How structures in bodies develop in the embryos have often been found to reflect how those structures changed during evolution. Embryonic humans at early stages, for example, look a lot like embryonic fish, and later in development as embryos of other mammals, betraying our origins on the evolutionary tree. The bird brain is divided into a number of sections, each with a different function. The cerebrum or telencephalon is divided into two hemispheres, and controls higher functions. The telencephalon is dominated by a large pallium, which corresponds to the mammalian cerebral cortex and is responsible for the cognitive functions of birds. The pallium is made up of several major structures: the hyperpallium, a dorsal bulge of the pallium found only in birds, as well as the nidopallium, mesopallium, and archipallium. The bird telencephalon nuclear structure, wherein neurons are distributed in three-dimensionally arranged clusters, with no large-scale separation of white matter and grey matter, though there exist layer-like and column-like connections. Structures in the pallium are associated with perception, learning, and cognition. Beneath the pallium are the two components of the subpallium, the striatum and pallidum. The subpallium connects different parts of the telencephalon and plays major roles in a number of critical behaviours. To the rear of the telencephalon are the thalamus, midbrain, and cerebellum. The hindbrain connects the rest of the brain to the spinal cord. The process of evolution by natural selection was first explained by Charles Darwin, a celebrated British Naturalist. Ironically, his study of the variation in bill shape between Finch species in the Galapagos Islands was one of the major clues for his theory! Recent Discoveries and Fossil Evidence

Dinosaur fossils are old, and have been studied intensively since the late 1800s. You might think that all the important discoveries would have been made long ago, but some remarkable bird-related fossils have been found in recent years. These fossil finds have shown how similar birds are to some of their extinct dinosaur ancestors. Before you read on, take a moment to think about what characteristics make birds different from a Velociraptor or Tyrannosaurus. What makes a bird a bird? Here are a few of those traits; you could probably think of others. Collage of bird anatomical illustrations with the different vertebral sections color-coded across various species. The species included are as follows: Top row (left to right) Struthio camelus and Sagittarius serpentarius (formerly Gypogeranus serpentarius) Bottom row (left to right) Megascops choliba decussatus (formerly known as Strix decussata) and Falco rusticolus islandus (formerly Falco islandus). Sections of the vertebral column in anatomical bird diagrams Color Hu, H.; Sansalone, G.; Wroe, S.; McDonald, P.G.; O'Connor, J.K.; Li, Z.; Xu, X; Zhou, Z. (2019). "Evolution of the vomer and its implications for cranial kinesis in Paraves". PNAS. 116 (39): 19571–8. Bibcode: 2019PNAS..11619571H. doi: 10.1073/pnas.1907754116. PMC 6765239. PMID 31501339. a b c d Whittow, G. Causey (2000). Sturkie's Avian Physiology. San Diego, California: Academic Press. pp. 233–241. ISBN 978-0-12-747605-6.Stettenheim, Peter R (2000). "The Integumentary Morphology of Modern Birds—An Overview". American Zoologist. 40 (4): 461–477. doi: 10.1093/icb/40.4.461. However, in the later stages of development in dinosaurs, crocodilians and other reptiles, the skull bones no longer mirror the brain growth as closely. Later in development cranial muscles and other tissues also begin to affect skull shape.

These characteristics might seem to be unique to birds, but the surprising thing is that all of them have been found in nonflying fossil dinosaurs. While it's been clear for many years that birds descended from dinosaurs, researchers have only recently learned how much those prehistoric dinosaurs resembled birds. Some of these characteristics have traditionally been explained as adaptations to flight, but if the characteristics first appeared in dinosaurs long before the evolution of flight, then they weren't adaptations to flying. Dr Abzhanov said: “Evolution is the accumulation of and selection on changes made to the developmental process. Studying birds is a fascinating subject because they retain features of ‘young’ dinosaur ancestors, while also clearly adding their own adaptations, such as toothless beaks and wings. a b Arnqvist, G.; I. Danielsson (1999). "Copulatory Behavior, Genital Morphology, and Male Fertilization Success in Water Striders". Evolution. 53 (1): 147–156. doi: 10.2307/2640927. JSTOR 2640927. PMID 28565197.Lierz, Michael (January 2003). "Avian renal disease: pathogenesis, diagnosis, and therapy". Veterinary Clinics of North America: Exotic Animal Practice. 6 (1): 29–55. doi: 10.1016/S1094-9194(02)00029-4. ISSN 1094-9194. PMID 12616833. Scientists have been trying to decipher the intricacies of this process for centuries. A research team in the Museum, led by Prof Anjali Goswami, is working to find out more about how and why we evolved the way that we have. The bursa of fabricius, also known as the cloacal bursa, is a lymphoid organ which aids in the production of B lymphocytes during humoral immunity. The bursa of fabricius is present during juvenile stages but curls up, and in the sparrow is not visible after sexual maturity. [92] Anatomy [ edit ] The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system (see illustration on the left). [57] [59] The partial pressure of oxygen in the parabronchi declines along their lengths as O 2 diffuses into the blood. The blood capillaries leaving the exchanger near the entrance of airflow take up more O 2 than do the capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air, [57] [59] but is nevertheless less than half that of the inhaled air, [57] thus achieving roughly the same systemic arterial blood partial pressure of oxygen as mammals do with their bellows-type lungs. [57]

There are only a few muscles in the trunk and the tail, but they are very strong and are essential for the bird. These include the lateralis caudae and the levator caudae which control movement of the tail and the spreading of rectrices, giving the tail a larger surface area which helps keep the bird in the air as well as aiding in turning. [39]The thoracic vertebrae number between 5 and 10, and the first thoracic vertebra is distinguishable due to the fusion of its attached rib to the sternum while the ribs of cervical vertebrae are free. [7] Anterior thoracic vertebrae are fused in many birds and articulate with the notarium of the pectoral girdle. [13] Diagram of a general bird pelvic girdle skeleton including the lower vertebral column sections. Note that the caudal vertebrae (5–10) are not fused in this diagram but can be in certain species. Synsacrum [ edit ]