A life list is a compilation of all the bird species a birder has identified with absolute certainty during their whole lifetime of serious birding. Being "serious" implies knowing about look-alike species and subspecies, the various plumage states, and having a systematic-enough mind to not be sloppy and haphazard when it comes to making the lists.
This is my third year of "serious" birding. By serious I mean I'm fairly obsessed with it, enough to spend a good amount of time at it. The first year I decided to keep track of things. With a good mentor and lots of wood lots and parks nearby, the first hundred birds came fairly easily. The next summer brought about fifty more and then it became hard and harder. A winter trip to northern Minnesota and the Sax/Zim bog area brought another dozen. Then a March first ever trip to Florida brought a new bonanza of "lifers"which put me over the 200 mark. Take a look at a few of them.
Little Blue Heron
Painted BuntingIf you should start your own Life List right now, and for some reason during the rest of your life you never travel beyond your neighborhood, you might well end up listing a hundred species or more -- many species being spotted during migration. If you should expand your birding trips to include local parks and nature reserves, you could end up with 300 or more species. If somehow you were to see all the bird species ever sighted in all of North America, your list would hold about 850 names. So far no one has seen all the earth's more than 9,000 bird species. Hmmmm. I think I should have started this counting business sixty years ago. At this stage of life 9,000 looks like a pretty long shot for me. :)