25.11.2015 change 25.11.2015

We know how gulls use ducks to get food

Photo by Dominik Marchowski Photo by Dominik Marchowski

Gulls have learned to use the presence of ducks to gain access to the mussels brought by ducks from the bottom of bodies of water. The results of observation of their behaviour, conducted on the Szczecin Lagoon, have been described in "The Auk: Ornithological Advances".

Gulls are poor divers and mussels are usually beyond their reach. Observations conducted in the waters of the Szczecin Lagoon suggest, however, that these birds know the strategies to gain valuable, normal inaccessible food. This confirms that gulls are highly adaptable group of birds, able to exploit a wide variety of food resources… and respond to new opportunities.

Ducks wintering on the Szczecin Lagoon search for food on the bottom, in the benthos zone. They are interested mainly in zebra mussels. When bringing clumps of mussels to the surface, they regularly lose their fragments.

To determine whether the gulls on the lagoon take advantage of this or if their presence among the foraging ducks is only a coincidence, Dominik Marchowski, Łukasz Jankowiak and Dariusz Wysocki of the University of Szczecin observed the behaviour of the birds for one year. They focused on three species of duck--the Common Pochard (Aythya ferina), Tufted Duck (A. fuligula), and Greater Scaup (A. marila). They recorded the intensity of the ducks foraging and checked whether any Herring Gulls (Larus argentatus) and Mew Gulls (Larus canus) were present in the flock.

They noticed that that the more intensively the ducks were foraging, the greater was the probability of gulls presence in their flock.

Biologists also analysed gull pellets - lumps of undigested food (in the case of gulls they contain the remains of plants, invertebrates and small vertebrates) - to determine what gulls actually eat. Analysis of gull pellets allowed to confirm that when there were numerous wintering ducks on the lagoon, gulls switched from fish to a new type of food: mussels.

When feeding with ducks, gulls use two strategies: they pick up mussel fragments that the ducks have lost, but also steal fragments from the ducks directly. The first of these tactics, in which a species derives significant benefits, but does not harm others, is called commensalism, the second is called kleptoparasitism.

"Gulls were initially on the margins of our research. Initially, their interaction with the ducks seemed obvious, but after analysing the available literature, it turned out that little is known about it" - said the initiator of the research project, Dominik Marchowski. "We developed behavioural studies of birds and an analysis of pellets to confirm the scale of the phenomenon. It\'s worth watching the seemingly obvious behaviour of birds more closely, because they can hide interesting interactions" - he emphasised.

"This investigation provides rare and convincing evidence that demonstrates how interspecific feeding interactions between Larus gulls and diving ducks influence community structure in the vicinity of the Baltic Sea" - commented Dr. Timothy White of National Oceanic and Atmospheric Administration (NOAA), an expert on sea duck foraging. "It (…)broadens our understanding of community patterns, seabird feeding ecology and social interactions".

Video from observations can be viewed here.

PAP - Science and Scholarship in Poland, Anna Ślązak

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