Amazingly Adaptable Seagulls!

It is low tide on the Clyde coast, a group of bright - eyed inquisitive herring gulls are searching under every stone and in every crack on the rocky shore. Suddenly a cry of triumph, a quick thrust of a beak and the gull holds aloft a large sea-urchin. After manipulating it with its beak it decides to fly high, then drops it onto a road below and feasts on the revealed contents.

Larger gulls such as herring gulls, greater and lesser black-backed gulls are highly inquisitive and adaptive. They can exploit different habitats and many gull species are equally at home feeding on worms in fields as on rocky and sandy shores looking for food when the tide is out. Intertidal habitats have a wide range of different potential prey items including fish, sea urchins, starfish, molluscs, periwinkles, crabs, marine worms and dead, washed-up creatures.

Having such a wide range of different food items means that gulls have to develop different techniques to capture and eat them – including the use of roads and piers to break hard items of food to reach the edible contents.

Smaller gulls such as common gulls and black headed gulls are typically more specialist feeders feeding on insects and smaller marine organisms. You may see black headed gulls in shallow water spinning around as they look intently in the water beneath them. They are looking for small creatures to eat that are disturbed by the water currents created by their ‘paddling’ feet.

The adaptability of gulls as a whole means that they will exploit any potential food source –including artificial ones created by mankind such as rubbish tips or even chips dropped on an esplanade. They just represent easily exploitable food sources in the battle for survival, but birds engaging in this behaviour can become a nuisance from a human perspective. Our general perception is that there are increasing numbers of gulls, but the reality is that there has been a decline in several gull populations over the last few decades with the reduction in the size of the fishing fleet on the Clyde.

In addition to food supply, a wide range of other factors influence seabird populations including diseases such as botulism. The reported cases of dead gulls seen around ground-nesting colonies on Little Cumbrae this year are potentially due to Avian botulism. This is a paralytic disease caused by ingestion of a toxin produced by the bacterium Clostridium botulinum.

This bacteria is naturally widespread in soil and requires warm temperatures, a protein source (such as dead organisms) and an anaerobic (no oxygen) environment in order to become active and produce toxin. This acts by blocking nerve function and can lead to respiratory and musculoskeletal paralysis. Birds either ingest the toxin directly or may eat organisms containing the toxin.

Affected gulls will be unable to fly or walk properly (due to wing and leg paralysis), be very weak, and appear to have difficulty breathing. Birds which have been exposed to low toxin levels may recover, higher levels of exposure may result in death. It can be distressing seeing a colony affected by botulism – there can be a mix of healthy nesting birds, ill birds and dead birds in various states of decay.

A cycle develops in a botulism outbreak when fly larvae (maggots), feed on the carcasses and ingest toxin and are then be eaten by other birds. Outbreaks decrease with the dispersal of birds after nesting and with the onset of colder weather. Such diseases are natural occurrences and regulate wild populations.

If you would like to learn more about the wonderful wildlife on the Clyde why not join a local Club or society; such as one of the local branches of the RSPB, Scottish Wildlife Trust or Scottish Ornithological Society who will welcome new members. The Field Station at Millport also runs a variety of different wildlife courses through the year. (e-mail: enquiries.sco@field-studies-council.org).