Ernst Mayr is one of biology's greatest sons (can I say that he is a son of biology?). He made many contributions to evolutionary biology, in particular, the biological species concept. The obvious outcome of this was that it allowed biologists to draw a line in the sand and say that this is this species and this is another species, although some argue that it is arbitrary and is more problematic than useful. For example, it is possible for two defined biological species to interbreed (hybridize) and have viable, fertile offspring. Commonly, people will cite lions and tigers hybridizing (the infamous Napoleon Dynamite Liger) in zoos or other man made facilities. However, this is highly improbable in a contemporary world since everybody knows (?) that tigers live in Asia and lions in Africa. None the less, it does speak to the potential power of the process. But, if we are in search of viable hybrids, we need to think less like a human. By this I mean we need to steer away from charismatic organisms and look at other life forms. Plants are a great place to start. There are countless example of naturally occur plant hybrids, and excitingly it occurs at relatively high taxonomic levels (i.e. inter-genus and even inter-familial hybridization). There are also examples of interfamilial bird hybrids such, particularly in the unfortunately named game birds.
I seem to have digressed from my initial topic but species concepts and modes of speciation are currently a particular interest of mine. Anyways all i really wanted to say was that Ernst started out as a bird watcher as a young man. He discusses it in the video below. I really love how, even though he is his 90s, he tells the story of observing a rare bird with such excitement, as though it was yesterday. The video comes from Web of Stories which is a website with videos of a wide range of interesting and influential people, including Ernst Mayr. The videos are broken down into small digestible sections only a few minutes long and are definitely worth a look.
Here is a piece Mayr wrote for Science in 2004 when he was 100 about the Modern Synthesis. He died the following year. Even after 100 years, 81 of which were spent as a student of biology his hunger for discovery, evolutionary biology in particular, is beautifully revealed by his last sentence "I only regret that I won't be present to enjoy these future developments".