Canary in the Coal Mine?

A coworker of mine came by this morning to personally deliver the news of his resignation. I knew about it yesterday, but it was nice that he wanted to come tell me himself and talk about his situation with me a bit. He has been here for over 5 years and he is someone I have a lot of respect for. I don't always agree with him or think he is doing the right thing in a given situation, but he is very intelligent and was always looking to create value here. I will miss having him around and talking about college football - he is a Michigan alum and a huge fan - but I will really miss his insight and his understanding of the company's culture. And, his departure makes me think about what the future might hold for this company and for me. When a former coworker of ours left, he told her that she was the "canary in the coal mine". My question is, what does that make him?