The thick-billed murre are a species of seabird that mate in a very narrow time window and remain monogamous through many mating seasons, but these birds sometimes divorce. There are five hypotheses that Marianne Gousy-Leblanc et al believe could explain this phenomenon. Divorce can occur for the purpose of finding a more compatible, or higher quality mate. Forced divorce occurs when a third individual forces one of the original members out of the partnership. The last two hypotheses have more to do with timing and environmental impacts on mating. The musical chairs hypothesis says divorce happens when one mate arrives at the breeding site at a different time than the other, and the last one involves factors that can randomly impact the arrival of individuals at the breeding site.
These birds are monogamous and both sexes perform parental care for their offspring. It is in the interest of the bird to pick a good mate the first time around, rather than divorce and try again. And yet divorce still occurs.
A recent study by Marianne Gousy-Leblanc of McGill University studied this phenomenon using data collected from 1991 to 2019. The birds mate at Coat’s Island, Hudson Bay, Nunavut and were identified visually by band and metal band numbers. Researchers observed plots from mid-July to mid-August, the brief breeding season for thick-billed murre. If a chick left the nest within 14 days of hatching, it was considered a successful fledging. Data for successful fledging was only collected from 1991 to 2010.
In this study, divorce was specified as an instance in which both individuals are still alive, but there is a subsequent breeding attempt by at least one bird with a new individual. Divorce is more common in pairings that had not experienced a successful breeding the year before and does not seem to be due to the availability of attaining a better mate or nesting site. The yearly average divorce rate is about 9% in this community, and was mainly seen in pairs with younger and more inexperienced individuals with low quality nest sites. Data does not support the hypothesis that divorce will increase the reproductive success of an individual due to a more compatible partner as reproductive success of divorced individuals was not immediately better, but actually worse. Only one third of new pairings resulted in a successful reproduction. Future studies should look at the long-term effects of divorce on reproductive success, but it seems evident that there is not an instant change in reproduction.
Divorced birds tended to repair with similarly aged and experienced birds, while widowed birds seemed to repair with younger birds. Gousy-Leblanc questioned whether this was due to widowed birds waiting for their partner to return, and being left to choose among the less desirable partners. Understanding the reason for divorce in these birds helps us to understand important aspects of mating.
Reference:
Gousy-Leblanc, Marianne, et al. “Go Your Own Way? Reasons for Divorce in a Monogamous Seabird.” Animal Behaviour, vol. 204, Oct. 2023, pp. 13–24. ScienceDirect, https://doi.org/10.1016/j.anbehav.2023.07.007.
