By Paul Rincon
BBC News Online science staff
Drops in the number of fish thrown back by trawlermen are forcing a scavenging seabird to prey on fellow birds, scientists say.
The great skuas threaten other bird populations
The great skua is known to feed extensively on discards from commercial fisheries.
But a decline in this dietary source appears to have sent the animal after other seabird communities - such as puffins, guillemots and black-legged kittiwakes - to maintain its food intake.
The international team of researchers have published their findings in the journal Nature.
In addition to discards, the great skua (Stercorarius skua) feeds on small fish such as the sandeel.
The bird is a top predator in its habitat and has experienced rapid population growth in the last century, partly due to conservation efforts.
Researchers led by Dr Stephen Votier of the University of Glasgow used a statistical method to show that declining discards and reduced availability of sandeels have resulted in increased predation of other birds by great skuas.
The birds can behave as scavenger or predator
Declines in the availability of discards have coincided with long-term declines in young sandeels. This has exacerbated the tendency of great skuas to switch from scavenging on discards to preying on other seabirds.
Scientists collected data on the great skuas' diet by examining their pellets.
"Some of these pellets are just a bunch of feathers; it's very difficult to separate out species and ages. What we've gone for in this study is broad quantities and predation rates," Dr Votier told BBC News Online.
The authors of the report claim this could pose a serious threat to many seabird communities.
Other seabirds thought to be most at risk from predation by great skuas include Leach's storm petrel, the European storm-petrel and the northern fulmar.
Dr Euan Dunn of RSPB Scotland said the species most at risk was the black-legged kittiwake.
"This is partly because the species is easily predated by great skua on the cliffs but also because the kittiwake's own breeding productivity is suffering badly," he explained.
Puffins are one of the birds under threat
Ironically, this is being caused by the decline in sandeels, which are the mainstay of the kittiwake's diet. Unlike the great skua, the kittiwake cannot switch from eating sandeels to preying on other birds.
Declining North Sea fish stocks and the closure of fisheries could be partly to blame for low levels of discard.
Changes in fishing policy, in fish population structure and even the types of nets used can either mitigate or exacerbate the problem.