There is a whole category of things I look for that could be there, but mostly aren't. Tonight I saw one. Just before the moment when it would have been impossible to stop the stir fry I was cooking for dinner, I saw it: the Common Nighthawk.
I have never seen one in my entire life. Peterson lists them as "Uncommon" and so they are here in Eastern Canada. But there it was, with its characteristic flight pattern, white wing bars, and five others with it, all eating insects on the wing. I knew what it must be despite having nothing to compare it to in my history of looking: it is what is commonly there when you see the uncommon.
This sort of seeing, being willing to look hard at what is common, is key to the discovery of the uncommon. I watch Swallows do the same thing over our lake every night in the summer. But tonight wasn't every night, and this wasn't a Swallow. It was not a Swallow before it was a Nighthawk.
The Nighthawk is something I look for, but not something I see. I know what to look for, and that the looking is appropriate. However, this sort of looking can take a lifetime, and it's over in moments. The experience of seeing what you know must be there is something you take into the next evening of looking.