Giant seagulls are moving in to Welsh towns – and feeding on the eggs and chicks of nuisance gulls.
Great black-backed gulls – up to twice the size of the gulls we normally see in towns – are the top predators at sea colonies in the wild.
Now some are breeding on urban roofs, surrounded by the ready meals in smaller gulls’ nests.
But the newcomers are gentle giants, as far as human residents are concerned. Unlike herring-gulls and lesser black-backed gulls, they don’t harass or dive-bomb people, even if they have chicks or nests nearby.
And they could be a more effective way of reducing the smaller gulls’ breeding success than devices like plastic owls or loudspeakers playing recordings of distressed gulls.
But Europe’s top expert on urban gulls said there aren’t enough of the big gulls to make inroads into the numbers of smaller gulls, at least not in his lifetime.
“This is a recent phenomenon,” said Peter Rock, who has studied gull populations in towns and cities across Britain.
“Roofs are safe places for them to nest because there are no predators. You can’t see them from the ground. They will munch eggs and chicks of the other two species of gull.”
The great black-backed gull (larus marinus) is the largest gull in the world. It is 64-79 cm long with a wingspan that can reach 1.7 metres.
It lives quite a long time for a bird, with the maximum recorded age being 27.1 years. Like most gulls, it is an opportunistic feeder, eating almost anything it can swallow.
It gets most of its food scavenging.
The big gulls’ diet was highlighted this week in a report for Cardiff councillors, who are considering what can be done about the city’s growing population of aggressive urban gulls.
The first pair of great black-backed gulls to breed in Cardiff was spotted in 2006. By 2011, three pairs were known to be breeding in the south of the city.
Another four pairs are breeding further up the Severn in Gloucester, and a pair has nested in Bridgend since about 2003.
Urban gulls treat towns and cities along the Severn Estuary as their home patch. Cardiff gulls will sometimes fly to Gloucester and back in a day for food, and vice versa.
Cardiff has the biggest population in the area, which had already passed 3,300 breeding pairs of gulls by 2011.
This may rise to 5,000 by 2020 – to the dismay of householders fed up of being attacked by defensive gulls in the breeding season or woken up by frenzied flocks tearing apart rubbish bags.
Mr Rock advised people and authorities to take no action against great black-backed gulls, since they were not aggressive towards people and ate other gulls’ eggs and chicks.
He said great black-backed gulls raised on rooftops would reach maturity in four years, and may well see townscapes as ideal breeding areas.
Smaller gulls are a problem around the Welsh coast. In resorts such as Llandudno, some tourists think it’s fun to feed chips to gulls. This has led herring gulls to expect food, which they will snatch from people’s hands.
In 2011 a Gwynedd councillor said he feared gull attacks might eventually kill someone, after a pensioner broke his hip while trying to avoid a dive-bombing gull in a Caernarfon car park.
In 2002 a pensioner in Benllech, Anglesey, died of a suspected heart attack after gulls forced him to retreat down a ladder as he tried to clear bird droppings from his roof.