Adaptive radiation is the evolutionary process which allow the evolution of many species from a single species because some population of that species get into an environment which is different from their old environment in terms of food availability, predators, etc.
So the first bird that evolved the ability to fly branched into many different species due to habitat change is an example of adaptive radiation. The most famous example of adaptive radiation is Darwinian finches which evolved from an ancestral species of south America that reached Galapagos island.
