Disentangling the causes of age-assortative mating in bird populations with contrasting life-history strategies

I have published my first paper which forms one chapter of my PhD thesis. We investigated why individuals in animal populations often end up breeding with partners that are a similar age to themselves, a pattern known as age-assortative mating. Age can have a strong effect on reproductive success, so understanding why these pairings arise could be important for understanding how the age structure of a population influences its overall reproductive performance.

In this study, we compared long-term breeding data from two species with very different life-history strategies. Firstly great tits, using 51 years of data, and secondly mute swans, using 31 years. Interestingly, although both species showed age-assortative mating, we found that the processes producing it were quite different.

In great tits, age assortment appeared to be largely passive. Their relatively short lifespans mean that populations can contain large cohorts of young birds, which are then particularly likely to pair with one another simply because they make up a large proportion of the available breeding population. We found little evidence that great tits were actively selecting mates according to age. In contrast, the much longer-lived mute swans showed a stronger influence of pair fidelity, as established pairs remain together for many years, as well as some evidence that individuals actively form pairs with similarly aged partners.
The study therefore highlights how very similar patterns observed across species can arise through quite different ecological and demographic processes, and how the life-history of a species can fundamentally shape the social structure of its populations.
​The paper can be found here.