Sjhaana Clovoc, via e-mail Avian hearing has been famously credited with alerting humans to impending danger. What is audible to them clearly affects their survival, sometimes from very early on. Some species can vocalize before they hatch, and the hearts of young song sparrows beat faster upon hearing their species' song for the first time. Many birds can identify mates or other group members by voice alone. One study found that the male mockingbirds with the largest repertoires not only get the best territories but also attract mates more readily.
Woodpeckers can pinpoint beetle larvae munching inside a tree.
Swiftlets and oilbirds can navigate dark caves by echolocation. But on the whole, birds hear within the same general frequency range as we do. In structure, the avian hearing organ is remarkably similar to that of reptiles, reflecting birds' emergence from dinosaurs, ages before they were enlisted to stand vigil for mankind. The prickly, skin-puckering sensation you experience when you're frightened or titillated or shivering from the cold is caused by constriction of tiny arrector pili muscles at the base of the hairs on your arms, your legs, and the back of your neck. In the old days, chilly weather made the voluminous hair of primordial hominids bristle, providing natural insulation by creating pockets of warmer air between the skin and the outside cold.