The avian bill is a textbook example of how evolution shapes morphology in response to changing environments. Bills of seed-specialist finches in particular have been the focus of intense study demonstrating how climatic fluctuations acting on food availability drive bill size and shape. The avian bill also plays an important but under-appreciated role in body temperature regulation, and therefore in energetics. Birds are endothermic and rely on numerous mechanisms for balancing internal heat production with biophysical constraints of the environment. The bill is highly vascularised and heat exchange with the environment can vary substantially, ranging from around 2% to as high as 400% of basal heat production in certain species. This heat exchange may impact how birds respond to heat stress, substitute for evaporative water loss at elevated temperatures or environments of altered water availability, or be an energetic liability at low environmental temperatures. As a result, in numerous taxa, there is evidence for a positive association between bill size and environmental temperatures, both within and among species. Therefore, bill size is both developmentally flexible and evolutionarily adaptive in response to temperature. Understanding the evolution of variation in bill size however, requires explanations of all potential mechanisms. The purpose of this review, therefore, is to promote a greater understanding of the role of temperature on shaping bill size over spatial gradients as well as developmental, seasonal, and evolutionary timescales.

Embryonic staging tables provide a standard of developmental stages that can be used by individual investigators and provide approximate time points for the study of developmental phenomena. Surprisingly, despite the presence of a plethora of studies on the chicken skull and its role as a model species in developmental research, a staging table of the development of the chicken skull remains lacking. A detailed photographic staging table of the osseous portion of the chicken skull is thus presented here based on cleared and stained HH stages spanning HH 35 (first appearance of skull ossification) to the final stage before hatching (HH 45). This table documents the development of most of the cranial elements in the skull from the start of ossification until the element takes its final shape. The table shows that the elements of the lower jaw and ventral side of the skull begin ossifying before the skull roof and that most elements take roughly 5 days to reach their final shape, whereas others take up to 9 days (e.g., the frontal). The obtained results lead to several hypotheses about chicken skull development and provide a timeframe for future studies on chicken skull development.

Sutures are fibrous joints that occur between bone elements in vertebrate skulls, where they play a variety of roles including facilitating skull growth and function. In addition, a variety of studies examining sutures from diverse perspectives in many taxa have enabled the determination of anatomical homologs. Surprisingly, one important aspect of sutures—histology—remains unknown in the key model organism of the chicken. To fill this gap in our knowledge, we generated histological sections of six different cranial sutures across a range of developmental stages in embryonic chicken. Despite having a skull that is largely co‐ossified or fused as an adult, we found that the types, components, and ontogeny of sutures in chicken skulls are very similar to sutures in other vertebrates. We did, however, find a new transient stage in the ontogeny of sutures between endochondral bone elements, in which one element has ossified and one was still cartilaginous. Moreover, to better understand the morphogenetic events at the onset of suture formation, we compared the developmental histology of six sutures with that of the space between the two ossification centers of the frontal—a location expected to be void of suture structures. We found that the mesenchymal cells in sutures condense and form a middle vascular layer. This was not found to be the case in the space between the two ossifications of the frontal, where instead only osteoid occurs.

The use of differing definitions of homology has led to controversies about the identity of bony elements in vertebrate skulls. An example is the controversy concerning the identity of the avian frontal, which has been assigned that name based on its location and shape. However, developmentally, it has been suggested to be a fusion of the frontal and parietal (i.e. frontoparietal). To assist in resolving this controversy, a search for a suture in the gap between the two ossification centres of the chicken's frontal was attempted. The production of a staging table of the chicken skull revealed a putative suture would be found between stages HH36 and 37. Histological examination of the developing frontal failed to find any evidence of a frontal-parietal suture, leading to the conclusion that the avian frontal is a single element formed from two ossification centers and is not a frontoparietal. iii Acknowledgements I would like to acknowledge and thank several people who have helped make this thesis possible and encouraged me throughout the two very difficult years of my masters. Chief among these people is Hannah Feldmann whose encouragement, patience, dedication and belief in me has made it possible to finish this thesis despite the extreme difficulty in completing it. I would also like to thank her for helping me in a crucially important stage of the experiment where I needed a companion to stay with me overnight in the lab. I would also like to thank my family for emotional and financial support throughout the two years and for making it possible for me to reach the masters level. My family's never-ending support made my thesis achievable. My best friend Olivier Heldwein deserves a lot of gratitude for help in so many ways and on so many occasions, including experimental steps and use of his camera. Olivier provided emotional and scientific support to me during the execution and writing the thesis, for which I am grateful.