Crèche Formation in the Common Eider

Abstract

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