010404-19 10.1038/nsu010404-19 6 April 2001 ecology & evolution A lighter shade of egg JohnWhitfield Laying a pale final egg in a clutch might warn off potential freeloaders.

A herring gull: her pale eggs may keep cheats in check
You'd think that only a mother bird would be able to tell one egg in a clutch from another. But in some species, including sparrows and herring gulls, the last egg laid sticks out like a sore thumb -- it is paler than the others. This pallid egg, new research suggests, could tell other birds: 'Don't bother dumping any eggs in here, I've begun incubating, and yours won't have time to hatch.'

Not only cuckoos attempt 'brood parasitism'. Many female birds -- perhaps those who haven't been able to find a nest site of their own -- try to lay eggs in the nests of others of their own species, tricking them into raising their offspring.

Now Mark Broom, a mathematician at the University of Sussex, Brighton, and colleagues have used mathematical game theory -- a way of working out the best strategy for a given situation -- to test the hypothesis that a final pale egg could be an attempt to deter this sneaky tactic. "It's plausible [but] didn't have any theoretical underpinning," says Broom1.

Broom's group simulated a population of birds that used various strategies. The population included those that lay pale eggs last, cheats -- which lay them first -- and those that do not lay pale eggs. Parasites either searched until they found a nest with no pale eggs, or dumped an egg in the first nest they came to.

Laying a pale egg last can be an evolutionary stable strategy, the mathematics reveals. That is, honest layers, and parasites that pay attention to egg colour, can produce more young than cheats who try to trick parasites, or parasites that ignore pale eggs. Under certain conditions.

These conditions are that pale eggs are more costly to produce than normal-coloured ones, and more costly to lay early in a clutch than late. Both conditions are feasible: abnormally coloured eggs may be poorly camouflaged, or prone to overheating or chilling. And early eggs are usually more valuable to their parents, as the chicks that hatch from them are more likely to survive to adulthood.

If pale eggs are very costly, or parasites very rare, it is less effort to risk incubating a foreign egg. If they are too cheap, cheating spreads, and "as soon as everyone starts cheating, it's not worth responding to the signal", says Broom.

Now Broom hopes there will be some experimental work to test whether potential brood parasites do really avoid nests with pale eggs, and to measure the costs of pale eggs.

Marion Petrie, a behavioural ecologist at the University of Newcastle-upon-Tyne, says that, as yet, there is no evidence that birds actually use egg colour as a signal in this way. She believes that the high cost of pigment is a more likely explanation for pale eggs -- by the end of the clutch, female birds may have simply run out.

"If a brood parasite really wanted to know whether a bird has begun incubating there are other ways of telling, such as the warmth of the eggs," she says.

Ruxton, G. D., Broom, M. & Colegrave, N. Are unusually colored eggs a signal to potential conspecific brood parasites? American Naturalist 157, 451458 2001.