While at the beach of Katwijk aan Zee on Sunday, I observed gull behavior that I had never seen before nor was aware of: Black-headed Gulls Chroicocephalus ridibundus stealing food from Sanderlings Calidris alba by chasing and harassing them — a process known as kleptoparasitism (see Wiki).
Hundreds of Sanderlings were fouraging in small groups at various places along the water edge. Among them, several Black-headed Gulls of all ages had gathered and were mostly fouraging themselves, minding their own business.
At various moments though, Black-headed Gulls could be seen chasing a Sanderling as soon as they spotted it with food.
Most of the time the chase was done by a single gull but at one point I saw one Sanderling being chased by 3 individuals.
Even though the Sanderlings were much more maneuverable because of their smaller size and tried to out-turn the much larger Black-headed Gulls, the gulls managed to keep up with them in most instances.
Sometimes the chase was successful, sometimes it wasn’t.
Some chases only lasted a brief moment while some Sanderlings were chased relentlessly, not prepared to give up their food.
It must have been quite a harrowing experience for such a small bird (weighing around 50 grams) to be bullied by such a large gull.
How common is cleptoparasistism in Black-headed Gulls?
The only example of cleptoparasitic behavior in Black-headed Gulls that I was aware of was towards Lapwings Vanellus vanellus and Golden Plovers Pluvialis apricaria after having read Gulls and Plovers — The ecology and behaviour of mixed-species feeding groups (1975) some time ago.
The behavior towards terns is described in “Kleptoparasitism of Sandwich Terns Sterna sandvicensis by Black-headed Gulls Larus ridibundus” (1977) and also in Living with gulls: the consequences for Sandwich Terns of breeding in association with Black-headed gulls (2001)
Birds of the Western Palearctic (BWPi, interactive version) states that Black-headed Gulls are “food-pirates and scavengers’ but surprisingly does not mention cleptoparasitism as part of the feeding behavior for this species of gull.
Why invest in this behavior?
While observing the gulls chasing the Sanderlings I was wondering how investing so much energy in chasing a small bird carrying such a small amount of food can be beneficial over finding the food themselves? To me it looked like there wasn’t a shortage of food around: food actually seemed to be in abundance on the beach.
I couldn’t see a clear need therefore to resort to this type of behavior.
Having said that, the birds that were feeding on the beach (the Sanderlings as well as various species of gulls) seemed to prefer specific areas instead of spreading out evenly over the available space. What was noticeable with the gulls especially was that each time they were disturbed by beach goers the gulls retreated onto the water some 20 meters and more from the beach, waiting for the moment when it would be safe again to return to the exact same spot.
That certainly limited the opportunities of obtaining food.
Competition between gull species and differences in hierarchy within a species (adults versus non-adults and males versus females) may also play a part in food availability.
I found it very interesting to observe this behavior therefore and will certainly make an effort to study it in more detail when I come across it again.