Foraging behaviour of gulls

Monika Reiss, PhD student at Liverpool John Moores University, who we interviewed recently, has produced the following blog for us about the foraging behaviour of gulls in general, and Liverpool's gulls in particular.

Juvenile herring gulls on the Mersey, April 2020. Photo credit: Debra Williams

 

Diet

Gulls are omnivorous and mostly opportunistic seabirds. Their natural foraging material found in the water can be any kind of marine fish, invertebrates, shrimps, cockles, crabs, crayfish, clams and mussels. Naturally found food in the terrestrial environment may include worms, insects and other birds, their offspring and eggs. It is known that herring gulls are cannibals and they often prey on the adults of their own species and their young. The biggest of all gulls – the great black-backed gull – can even eat common terns, Manx shearwaters and other big gulls. The common and black-headed gulls additionally eat seeds and grains. Their non-natural or anthropogenic food includes fast food like chips, fried chicken, burgers, human refuse gathered from rubbish bins or refuse tips, and discarded fish from fish-processing plants. Their nutritional intake and needs may vary between the seasons; for example, during the breeding period (April-August), the female needs food rich in calcium and protein as she is laying eggs. After the eggs hatch, the parents need to provide a sufficient amount of calories, not only for themselves, but also for their chicks. Gulls’ nutritional needs decrease during the winter when their chicks have fledged.

How do they get food?

Gulls can obtain food in the air, water and on the ground; thanks to many different techniques for sourcing their prey, they can meet their foraging needs. Most of those birds forage on the tidal flats during the low tide; exposed marine fauna is vulnerable and very easy to catch. Gulls can catch their prey by simply walking on the mud flats and picking up small fish or invertebrates. If they find something more difficult to get into, like, for example, shellfish, they pick it up in their beaks and drop it from height onto the rocks, until the shell cracks. Something similar may happen with small mammals or other birds’ chicks: they can either drop them or drown them in the water. Gulls very often fly near shores where they can effectively spot any available food and pick it up. When they find food, they often perform a long, loud call to let other birds know about this potential food source. This behaviour works along with the ‘tit for tat’ rule, and most likely, in the future, this favour will be returned. While swimming – or, rather, ‘loafing’ on the water – birds can passively search for food as well. When they notice a movement under the surface, they can shallowly dive to catch the prey. The common gull has an amazing ability to hover above the surface of the water, with its head down, and can catch fish without diving! Gulls also like to gather around freshly ploughed fields where they can forage on worms and insects. Big groups of gulls are often seen flying behind fishing boats, where they can often steal fish from the fishing nets. Another act of theft performed by urban gulls is stealing human discards from refuse dumps and small rubbish bins; bolder gulls even try to snatch food from people’s hands. But theft is not related to only anthropogenic sources of food – gulls can steal a meal off other birds, too, and sometimes this can cause an air fight.

Where do they get food from?

Gulls are seabirds so their natural food is usually found in waterways like the River Mersey or the Irish Sea, but sometimes they also use small ponds or lakes, for example, lakes in Sefton Park or Princes Park. They also enjoy the beaches at Formby and Crosby. Gulls prefer open spaces which are not full of trees, and they actively avoid dense forests; it is easier for them to spot a predator in an open space. Therefore, they also forage around Stanley Park in Anfield or Wavertree Playground. Urban gulls of Liverpool, especially during the summer months, take advantage of tourists willing to feed them along the docks and also in the city centre. Gulls very often sit on the rooftops, outside restaurants or fish and chip shops, and watch for any refuse or wait to steal the food out of customers’ hands. Another source of human discards is the ‘rubbish tip’ or recycling centre, for example, Otterspool and Old Swan Waste Recycling Centres. Birds can easily access food discards and can very often spend i several hours in these places. As mentioned before, gulls prey on different gulls’ chicks and eggs; this can happen during the breeding season on the rooftops of buildings (shopping centres, university buildings, houses) if they are urban gulls, or in natural colonies like the one in the salt marshes in Southport.

Given that we are now reducing the amount of food waste that is sent to landfill, and that there have been many fewer people and thus food refuse around our city centres recently due to the Covid-19 lockdown, it will be interesting to continue to monitor our urban gulls, to see how they react to this reduction in anthropogenic food sources.

Edited by Debra Williams