An estimated 20 billion birds in recent decades have altered their migratory habits in response to climate change, says a leading ornithologist, Miguel Ferrer, of Spain. This means approximately 70% of the world’s migrating birds changed either the routes they historically took or the length of their migration. In many cases, birds are flying shorter distances than before, and some could stop migrating altogether because of variances in their food supplies or other factors brought on by global warming.