They get reported on e-Bird, which is one of the main citizen science birding organizations. With thousands upon thousands of reports daily, E-bird is able to track the numbers and movements of bird species all over the world, and the data is available online. When you enjoy numbers the way I do, you'll understand how easy it can be to get lost for hours on their website.
It might seem overwhelming at first, but e-Bird encourages reports every day, and not just for unusual or exotic species. They also want to know what is happening in backyards and urban parks as well. Over time, we'll be able to see the effects of global warming on bird migration and populations, so the reports, however mundane they might be, are extremely important.
In any case, these musings about e-Bird happened because I quite unexpectedly hit a milestone of sorts. I was on my a regular morning walk on the Tuolumne River Parkway Trail when I saw a diminutive bird in the underbrush that was an unusual shade of blue. It behaved like a Ruby-crowned Kinglet, but the kinglets are olive-green in color. I was uncertain what it was but I got a couple of pictures and suspected in the back of my mind that it was a gnatcatcher, and this was later confirmed when I got the Sibley's guide out. It was a Blue-gray Gnatcatcher (Polioptila caerulea). I knew the bird from lists and guides, but when I looked through my notes and photos, I realized I had never seen one in the wild. And when I input the species on e-Bird, it registered as bird number 250 on my life-list, a nice round number for a person obsessed with counting things.
If you are curious about how many birds have been observed in your region, e-Bird is a good place to start. Use the zoom tool on the "Hotspots" link and map and you'll see dozens of places near your home where birds tend to congregate. You can click on a hot spot to see the birding history for that location. The history of the Tuolumne Parkway Trail can be found here. There is a printable checklist, and even a page with all the photos of birds from that particular location. It's a marvelous place to learn something new about your home base.