Ornithology: Science of Birds - Includes data on the science of wild birds, with an emphasis on conservation and education. Electronic Resources on Ornithology - Annotated links and ornithological articles by Christopher Majka. Birdnet - Ornithological Council's website with comprehensive data about North American ornithological resources. Bird Skull Collection - Images and data about skulls of western Palearctic and Tropical species, with complete skeletons of some species. List of Essays - Many articles on every aspect of bird life. A comprehensive and useful resource. Avian Reproduction: Anatomy and the Bird Egg - Notes on the bird reproductive system and the development of the embryo, with explanatory drawings and photographs. Avian Reproduction: Clutch Size, Incubation and Hatching - Notes on clutch size optimization in birds, reproductive success, embryo development, incubation and the hatching process, with links to other similar articles. Bird Respiratory System - Article explaining how air sacs play an important part in the gaseous exchange in birds, with many diagrams. A Qualitative and Quantitative Study of the Lung of an Ostrich - The colossal body size, the relinquishment of flight and the extreme thermal stresses on the ground make the examination of an ostrich’s lungs rewarding. Population Dynamics - Essay discussing the factors behind the variation in the number of individuals in a bird population over time. Site Tenacity - Many birds return to the same nesting site each season. This essay discusses why this should be so. Flapping Flight: Inertial Studies - Technical article by researchers at Bristol University who measured the wings of bats and birds and calculated their efficiency in flight. Flight in Birds - How do birds fly? An introduction to flight in birds examining wing shape, airflows, flapping flight and gliding. Development and Uses of Avian Skeleton Collections - Skeletons are essential for the identification of fossils, bones from archaeological sites and food items taken by predators as well as histological and physiological studies. Territoriality - Article describing how birds try to exclude others of the same species from their home range. Vocal Dialects - Article describing how the songs of many avian species show geographic variation. Vocal Copying - Article describing how birds learn their songs from adult tutors and may incorporate sounds from other species into their repertoire. Vocal Functions - Songs identify the species of the singing bird. The territorial song of males serves the dual purpose of territorial proclamation directed at other males and of mate attraction directed toward females. Sexual Selection - Female birds of some species tend to choose as mates those males holding the most desirable territories, but there is little evidence that females preferentially select males with different degrees of ornamentation. Polygyny - Most birds are monogamous, but in some species the occurrence of polygyny, where one male mates with more than one female, is related to high-quality territories with an abundance of resources. Parental Care - The young of passerines when first hatched are naked, blind, and helpless and require much care which involves feeding, training and protection from predators. Creches - Article describing how some birds adopt the strategy of grouping their young, leaving them with a guardian while both parents go off to forage. Incubation: Heating the Egg - Most birds create the required temperature by sitting on their eggs but some transfer heat through their webbed feet, and the megapodes of Australia bury them in a mound of decaying vegetation. Bird Milk - Article describing how the young of some birds are fed on special secretions from their parents. Urban Birds - Article discussing how birds have adapted to urban foraging amid the artificial concrete and steel ecosystems of cities. Sense of Direction - Article from the Princeton Weekly Bulletin outlining how Biologist Martin Wikelski is helping to settle the long-standing debate over how migratory birds manage to navigate in darkness and bad weather. When and How do Birds Migrate - Article discussing the varying strategies adopted by birds in finding their way when taking part in their long distance journeys. Chicken Embryo - A number of fine photographs of the development of a chick inside the egg. Bird Banding Laboratory - Based at the USGS Patuxent Wildlife Research Center, issues permits to researchers in the United States who place uniquely numbered bands (rings) on birds' legs. Site features the history of banding and reporting of bird band observations. BirdMinds: Exploring Bird Intelligence - Examples of avian intelligence are reviewed for many different bird species, based upon scientific and anecdotal evidence. Wing Image Collection - The Slater Museum of Natural History provides a comprehensive selection of photographs of bird wings.