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Dumping eggs on the neighbours

Dumping eggs on the neighbours

A huge study of over 69,000 nests around the west coast of Scotland has revealed for the first time the full extent of egg dumping by seabirds. The findings show that cuckoos aren’t the only birds to lay eggs in other birds’ nests, and may give an insight into how so-called brood parasitism evolved.

While the cuckoo is famous for laying its eggs in other birds’ nests, this habit is rare and poorly understood among seabirds.

Dr Clive Craik from the Scottish Marine Institute discovered that 13 different seabird species lay their eggs in other birds’ nests, which scientists call ‘egg dumping’. The birds that dump most frequently are the oystercatcher and the common eider duck.

‘Both oystercatchers and eider ducks dump their eggs in gull nests, possibly because gulls are the most common species. But they are unlikely to successfully rear the alien chick. There may be a slim chance that some unusual species combinations might work,’ says Craik.

’An eider duckling is more likely to be seen as food than family if it hatches in a herring gull nest, but there is a very small possibility that it could run through the undergrowth undetected and find a nearby crèche of eider ducklings,’ he adds. 

Over the course of his 14-year study from 1996 to 2009, Craik found 123 nests with more than one species of seabird egg in it, with 13 different species of seabird being parasitized by an alien seabird laying in their nest.

Over half of the cases of egg dumping were between only three pairs of species: common gull and oystercatchers (the victim was the common gull), herring gull and eider duck (victim the herring gull) and common gull and black-headed gull (the victim, once again, the common gull). Though the frequency of egg dumping is low (of the 69,775 clutches examined, only one in 500 nests had an alien egg), the common gull seemed particularly susceptible. 

Dr Craik commented: “Some seabird eggs look very similar to the untrained eye, some differing only in size. But it seems unlikely an oystercatcher would mistake a common gull nest for its own. Egg dumping of this sort may be a primitive version of what the cuckoo does – a crude attempt by one bird to make others raise its young.”

The study was published in the April 2011 edition of Seabird.

lutches at seabird colonies in west Scotland 1996-2009” in Seabird 23 (2010) 41-52

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