Crèche Formation in the Common Eider

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Abstract
The formation of creches was studied in the common eider (Somateria mollissima) nesting at high density on Bicquette Island [Canada] in the St. Lawrence River estuary. Aspects of social behavior relevant to creche formation are examined in detail. Data from 293 individually tagged females aided in elucidating the breeding history and in understanding the parental behavior of previously recognized categories of adult females (''B-Brooding,'' ''A-Associate,'' ''V-Visiting,'' and ''N-Neutral''). Creches resulting from the encounter between 2 or more broods begin to form immediately after the nest exodus and last well into the rearing period (about 10 wk). The encounters themselves may be stimulated by alarm or predation or may sometimes result from the mutual attraction of ducklings. The rate of fixation of such encounters into permanent creches decreases from a high of 88% in broods reaching the shore of the nesting island to a low of 0-6% in encounters between groups meeting during the latter half of the rearing period (between the 4th and the 10th wk). Agonistic interactions among B-status females tending creches provoke a hitherto unexplained lowering of the ratio of females to ducklings in such groups. Instead of a loose system of cooperation between females rearing creches as was previously surmised, it was found that the organization of these groups is based upon a hierarchical ranking of females involved in tending. The creche is thus seen as an accidentally expanded family.