Among animals, males and females in the vast, vast, vast majority of species do not form long-term pair bonds. Even when the field is narrowed down to animals with a backbone, the most common situation by far is for males and females to come together only long enough to mate. When there is an exception, an explanation is required.
Birds represent an exception. At least on the surface, birds of most species are monogamous, with males and females remaining together for an extended period, even in successive breeding seasons. The most frequently cited reason this prolonged pair bond is that both the father and mother are able to assist with raising the chicks to independence, even if the parents do not contribute in exactly the same way.
Biology is all about exceptions to exceptions, and our curious nature demands that these cases receive special consideration. If not monogamy, why not?
Juan Antonio Gil of the Fundación para la Conservación del Quebrantahuesos (Foundation for the Conservation of the Bearded Vulture) and his Spanish colleagues recently reported on a rare but interesting aspect of the natural history of Bearded Vultures. Also known as Lammergeiers, these birds most frequently breed as monogamous pairs. Polyandry, the situation in which a the breeding pair is joined by an additional male, is not at all uncommon amongst Bearded Vultures. The report of Gil et al. concerned the previously-unreported situation of polygyny in this species.
Studying Bearded Vultures in the mountainous southern slopes of the central Pyrenees, the research team observed three examples of “reproductive units” with more than one female. In the first case, a male and female were in a monogamous relationship from 2006 to 2012. In that interval, the female produced six clutches, but only two chicks hatched, and neither survived long enough to leave the nest. This pair were joined in 2013 by an additional female, but the trio produced no eggs. The first female departed in 2014, and the remaining pair successfully raised a chick in 2015. Through 2017, the first female remained in the area, but without a mate.
The second observed case of polygamy began with a pair that were monogamously mated from 2007 to 2009. They were not successful in that period. An additional female joined the pair in 2010, and an additional male did so in 2011. Through the 2017 breeding season, the foursome had no reproductive success.
The third case observed by Gil and his colleagues began when a female, five years old, occupied a breeding territory in 1999. She remained alone until 2004 when she was joined by a male. That pair built their first nest in 2007. An additional female joined the pair in 2011. The trio produced its first clutch of eggs inn 2016, but the breeding effort was a failure. The same three birds occupied the site in 2017, including the first female, now 22 years old. In all that time no chicks were fledged.
Why, then, are Bearded Vultures not all mated monogamously? An unbalanced sex ratio might explain the behaviour, but that was not the case in the Pyrenees population. Perhaps superior breeding habitat is limited, and so trios and even quartets form as an alternative to breeding on poorer territories. Again, that does not seem to be the correct explanation, because the polygynous groups did not occupy high-quality territories. The poor reproductive performance of the polygynous vultures suggests that the additional individuals were not gaining access to very high quality territories or very high quality mates.
Instead, Gil et al. felt that habitat saturation could be at play. Older birds without access to a breeding territory might be forced to join an already-mated pair “as an extreme strategy to produce offspring at any cost,” before they became too old to breed.
It is no wonder that Bearded Vultures need a conservation foundation.
Gil, J. A., G. Chéliz, Í. Zubergoitia and P. López-López. 2017. First cases of polygyny for the Bearded Vulture Gypaetus barbatus in the central Pyrenees. Bird Study 64:565-568.
Photo credits: www.pinterest.com
