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Reproductive strategies are important aspects of adaptation which can influence breeding success in different environmental settings. My PhD thesis investigates shorebirds (sandpipers, plovers, and allies) that exhibit an unusual diversity of reproductive strategies among birds. Previous research has concentrated on northern hemisphere shorebird species, which could have different responses to reproductive strategies than species in southern hemisphere. In Chapter 1, I introduce reproductive strategies and reproductive decisions, first for all birds and then for shorebirds specifically. I highlight the importance of shorebirds as a study species, and detail my thesis focus and objectives. In Chapter 2, I found that nest survival in relation to clutch size was significantly different within three Malagasy species (Kittlitz's, white-fronted and Madagascar plovers). There was no difference between species, suggesting that variations in nest survival rate are more affected by extrinsic factors than breeding strategy differences. In Chapter 3, I found that in these same three species of Malagasy plovers the parents may attend their young for up to at least 71 days. The three species varied in duration with white-fronted plovers providing less care than Kittlitz's and Madagascar plovers. Only hatch date predicted the duration of parental care with a correlation between earlier hatching dates and longer parental care. I discuss the possible explanations as to why extended care is provided in these species, including social learning (of foraging areas or anti-predator displays), reducing predation to the chicks (from anti-predator displays or parental knowledge of predators), and increased survival for juveniles surviving their first year (from reduced predation and increased feeding opportunities from social learning). In Chapter 4, I found smaller broods produced more male offspring, and different habitat sites produced different sex ratio biases in Madagascar and white-fronted plovers (but not in Kittlitz's plovers). This suggests that variation displayed in these two species is a function of brood size as well as the conditions that the brood experiences. This could be important for conservation efforts as it highlights that larger broods are preferrable for a higher female proportion of chicks, and that both fragmented habitats and large salt marsh areas are important to achieve parity in the overall population sex ratios of juveniles. In Chapter 5, I found that substitution rates and life history traits in bird species are correlated since age at first breeding (p < 0.05), clutch size (p < 0.001) and body mass (p < 0.001) predict substitution rates at both a species and family level. These results suggest that multiple life history traits are predictors of high substitution rates. Chapter 6 discusses the main findings of my thesis, relates this to the current trends in shorebird research, discusses reproductive strategies in a wider context and finally suggests future directions for shorebird research.

With up to 18,000 extant representatives and many more extinct, archosaurs are hugely species rich, have enormous ecological diversity and re-radiated spectacularly in the wake of the P-Tr, T-J and K-Pg extinctions. The success of the archosaurs has been attributed to a variety of factors, not least their unique locomotory adaptations, varied diet, hardiness to environmental change, unique dentition and, in some lineages, endothermy and derived metabolic adaptations. A factor that is often overlooked in comparing the radiation of the archosaurs with that of other amniote clades (eg. mammals and squamates) is their unique parental care behaviours. Some of these behaviours count among the greatest parental investments within the tetrapods, and the story of the evolution of parental care in the archosaurs is a fascinating field of inquiry. Here, I review current understanding of parental care in archosaurs, its origin and evolution. This collated information is presented as an introductory chapter. Following this are five pieces of novel research, presented in the form of scientific publications: (1) An analysis of the evolution of egg mass in extant birds, and the biological implications of previously masked allometric relationships within Orders. (2) An investigation into the parental sex roles in extant birds, with emphasis on the relationship between female biological investment and male behavioural investment. (3) An overview and novel analysis of the reproductive traits of extant crocodylians (4) An ancestral state reconstruction of the reproductive traits of extinct dinosaurs, with focus predominantly on the morphological changes to the egg in the theropod lineage ancestral to modern birds. (5) A description of the most complete juvenile spinosaur material recovered from the Kem Kem of Morocco. I conclude that: (1) Previous examinations of the relationship between egg and body mass in extant birds have masked underlying between-Order trends that offer significant evolutionary insight into the adaptive conditions and underlying genetic mechanisms present in the evolution of birds. (2) I report the first evidence of a significant negative relationship between male incubation and proportional female pre-laying investment in birds, with no such relationship recovered for other aspects of male care (feeding and brooding). (3) I report the first evidence of a latitudinal body mass trend in extant Crocodylia, as well as significant relationships between important reproductive traits (female mass, egg mass, clutch size, hatchling mass). No significant relationship was identified between breeding latitude and hatchling mass, suggesting a different type of selection from that in testudines. (4) I report strong evidence of a morphological change in the egg of theropod dinosaurs along the lineage that led to birds, consistent with previous revelations of changes to the adult morphology, nest and egg structure. (5) I describe seven new cranial and postcranial specimens of juvenile spinosaur from the Moroccan Kem Kem, suggesting a full individual size of 3-5m. I conclude that these juveniles, recovered in the same geological deposits as the adults, likely shared a common habitat, though this is less likely to have been the case for hatchlings.

Sexual conflict, the different interests of males and females over reproduction, is a potent evolutionary force. Here I investigate sexual conflict in the context of parental care by focussing on two questions: (i) which behavioural, morphological and environmental traits influence the parents’ decision to care for the brood or desert? (ii) How does sexual conflict influence the evolution of behaviour and morphology? I investigate both questions using a small, polygamous passerine bird, the Eurasian penduline tit Remiz pendulinus, which exhibits intense sexual conflict over parental care such that either the male, the female or both parents desert the clutch. Using detailed behavioural observations during the crucial few days preceding desertion, I argue that it may be in the parents’ best interest to conceal their intention to care for (or desert) their brood. The rapid resulting process that leads to variable parental care resembles a coordination game in which either parent may desert first. I developed a game-theoretical model that suggests that a key to resolving the conflict between parents is the sex difference in reproductive payoffs for given parental care strategies, rather than differences in parental quality per se. Environmental variables (e.g. food availability and mating opportunities) seem only subsidiary in the decision-making process of parents. My final chapter explores ramifications of sexual conflict at an evolutionary timescale. By comparing the highly polygamous Eurasian penduline tit with the monogamous Cape penduline tit, I show that morphological and behavioural differences between these two species are consistent with predictions of sexual conflict theory. During my PhD I also identified that there is considerable variation in breeding systems within different species of penduline tits. I argue that by studying these systems new insights will emerge into (i) the drivers of breeding systems, and (ii) neural and genomic traits that underlie breeding system evolution.