Birds more likely to reject costly cuckoo eggs

From ABC News in Science, 8 July 2015:

The greater the cost of a raising a cuckoo chick, the more likely a host bird is to kick cuckoo eggs out of the nest before they hatch, Australian researchers have found.

Brood parasitism — the practice of laying eggs in another species’ nest and leaving them to be raised by the unsuspecting parents — is common around the world, and is practised by species such as cuckoos and cowbirds.

Some bird host species are more likely than others to spot and reject a parasitic egg — either by pushing it out of the nest or abandoning the nest entirely — but until now it was not clear why this was the case.

In a study published today in Biology Letters, the researchers analysed data from previous egg rejection experiments in 198 species of birds “to understand the conditions under which egg rejection evolves.”

They report that egg rejection is much more common when the cuckoo chicks are significantly larger than the host’s chicks.

“If the difference in size is very big, it’s much more costly to take care of a cuckoo that is gigantic, then rejection will be more likely to evolve,” says lead author Iliana Medina, a PhD candidate at the Australian National University.

Cuckoo eggs are usually very small but the chicks develop and grow very quickly, so the luckless host is then stuck trying to feed and rear an enormous chick much larger than itself.

Cuckoo chicks also reduce their competition by pushing out any other eggs in the nest or even killing the other newborn chicks.

This more virulent parasitic behaviour is common in Australia and Europe, where the main parasitic species is the cuckoo, and it is also a predictor of whether the host species rejects the parasite egg. Read more.