Book excerpt: What is a bird? To answer that, we must understand how birds are different from all other living things and how they fit into the diversity of life on Earth. This excellent RSPB guide to bird anatomy looks at the avian body, system by system, how it evolved, and how it functions. Chapters explore traits that are unique to birds, including their remarkable one-way breathing cycle, their trimmed-down skeleton, how feathers permit flight, provide weather-proofing and add beauty, and the avian bill – a lightweight replacement for both teeth and food-handling forelimbs. Each chapter tackles a particular body system and includes detailed anatomical illustrations, from cells and organs to skeletons and muscles, to show how birds' anatomical adaptations enable all their physical feats and fascinating behaviour. Feature spreads offer more in-depth analysis on topics like birdsong, temperature control, ornamentation, unusual diets, social behaviour, nocturnal adaptations, mutation and natural selection. Featuring more than 300 diagrams and colour photos, this fascinating new book also looks at the human impact on the avian world and reveals how behaviour and anatomy work together to produce these vibrant living beings that delight and inspire us so much.