Because of its homogeneity, both taxonomical and ecological, the sea-bird family Alcidae constitutes an appropriate group for the study of adaptive radiation. This radiation involves mainly the acquisition of specialized feeding habits and the consequent specialization of the various species of the family at different trophic levels.
The plankton-feeders, exemplified by the Least Auklet Aethia pusilla, have a relatively wide beak with a fleshy tongue and a broad palate with numerous denticles. The fish-feeders, exemplified by the Razorbill Alca torda, have a narrow bill, a certain degree of tongue cornification and few, but sharp and regularly arranged palatal denticles. A few species, including the puffins and one auklet (Cyclorrhynchus), have characteristics intermediate between these extremes and feed partly on fish and partly on plankton.
Body-size in the predator is related to the size of the prey and these relations are examined within the family. Within a group with similar adaptations, species which are alone in their niche in their area tend to be of intermediate body size for that group. Finally, though the family may exhibit a series of parallel trends in bill shape when the bill is considered as a social releaser, it exhibits a gradation in shape and structure (adaptive radiation) when considered as a food-getting tool.