June 20, 2013 Why are gull chicks murdered especially on Sundays? How does man influence the size of gull populations? These and many other questions are answered in the doctoral thesis of Kees Camphuysen from the Royal Netherlands Institute for Sea Research NIOZ. Camphuysen will defend his thesis at the University of Groningen on 21 June 2013.
Kees Camphuysen has been doing research on seabirds since 1973, and since 2006 his focus has been on seagulls in the Texel dunes. There, he focused special attention on the European herring gull and the lesser black-backed gull. Since the sixties, the number of herring gulls has increased enormously, then stabilized and subsequently strongly declined. The lesser black-backed gull, which established itself in the Netherlands around 1930, suddenly became much more numerous later and this species has finally eclipsed the herring gull in numbers. Has the lesser black-backed gull supplanted the herring gull by taking up the best spots, or did it win the competition for food? Or does the decline in herring gulls and the increase of lesser black-backed gulls maybe have nothing to do with each other?
From Camphuysen's research it appears that both gull species have benefited from an expanded food offering caused by people. Herring gulls have learned to tap food in landfills, while lesser black-backed gulls in particular were attracted by the fish waste that was put overboard at sea. Now that the majority of landfills are covered and the fishing fleets are shrinking, both gull species are finding it more difficult to find food. It seems that the increase and decrease of both species is not directly influenced by each other.
Camphuysen also discovered a remarkable rhythm in the growth of the chicks and also that much more cannibalism took place over the weekend than on weekdays (gull chicks that were pecked to death by adult gulls and sometimes eaten). It turned out that gulls, especially during chick care, rely heavily on fish waste thrown overboard from fishing boats. Bad luck for these birds: at the weekend, the fishing fleet is largely in the harbour. This weekly rhythm is a problem, especially in the second half of the chick care period (in July), when there is barely enough food to be found for the hungry chicks.
The fleet is expected to shrink even more in the coming years. The problem of food shortage will continue to increase as a result, but then not only at the weekend. European policy, wherein by-catch may no longer be put overboard, will clearly have consequences for both gull species. How they will react to this is difficult to predict. A strong inland increase of gulls, looking for alternative food sources, is one of the likely effects.
Camphuysen has also equipped gulls with a GPS data logger, so as to be able to see where they look for food. It emerged that lesser black-backed gulls foraged much further afield than herring gulls. Lesser black-backed gulls also went oftener and further onto the North Sea, following fishing boats. One lesser black-backed gull, which had three youngsters that were not growing well, took a desperate measure and flew via Hoorn to Amsterdam, in order to hang out in the Leidsestraat there. Who knows, did she eat chips there, or a kebab roll? She then flew to the North Sea in order to follow a fishing boat far out at sea. The next day, her young had grown properly again.
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