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Gull Predation and Crèching Behaviour in the Common Eider

J. Munro and J. Bédard
Journal of Animal Ecology
Vol. 46, No. 3 (Oct., 1977), pp. 799-810
DOI: 10.2307/3641
Stable URL: http://www.jstor.org/stable/3641
Page Count: 12
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Gull Predation and Crèching Behaviour in the Common Eider
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Abstract

(1) The nest exodus of 3399 Somateria mollissima ducklings was followed on densely populated Bicquette and Fraises islands in the St Lawrence River estuary in 1973 and 1974. (2) In 1973, 2485 ducklings were attacked a total of 906 times by Larus argentatus and L. marinus between their arrival on the shore and their departure from the nesting island with the subsequent loss of 335 birds (overall predation success 37%). A thirty-four-fold decrease in predation was noted in 1974 when 974 ducklings suffered only twenty-seven attacks which led to the loss of eight young. (3) Predation acted as a proximate factor leading to amalgamation of several broods and/or crèches. Groups of eiders coming within close range of each other would amalgamate twelve times more often when their encounter coincided with predation than when it did not. (4) Predation was usually by gulls acting singly or in loose pairs, but a system of group predation involving from five to forty gulls was observed on sixteen occasions in 1973. Group predation always led to the destruction of the entire crèche attacked and increased in frequency as the hatching season progressed. (5) Larger crèches suffered more attacks than small ones but offered much higher duckling survival at least in the face of attack by lone gulls. (6) The advantage of crèching accrued through decreasing the risk to the duckling `hidden' into the group rather than through enhancing the detection of aerial predators. (7) Crèching remains advantageous even when the overall results of the two types of predation are combined. However, when group predation only is considered, crèching is clearly disastrous, for large crèches are much more vulnerable than small ones.

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