Have you ever seen a seagull dance?
Seagulls, or gulls, to use the proper terminology, can sometimes be seen stamping or tapping their feet in turn on wet grass in a series of moves that look a little like dancing.
But why do they do this? To find out, we must understand something about the behaviour of worms.
During heavy rain, earthworms instinctively rise from below the ground to the surface. You can often see them after a downpour, sometimes stranded on pavements or roads.
For years, it was assumed that they did this so as not to drown in their underground burrows. However, biologists now know that’s not true, because worms don’t breathe like humans. They don’t have lungs, and instead breathe through their skin, which is covered in mucus that helps them absorb oxygen and release carbon dioxide. To do this efficiently, the mucus needs to stay moist. If it gets too dry, the worm can’t breathe in any oxygen and it will suffocate which is why they rarely come to the ground except in wet weather.
But why, when they have a moist environment underground, do worms come to the surface at all and run the risk of drying out?
There have been a few suggestions put forward by scientists to explain the behaviour. It used to be thought that they came to the surface to mate, but it is now known that only a few of the 7,000 species of earthworm mate on the soil surface.
Another explanation is that the drumming of the rain on the surface of the earth is similar to the vibrations made by burrowing predators, such as moles. When they feel vibrations below the ground the worms move upwards away from the perceived threat. But this idea has in the main been dismissed. No experiments have proved this to be the case, and we know from observation that worms rise to the surface even in very light drizzle, when any vibrations would be imperceptible.
More recently, another theory has been proposed; that the worms are migrating to find fresh territories. Moving around in soil is slow and takes a lot of energy, even for earthworms. Travelling over ground is easier and much longer distances can be covered but they can only do this when it rains. Although there is a risk of them drying out or getting eaten by birds and other predators, as with many behaviours driven by evolution, there is a pay-off, as the worms that do make it have a better chance of survival in their new home.
The art of worm-charming, also known as worm-grunting or worm-fiddling, has been practised by humans for centuries, usually by fishermen to collect bait. They use various methods to vibrate the soil from simply tapping the earth to rubbing a strip of metal across the top of a wooden stake driven into the ground. Worm-charming is also a competitive sport in the UK as well as in the United States and Canada.
And when gulls stamp their feet on the ground they are also worm-charming. The gulls produce vibrations that mimic the sound of raindrops which entices worms to the surface, providing the birds with a convenient snack. During the dance you will often see them pause and move their heads from side to side as they scrutinize the ground in search of their next meal.
Gulls are not the only animals to practise worm-charming. The behaviour has also been observed in woodcocks, ducks, thrushes, blackbirds, and robins, as well as wood turtles.
Nikolaas Tinbergen, a Dutch ornithologist, who won the 1973 Nobel Prize in Physiology or Medicine along with Karl von Frisch and Konrad Lorenz for their “discoveries concerning organization and elicitation of individual and social behaviour patterns”, put forward the idea that the dance is learned behaviour with older gulls teaching younger gulls the movements.
Environmental factors also influence the behaviour as it is more often seen in birds that have to supplement their diet or who have moved inland from the coast.
2 Responses
Seagulls in Exeter, England also worm-charm. It seems to summon worms up for dinner.
I live in Tweed Heads Australia and have observed seagulls stamping their feet on sand in shallow water to bring small crustaceans to the surface to eat.