Dexterous
herring gulls learn new tricks to adapt their feeding habits
University of
Southampton
Observations of
herring gulls have shown how the coastal birds have developed complicated
behavior to 'skin' sea creatures to make them safe to eat. Researchers think
this feeding habit may be a response to urbanization and changes in food
availability.
The gulls (Larus
argentatus) of Dún Laoghaire Marina at Dublin in Ireland have found a novel
way of disposing of the tightly fixed outer layer of sea squirts -- an organism
which they have learnt to pluck from the underside of pontoons by diving or
floating on the water.
Sea squirts, or
ascidians, are filter feeding creatures which have an outer layer, known as a
'tunic', designed to give protection from predators. This tunic is unpalatable
to gulls, however, those living around Dún Laoghaire have overcome this using a
particularly clever method.
The birds grasp the
sea squirt's tunic in their beaks, shake it to loosen the soft edible inner
body, then switch to hold the inner body (again with their beaks) and shake
again until the inedible outer layer is removed.
The team's report, published in the journal Ecosphere, is the first recorded observation of Herring Gulls feeding using this unusual behaviour.
Project leader Luke
Holman, a postgraduate researcher in the School of Ocean and Earth Science at
the University of Southampton, comments: "As our coasts have been
urbanised over the centuries, gull species have become opportunistic when
looking for food. We have all seen them raiding rubbish bins in seaside towns
and even swooping to take food straight from people's hands. But our study
shows how the gulls of Dún Laoghaire are adapting in a different way --
exploiting a new food source by developing a novel, sophisticated feeding
strategy.
"Their dexterity
and handling ability is unprecedented and they are able to separate the two
halves of the organism either while on land or surface swimming. Their complex
technique reinforces the general view that gulls are highly versatile predators
-- skilled at handling objects and foraging."
Sea squirts are very successful fouling organisms that cling to the damp surfaces of coastal structures, such as pontoons, and often become invasive when artificially introduced to new areas.
Increased development of coastal areas provides them with ideal habitats for proliferating. In addition, shipping and recreational boating has encouraged their spread. This has led to them emerging as a potential new food source along increasingly urbanised coastlines.
The researchers are
now keen to know if the behaviour of the gulls in Dún Laoghaire is happening in
isolation or whether it represents a wider feeding pattern. The Herring Gull is
on the Red List of the British Trust for Ornithology (BTO) 'Birds of
Conservation Concern 4' because of severe, long-term population decline in the
UK and as such it could benefit from new ways of feeding.
This study was led by
the University of Southampton in collaboration with University College London
and the Institute of Zoology (Zoological Society of London).
