Crèche Formation in the Common Eider

Abstract

The formation of crèches 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 crèche 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'). Crèches 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 crèches 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 weeks). Agonistic interactions among B-status females tending crèches 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 crèches as was previously surmised, we found rather that the organization of these groups is based upon a hierarchical ranking of females involved in tending. The crèche is thus seen as an accidentally expanded family.