| Summary: | Among the North Atlantic marine birds, only one species fills the plankton-feeding niche (Plautus alle L., Alcidae) while in the North Pacific and adjacent seas, no less than five alcids occupy it. A comparison of the feeding and nesting habits was made in order to understand how the food resources and the nesting habitat were partitioned between three of these species, the Crested auklet (Aethia cristatella (Pallas)), the Least auklet (A. pusilia (Pallas)) and the Parakeet auklet (Cyclorrhynchus psittacula (Pallas)), The study was made on St. Lawrence Island, Alaska, during the summers of 1964- to 1966. The two congeneric species differ markedly in size (pusilla 90 g; cristatella. 300 g) the Crested and the Parakeet auklets are of equivalent body size. The three species have diurnal habits. The two Aethia are active on the nesting colonies during the morning and the evening and feed at sea in early afternoon and early morning. Cyclorrhynchus is present on the colonies in the morning and early afternoon only and spends the rest of the day at sea, feeding. Aethia cristatella and A. pusilla exhibit the same type of response to the food source : both have a diversified diet in May-July (caridean larvae, hyperiids, mysids, gammarids) and restrict themselves to one dominant prey item during the chick-rearing period (August-September). A. pusilla then concentrates its feeding on Calanus finmarchicus and A. cristatella on Thysanoessa spp. The two auklets have largely overlapping habits and share the same feeding grounds. They differ markedly in diet, but more so in the size of the prey organisms used and these differences can be accounted f or by differences in bill-size alone. Slightly over half of the diet of Cyclorrhynchus was made of carnivorous macroplankton (large hyperiids, fish, etc.). The Parakeet auklet differs from its possible competitor, the Crested auklet, by occupying a slightly higher trophic level and by devoting more of its time to feeding. The two species, however, are found on the same feeding grounds and are presumed to utilize the same feeding depth-range. Differences in diet between the two are provisionally attributed to differences in bill structure and bill shape. In Aethia the reversal to monophagy during the chick-rearing period seems to reflect a sudden increase in the availability of palanus and Thysanoegsa. This, in turn, is believed to have importance in determining the timing of the breeding season. No obvious factor is responsible for timing in Cyclorrhynchus. which breeds slightly later than either of the two species of Aethia. The Crested and the Least auklets seem to depend upon prey organisms that oscillate widely in abundance and availability. The Parakeet auklet depends to a greater extent upon organisms whose supply is more or less constant throughout the year. Segregation in nesting is complete between the two genera. Cyclorrhynchus is a cliff-nester while Aethia occupies talus slopes. In the latter habitat, the marked difference in body size between the two species is again responsible for segregation through the action of one principal factor, the average rock diameter on the slopes. The density of A. criptatella increases in early with increasing boulder size : the density of A. pusilia decreases both with decreasing boulder size and with the decreasing abundance of its large congener from a knowledge of the average size of the particles in the nesting habitat, one can predict accurately the relative abundance of the two species of Aethia. The position of the plankton-feeders in the community of diving marine birds is examined. Feeding adaptations (degree of tongue cornification, palatal breadth, number and arrangement of palatal papillae) follow a gradient or a regular modification throughout the family. On the basis of these variations, which can be expressed as a ratio (Bill-width / Gape), a model is constructed that gives a graphical representation of the breadth of the ecological field occupied by the family. Two distinct levels emerge : the fish-feeders (uria,. Alca) and the plankton-feeders (Aethia,. Plautps): an intermediate level can be distinguished (Fratercula. Lundq) The species of the latter group preserve adaptations that allow them to utilize plankton and fish-foods. The model allows us to recognize and define special adaptations with respect to other members of the family and to recognize the main trends in evolution of body size and feeding adaptations within this taxonomic group. In comparison with younger families or orders, most members of the Alcidae are discrete and well-defined ecologically. Among the three plankton feeders studied, the overlap in requirements is very small and no sign of competition for food or for nesting was found. Science, Faculty of Zoology, Department of Graduate
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