At the moment, I am reading a book about swifts in which it says these have been around since the Eocene era i.e. about 50 million years. It is hard to grasp. This means these birds were flying high in the sky before humans came on the scene. I find this quite humbling. I guess other migratory birds like swallows and martins go back a very long time too.
Most swifts, swallows and martins have now embarked on their long journeys south. I also keep an eye out for the first to come back each spring, probably as they have done for millions of years before me.