House sparrow in the context of "Euparkeriid"

The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropod dinosaurs named Paraves. Birds are categorized as a biological class, Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked reptile clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives (Palaeognathae), waterfowl (Anseriformes), ground-living fowl (Galliformes), and "modern birds" (Neoaves).

Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, Passer domesticus), and either Archaeopteryx, or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of Archaeopteryx to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between non-avian dinosaurs, Archaeopteryx, and modern birds is still under debate.

Plumage (from Latin pluma 'feather') is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with sex and age classes. Within a few species, there can be different colour morphs. The placement of feathers on a bird is not haphazard but rather emerges in organised, overlapping rows and groups, and these feather tracts are known by standard names.

Most birds moult twice a year, resulting in a breeding plumage and a non-breeding plumage; one of the moults, usually the one just after breeding, is a complete moult replacing all the feathers; the other, usually the one just before breeding, is often only a partial moult, with new small body feathers but not replacing the larger flight feathers in the wings and tail. Some very large birds, like eagles, replace their flight feathers slowly but continuously throughout the year, to minimise loss of flight efficiency. Many ducks and some other species such as the red junglefowl have males wearing a bright plumage while breeding and a drab eclipse plumage for some months afterward. Many passerine species have only one moult per year, with changes in plumage resulting from the wear of differently-coloured feather tips. Young birds have a juvenile plumage, which is replaced in the months after fledging by the first-winter plumage; in long-lived birds with slow maturation like gulls, this is followed by a succession of second, third, and sometimes fourth year immature plumages.

Deinonychosauria is a clade of paravian dinosaurs which lived from the Late Jurassic to the Late Cretaceous periods. Fossils have been found across the globe in North America, Europe, Africa, Asia, South America, and Antarctica, with fossilized teeth giving credence to the possibility that they inhabited Australia as well. This group of dinosaurs are known for their sickle-shaped toe claws and features in the shoulder bones.

Deinonychosauria is commonly defined as all dinosaurs more closely related to dromaeosaurids (such as Deinonychus antirrhopus) than to birds (such as Passer domesticus). It traditionally includes the families Dromaeosauridae and Troodontidae, which each possess enlarged "sickle claws". However, troodontids may instead be closer to birds than to dromaeosaurids, so they would lie outside Deinonychosauria under that hypothesis. This would also render Deinonychosauria equivalent to Dromaeosauridae, under a broad definition of the family. As the structure of the paravian family is still undergoing debate, the components of Deinonychosauria is unstable beyond dromaeosaurids.

The common kingfisher (Alcedo atthis), also known as the Eurasian kingfisher and river kingfisher, is a small kingfisher with seven subspecies recognized within its wide distribution across Eurasia and North Africa. It is resident in much of its range, but migrates from areas where rivers freeze in winter.

This sparrow-sized bird has the typical short-tailed, large-headed kingfisher profile; it has blue upperparts, orange underparts and a long bill. It feeds mainly on fish, caught by diving, and has special visual adaptations to enable it to see prey under water. The glossy white eggs are laid in a nest at the end of a burrow in a riverbank.

The saguaro (/səˈ(ɡ)wɑːroʊ/ sə-(G)WAR-oh, Spanish: [saˈɣwaɾo]; Carnegiea gigantea) is a tree-like cactus species in the monotypic genus Carnegiea that can grow to be over 12 meters (40 feet) tall. It is native to the Sonoran Desert in Arizona, the Mexican state of Sonora, and the Whipple Mountains and Imperial County areas of California. Saguaro typically grow at elevations ranging from sea level to 4,500', although they may be found at up to 5,000'. The saguaro blossom is the state wildflower of Arizona. Its scientific name is given in honor of Andrew Carnegie. In 1933, Saguaro National Park, near Tucson, Arizona, was designated to help protect this species and its habitat.

Saguaros have a relatively long lifespan, often exceeding 150 years. They may grow their first side arm around 75–100 years of age, but some never grow any arms. Arms are developed to increase the plant's reproductive capacity, as more apices lead to more flowers and fruit. A saguaro can absorb and store considerable amounts of rainwater, visibly expanding in the process, while slowly using the stored water as needed. This characteristic enables the saguaro to survive during periods of drought. It is a keystone species, and provides food and habitat to a large number of species.

Aphanosauria ("hidden lizards") is an extinct group of reptiles distantly related to dinosaurs (including birds). They are at the base of a group known as Avemetatarsalia, one of two main branches of archosaurs. The other main branch, Pseudosuchia, includes modern crocodilians. Aphanosaurs possessed features from both groups, indicating that they are the oldest and most primitive known clade of avemetatarsalians, at least in terms of their position on the archosaur family tree. Other avemetatarsalians include the flying pterosaurs, small bipedal lagerpetids, herbivorous silesaurids, and the incredibly diverse dinosaurs, which survive to the present day in the form of birds. Aphanosauria is formally defined as the most inclusive clade containing Teleocrater rhadinus and Yarasuchus deccanensis but not Passer domesticus (house sparrow) or Crocodylus niloticus (Nile crocodile). This group was first recognized during the description of Teleocrater.

Although only known by a few genera, Aphanosaurs had a widespread distribution across Pangaea in the Middle Triassic. They were fairly slow quadrupedal long-necked carnivores, a biology more similar to basal archosaurs than to advanced avemetatarsalians such as pterosaurs, lagerpetids, and early dinosaurs. In addition, they seemingly possess 'crocodile-normal' ankles (with a crurotarsal joint), showing that 'advanced mesotarsal' ankles (the form acquired by many dinosaurs, pterosaurs, lagerpetids, and advanced silesaurids) were not basal to the whole clade of Avemetatarsalia. Nevertheless, they possessed elevated growth rates compared to their contemporaries, indicating that they grew quickly, more like birds than other modern reptiles. Despite superficially resembling lizards, the closest modern relatives of aphanosaurs are birds.