LAUSR

Choosing the right mate is crucial for successful breeding, particularly in monogamous species with long and extensive bi‐parental care, and when the breeding pair is presumed to last many seasons. We investigated the degree of assortative mating in the Little Auk Alle alle, a long‐lived seabird with long‐term pair bonds and bi‐parental care for fixed (morphological) and labile (physiological, behavioural) traits. Using randomization tests, we suggest assortative mating with respect to wing length, extent of the white area on the upper eyelid and hormonal stress response (the difference between stress‐induced and baseline corticosterone levels). We discuss how the assortative mating patterns that we found in the Little Auk may be adaptive.