Crèche Formation in the Common Eider

-The formation of creches 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 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 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 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 weeks). 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, we found rather 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. Departement de Biologie, Faculte des Sciences et de Genie, Cite Universitaire, Universite Laval, Quebec, Que. , Canada GIK 7P4. Accepted 17 May 1976. A NUMBER of Anatinae display creching behavior (see Gorman and Milne 1972 for a recent review). Creche was defined elsewhere as the grouping of any number of parentally unrelated adult female(s) and young (Munro 1975). In the Common Eider (Somateria mollissima) of the St. Lawrence estuary, amalgamation of broods into creches sometimes resulted from accidental encounters between two or more broods and sometimes represented a spontaneous tendency to regroup in the face of larid predation. In the latter case, the behavior conferred a distinct survival advantage to the young in most cases (Munro 1975). Once formed, a creche was very stable from the time of settling in the coastal rearing areas at about 1 week of age, to fledging, at about 10 weeks of age (Munro 1975). Even a creche consisting of up to 35 ducklings and 2 females behaved as a cohesive family unit. In the present paper, we seek to understand the social and ecological conditions that prevail at and shortly after hatching and lead, within a few days, to the fixation of familial bonds and thus to numerically aberrant family units (creches). To this end, we describe in fairly minute detail various aspects of the behavior of the various categories of birds involved in creching. We also discuss the influence of duckling age on creche formation and the proportion of encounters resulting in permanent amalgamation. STUDY AREA AND METHODS About 20,000 pairs of Common Eiders breed in the St. Lawrence estuary, 92% being found on 6 nesting islands (see Reed 1973 and Munro 1975 for details). A discrete peak of hatching occurs between 10 and 20 June and the majority of broods and/or creches are rapidly led by adult females to mainland littoral areas for the duration of the rearing period. The length of the sea crossings involved varies between 3 and 14 km. Most ducklings are reared along the southern shore of the river between Kamouraska and Matane, with smaller numbers remaining around the largest islands or moving to the northern shore between the Saguenay River and Forestville. Over 1,400 breeding females were marked with patagial tags during 1972 and 1973, and a few entire broods andlor creches were marked as they were leaving nesting islands. The ducklings were marked with 759 The Auk 94: 759-771. October 1977 This content downloaded from 157.55. 39.128 on Mon, 18 Jul 2016 06:11:48 UTC All use subject to http://about.jstor. org/terms 760 MUNRO AND J. BtDARD [Auk, Vol. 94 colored nape tags, which did not disrupt brood unity but did cause a significant increase in predation. Therefore, the tagging of broods was discontinued after marking 140 young in 24 different groups. Four discrete categories of female behavior were recognized. (1) Broody (B). Assumes the leadership of the brood/creche. She does all or most of the vocalizations and surveillance and swims in a central or fore position in the group; does not abandon ducklings except under most extreme disturbance. Two, or rarely three, B-status birds may accompany a creche on a permanent basis with little or no aggression between them. (2) Associate (A). Assumes a subordinate role to the B-female. Swims in a peripheral or rear position in a creche. Readily leaves the ducklings in the case of moderate disturbance. Generally associates with the same creche over several consecutive days. (3) Visiting (V). Has a very low and transitory broodiness. Such females are temporarily attracted to a nearby brood or creche. Show ambiguous (care-giving and agonistic) behavior towards ducklings. May swim alongside a creche for variable lengths of time but leave at the slightest disturbance. May visit a number of different creches in the same day. (4) Neutral (N). Displays no broodiness whatsoever and is not attracted by a nearby creche or brood. Some of the V-females may probably assume an N-status at certain times and vice versa. Generally 5 min of continuous observation are sufficient to classify a female. Further explanations of these categories are found in