Female bearded vultures in polyandrous trios prefer mating with the alpha male, but will also mate with the beta to increase the likelihood of successful nesting.

When a male bearded vulture is unpaired or free-floaitng, he will often join a pre-existing male and female pair, creating a polyandrous trio. These trios are commonly recorded in the Pyrenees mountain range of Spain and France.

Female bearded vultures in polyandrous trios prefer to mate with alpha males, but will also mate with beta males.

Mating with a larger number of males may benefit the female by providing her with more parental care for her young. Extra-pair copulations may be a way to increase the likelihood of successful nesting if the first male is infertile, or may increase genetic diversity within the brood. Females may also mate with both males to avoid harassment or aggression.

Reverse mounting is also common among polyandrous trios. After the alpha male drives off the beta male, he is mounted by the female.

Sources: (Antor, et al., 2007; Bertran & Margalida, 1999, 2003, 2006; Bertran, Margalida, & Arroyo, 2009; Ferguson-Lees & Christie, 2001; Tenenzapf, 2011)
Image: Arjan Haverkamp