MALE-MALE MOUNTINGS BY A SEXUALLY MONOMORPHIC BIRD - MISTAKEN IDENTITY OR FIGHTING TACTIC

Male-male mountings were commonly performed by sexually monomorphic Ra zorbills Alca torda in mating arenas where males aggressively competed over extra-pair copulations (EPCs) with visiting females. In two bree ding seasons, 417 male-male mountings were recorded. Forty-one percent of all non-pair mountings were of males, 66% of males performed male mountings, and 91% of males received mountings. Several lines of evide nce suggest that male mountings were a form of fighting over EPCs rath er than a case of mistaken identity: (1) In a multiple regression, the percentage of all mountings that were of males increased with the mal e-male aggression rate rather than with the male:female sex ratio; (2) the frequency of male mountings also increased with the aggression ra te, rather than with the frequency of EPC attempts of females; (3) a l arge percentage (41%) of male mountings occurred after the mounted mal e had been settled in the arena and had time to be recognized, and not only immediately upon arrival, when mistaken identity may have been e xpected; and (4) males made significantly more, rather than fewer, mal e mountings as their age increased, suggesting that lack of experience did not lead to mis-identifications. The number of male mountings per formed was positively correlated with EPC success, as well as with thr ee other variables associated with EPC success, suggesting that male m ounting belongs to a suite of behaviors aimed at obtaining extra-pair copulations. A possible benefit of this behavior is to inhibit competi tors from attending the mating arenas, because there was a negative co rrelation between the attendance of males and the number of mountings they received per day. This is the first study to report quantitative evidence that male-male mounting by birds is beneficial.