Socially monogamous partnerships in birds: causes, consequences, and pair-bond strength

<p dir="ltr">Social monogamy is common in birds, with many species forming long-term partnerships. Remaining with the same partner reduces sexual conflict and provides additional benefits as partners align behaviourally and physiologically. Understanding the factors influencing mate retention and divorce, along with its consequences, provides insight into the benefits of social monogamy and how mating systems are shaped more generally. Divorce is often considered adaptive, allowing individuals to improve reproductive success with a better partner. However, non-adaptive explanations also exist, including the hypothesis that environmental factors, such as extreme weather, influence divorce rates. Research on avian divorce has several shortcomings, as factors like initiator identity, individual age, widowhood, and long-term or even intergenerational fitness consequences are often neglected. Additionally, while behavioural coordination of pairs has been extensively studied in parental care, the significance of partnerships outside reproductive contexts remains understudied.</p><p dir="ltr">My PhD thesis examined partnership dynamics in two wild passerine species: the Seychelles warbler (<i>Acrocephalus sechellensis</i>) and the chirruping wedgebill (<i>Psophodes cristatus</i>). Using long-term multigenerational data, I investigated mate switching in Seychelles warblers whilst addressing shortcomings in the literature. Divorce was more frequent with shorter pair tenure and poor reproductive success, and among young and old males. However, divorce did not yield short- or long-term fitness benefits. Instead, early-life divorce in females was associated with lower survival, suggesting that while divorce may help to escape poorly performing partnerships, it is not necessarily advantageous. To assess environmental influences on divorce, I examined the effects of extreme rainfall. I found that divorce was more frequent under both very dry and very wet conditions. This was not mediated by reproductive failure, implying other processes shape divorce in this context. This provides the first evidence that abiotic environmental factors influence divorce in terrestrial bird species. Finally, I made the first attempt to quantify intergenerational effects of mate switching and fidelity in wild animals by looking at short- and long-term fitness proxies of offspring, but found no effect of parental pair-bonds on offspring fitness.</p><p dir="ltr">In chirruping wedgebills, I explored partnerships on a fine-scale behavioural level using movement and vocalisations. Using automated tracking data, I revealed that pair-bonded chirruping wedgebills have extremely similar home ranges, maintained consistent proximity and followed each other as they moved during both breeding and non-breeding periods, underscoring the significance of behavioural coordination in pairs beyond parental care. I also examined duetting as a measure of vocal coordination to test whether more coordinated duets elicited stronger territorial responses using playback experiments. Contrary to predictions, neither response intensity nor coordination differed, suggesting duet coordination does not indicate perceived threat. This highlights the complexity of duet function across species and different aspects of duets serving distinct roles.</p><p dir="ltr">My research advances our understanding of partnership dynamics in socially monogamous birds. I demonstrate that divorce is a flexible behavioural strategy influenced by intrinsic, social, and environmental factors, and that strong partnerships extend beyond the reproductive context. These findings highlight the need for further research to address methodological and biological gaps in our understanding of monogamous mating systems.</p>