There are a number of characteristics that enable these warm-blooded vertebrates to fly. Highly-efficient lungs and hearts allow them to absorb high levels of oxygen and to engage in sustained physical activity. Hollow bones (and a lack of teeth or tail vertebrae) help reduce total body weight. Lightweight feathers help them float on currents of air and, at the same time, provide them with warmth and tactile sensation.
Other common characteristics of birds include beaks and scaly legs, and the ability to lay hard-shelled eggs.
To navigate through the skies, researchers believe that birds use a combination of cues including magnetic fields, smell, and visual information from the sun and stars. This range of sensory perception comes in handy in the course of long migrations when birds are likely to encounter fog, electrical storms, and other unexpected navigational hurdles.
Birds also have an amazing ability to track movment. This characteristic enables birds to hunt for prey while flying and may also play a role in courting and mating practices.
Evolution is characterized by a slow process of change. Over millions of years, birds evolved from certain species of dinosaurs. That is why today's birds and the reptilian dinosaurs have in common scaly legs. In fact, some scientists believe that bird feathers are highly modified (i.e., evolved) forms of scales.
The relationship between dinosaurs and birds can lead scientists to ponder larger questions of evolution and its opposite—extinction. Extinction is defined as the complete disappearance of an entire species. This process can be a natural part of a species’ evolution as genetic forces play out and species fail to adapt to environmental changes. In the 20th century, however, the rate of species extinction has greatly increased, in many cases due to the human impact of hunting and habitat intrusion.