Most birds fly at a speed of around 20 to 30 miles per hour. If birds are in danger or hunting, their flight speeds can increase greatly.
The rate at which a bird can fly depends upon a few different factors, and, surprisingly, size or species does not affect speed. Tiny birds such as the hummingbird can fly just as fast as larger birds like geese. Flight speed is, however, affected by things like weather conditions and flight duration. A bird that has been flying for several hours will not fly as fast as a bird that has only been in the air for a few minutes.
When birds are being chased or are hunting, they can reach much faster speeds. For example, a peregrine falcon normally cruises at a speed of around 100 miles per hour. If that same bird is hunting, it may reach speeds as high as 200 miles per hour.