Interactive Behaviour of Puffins (Fratercula Arctica L.) and Skuas (Stercorari Us Parasiticus L.)

In: Behaviour
P.R. GrantBiology Department, McGill University, P.Q., Canada

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Puffins fly to their burrows with food (fish) in their beaks for the single chick they raise. Skuas chase puffins and induce them to drop their food. If the skuas do not catch the falling food before it reaches the ground gulls drive the skuas away from the food and secure it for themselves. These interactions were studied quantitatively for two weeks at a breeding colony of puffins at Vik on the mainland of Iceland. The only conspicuous response of the puffins to the skuas was one of avoidance at the end of the breeding season when the frequency of arrival of breeding puffins at the colony was low; the puffins turned around in flight and flew back towards the sea. Some of the characteristics of chases which ended with food being released by the puffins were identified. Chases involving more than one skua tended to be more successful (i.e. food released) than those with one skua. Chases of puffins away from the colony were more successful than those towards the colony. Chases of incoming puffins were more successful when initiated far from the colony than when initiated close to it. Low flying puffins appeared to be chased preferentially. From these observations some idea can be gained of the way in which selection acts upon the puffins and skuas. For the skua there appear to be optimal positions from which to start chasing a food-carrying puffin, and selection acts against those skuas which choose incorrect positions and gain no reward after expending much energy in a chase. For the puffin there is a selective advantage when returning to the colony with food for the chick in flocking with non-breeding birds without food. This stratagem maximizes the difficulties of the skua detecting a food-carrying puffin early enough to start a chase with a chance of success. Late breeding by the puffin is selected against because, when the other parents have left the colony, it is comparatively easy for a skua to identify and successfully attack a food-carrying puffin. The difficulties of avoiding a chase by a skua under these conditions are reduced if the puffin delays its final flight to the colony until the skuas are either absent or 'out of position'. Selection acts against those puffins which do not take this avoiding action.

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