Are Hazel Grouse Bonasa bonasia monogamous?

The apparently monogamous social organization of Hazel Grouse Bonasa bonasia previously had been inferred only from observations of unmarked birds. Using radio‐marked Hazel Grouse, we confirmed that the social organization during the breeding season was indeed monogamous, the pair being the social unit. The pair bond, however, appeared relatively weak, as birds were together only 37% of the time during the prelaying period. We could not document mating behaviour, but observations suggested that most males mated monogamously and that at least 37% of the females had an opportunity to mate polyandrously because of (1) the seemingly weak pair bond, (2) attentive neighbouring males, (3) larger territory size of unpaired males, (4) extensive movements of some unpaired males, (5) sharing of a female by two males and (6) a potentially male‐biased sex ratio.