of the pigment chemical rise as stress levels increase in the female, which would explain the greater coloration in the eggshell when it is laid. Egg speckling could be a cue for male blue tits to work harder to feed underweight chicks, says Merino. Alternatively, speckling may be a cue to abandon the nest and find a healthier female.
"This could be really useful, since almost all of the endangered Hawaiian birds we work with have spotted eggs," says Alan Lieberman at San Diego Zoo in California.
From issue 2604 of New Scientist magazine, 21 May 2007, page 22
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