Behaviourally driven mating system variation in blue tits

Schlicht, Emmi (2024): Behaviourally driven mating system variation in blue tits. Dissertation, LMU München: Fakultät für Biologie

Abstract

In birds, mating is central to fitness and thus key to understanding the evolution of any behaviour. There are two levels to understanding avian mating patterns. First, social mating systems are used as categories for species or populations. They describe the social units formed by males and females and arise from variation in sex role divergence. Evolutionary theory suggests that variation in social mating systems and sex role divergence emerge as a consequence of sex differences in optimal allocation patterns between time invested in mating and time invested in other activities, such as parental care. If sex role divergence is viewed as an evolutionary gradient, then the social mating system provides a guideline in which general area of this gradient a species can be found. Second, as a consequence of phenotypic plasticity, the empirically realised mating patterns of a species may shift within that general area such that mating patterns diverge a fair amount among species with the same social mating system. In birds, both mating and parenting are fundamentally rooted in social behaviour, a key property of which is flexible, context‐dependent adjustment. Plasticity in mating behaviour is expected to be particularly relevant in species subjected to variable ecological conditions, in species with highly evolved social behaviour and in species with well developed cognitive skills, all three of which are typical for many bird species. This thesis investigates how plasticity in mating behaviour influences mating patterns in a socially monogamous passerine, the blue tit Cyanistes caeruleus. Among socially monogamous species, the two most common modifications of the social mating system arise from adjustments in social bonds via facultative social polygyny and from adjustments in copulation patterns via extra‐pair paternity. The aim of the studies presented here is to elucidate the factors underlying individual adjustment of mating decisions in blue tits, focusing on within‐ and between‐individual variation in social associations, in familiarity, in experience and in age as well as in age‐related competitive effects. Chapter 1 presents a descriptive study on the origin and outcome of facultative polygyny in the blue tit. Regarding the origin of facultative social polygyny, it is found that laying date is a good proxy for mating order and allows reliable identification of the primary and secondary female. Time constraints are probably important in the origin of facultative social polygyny, especially in the context of replacement polygyny, while local breeding density, variation in habitat quality and individual traits (body size, age and arrival date) are less relevant. Pre‐breeding interactions indicate that in the study population facultative social polygyny may arise, because secondary females prefer to associate with a familiar male or with a male at a familiar location. Chapter 1 also considers the immediate and longer‐term consequences of facultative social polygyny for individuals. Socially polygynous males have smaller clutches in each of their broods than socially monogamous males in their single brood, possibly due to female anticipation of reduced paternal care: socially polygynous males contribute less paternal care at each of their two nests and at both nests combined than socially monogamous males at their single nest. This may partially be a result of coordination costs, because socially polygynous males rarely divide parental care between their two nests and, if they do, they seldomly intermingle visits, but instead focus their care first on one brood and then on the other brood. Secondary broods are more likely to contain extra‐pair young and circumstantial evidence suggests that secondary female may have copulated with additional males, either when they were originally mated with another male, which then disappeared, or when they were attempting to establish a pair bond after mate loss. Fledging success is reduced for both primary and secondary broods, mainly due to a higher rate of nest failure. The net effect of social polygyny on male reproductive success is negligible in this study population: due to a combination of smaller clutches, higher paternity loss and reduced fledging success socially polygynous males sire a similar number of fledglings as socially monogamous males. Facultative social polygyny then probably reflects females under time constraints making the best of a bad situation, whereby social inertia and the fundamental mechanisms of settlement and pair bonding determine the behavioural options available to males and females. Chapter 2 explores the effect of male age on extra‐pair siring success. Age is the most consistent predictor of plasticity in mating success via extra‐pair paternity across studies, but the effect is not well understood. The analysis in Chapter 2 separates within‐ from between‐individual effects and establishes that in the study population extra‐pair siring success increases, when a male moves from yearling to older age. The shape of the age trajectory indicates that there is no ongoing improvement after that and, in comparison to the initial change, negligible senescence. There thus is a threshold effect mediated via age class that separates yearling from older individuals, an effect later also found for other traits relevant for reproduction (clutch size, arrival date; Appendix 2). A literature review also suggests that the consistency of the age effect across studies may be driven by a similar threshold in other species. Interestingly, the occurrence of extra‐pair paternity in a nest is independent of the age of either the male or the female tending the nest. The threshold effect observed between naïve yearlings and older individuals points to an influence of experience or of incomplete maturation on male competence or on female preferences when either males or females are seeking extra‐pair copulations. Chapter 3 seeks to test, if the age effect on extra‐pair siring success studied in Chapter 2 is based on absolute or relative processes. That is, due to selection for life history optimisation or due to developmental constraints, yearlings may be incapable, incompetent or fundamentally unattractive in the extra‐pair context, independent of their social surroundings. For instance, learning to perform territory defence and courtship may fully absorb their time or energy reserves, preventing them from investing additionally in extra‐pair behaviour. Alternatively, yearlings may only fail, when inhibited by older males, because these directly or indirectly affect their success in competition or mate attraction. To separate these two processes, Chapter 3 presents a large‐scale field manipulation which altered the presence of older males in the population. Removal of older males in the months preceding the breeding season created a population of breeding males in 2022 that consisted almost exclusively of yearlings. Following a preregistered research plan, the effects of the manipulation on the extra‐pair siring success of yearlings, on the population‐wide frequency of extra‐pair paternity and on the spatial patterns of extra‐pair paternity were inspected. While extra‐pair siring success of yearlings increased, the rate of extra‐pair paternity and its spatial organization remained unaffected. At the same time, the rate of polygyny increased. These results support the idea that yearlings can be as successful as older males in siring extra‐pair young, if the latter are not present. In the study population the age effect on extra‐pair paternity is likely to be mediated by social effects of competition between age classes. Taken together, the results presented in the three chapters of this thesis emphasize the interacting roles of social effects, experience and age in modulating social bonds (via facultative social polygyny, Chapter 1 and Chapter 3) and copulation patterns (via extra‐pair copulations, Chapter 2 and Chapter 3) in a socially monogamous bird species. They indicate that the social mating system is not fixed, but flexible, and that behavioural plasticity is an important ingredient in shaping mating patterns in nature. Empirical studies can therefore contribute important insights that add to the theoretical underpinning provided by modelling approaches and further our understanding of avian mating systems.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: age, behavioural plasticity, blue tit, extra-pair paternity, facultative polygyny, mating system
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 3. Mai 2024
• 1. Bericht­erstatter:in: Kempenaers, Bart