A bird is a warm-blooded vertebrate that is distinguished by having feathers, possessing a toothless beak and laying hard-shelled eggs. Unlike mammals, birds lack teeth and do not nurse their young, and unlike reptiles, they regulate their body temperature internally and possess specialized lungs that support sustained flight. Birds are also distinct in that they are considered by scientists to be the only surviving dinosaurs.*
There are roughly 11,000 species of birds, grouped into about 240 families and more than 2,300 genera. They thrive in virtually every biome, from dense forests to urban centers, and frequently function as dynamic ecological connectors between disparate regions through transcontinental and trans-oceanic migration, facilitating large-scale nutrient cycling.
Many species function as pollinators, particularly in tropical ecosystems where certain plants rely almost entirely on birds for reproduction. Others serve as seed dispersers, carrying plant material long distances and aiding forest regeneration after disturbances. Birds of prey help maintain balances within food webs by controlling populations of rodents and other small animals, while scavengers reduce the spread of disease by consuming carrion (decaying animal flesh). Because birds respond quickly to habitat loss, pollution and climate shifts, changes in their populations often signal broader ecological disruption and serve as early warning systems for environmental decline.
Agriculture also benefits greatly from bird activity. Some species consume vast quantities of insects, reducing the need for chemical pesticides and supporting more sustainable farming practices. Some species assist in pollination of fruit crops, while others contribute to soil health through nutrient cycling. Although certain birds may also feed on crops, their overall contribution to agriculture tends to outweigh localized negative effects.
Birds face growing threats worldwide. Current estimates suggest that around 14 percent of bird species are threatened with extinction, with many more experiencing population declines. This is due to habitat destruction (including deforestation and land conversion for agriculture and urban development), climate change (which alters migration patterns and disrupts the timing of food availability), pollution (particularly from pesticides, plastics, artificial night lighting and structural hazards such as window collisions), invasive species (such as rats and cats that prey on eggs and chicks) and hunting.
When birds disappear, the ecological functions they perform often diminish or vanish entirely, leading to cascading effects within ecosystems. Reduced seed dispersal can slow forest regeneration, while the absence of insect-controlling species can result in pest outbreaks that affect both natural habitats and agriculture. These changes can ultimately affect human well-being by undermining food security, increasing reliance on chemical inputs and weakening ecosystem resilience.
_______
*Birds are considered to be dinosaurs because they evolved directly from a group of two-legged, feathered dinosaurs (theropods) that survived the Cretaceous-Paleogene mass extinction event 66 million years ago. In contrast, the ancestors of mammals emerged more than 300 million years ago, long before dinosaurs first appeared. Likewise, today's living reptiles such as turtles, snakes and lizards are descendants of ancient, non-dinosaur lineages that also emerged more than 300 million years ago.
While crocodilians (crocodiles, alligators and gharials) are often casually grouped with other reptiles, they are actually birds' closest living relatives. Both belong to the group Archosauria ("ruling reptiles"). Crocodilians split from the lineage that produced dinosaurs around 250 million years ago; thus, while they share a much closer ancestral link to dinosaurs than do snakes or turtles, they are not dinosaurs themselves.