If your bluebird boxes and wren houses aren't up, you're late. Get them up tomorrow.
Clean and fill your bird feeders.
For those of you who don't know, ornithology is the study of birds (not to be confused with optometry, the study of eyes, or otology, the study of ears). No matter how far they travel, ornithologists can't gather nearly as much data as a bunch of people scattered across the globe submitting information about the birds in their area can. Rather wisely, there are several programs you can use to help them do this.
One of these is eBird, a program in which you walk a certain distance or sit by your bird feeder and record all the birds you see. Also take note of any interesting behavior or of the gender: "This is a female cardinal;" "These mourning doves were preening each other." It's likely that the folks at Cornell will know what this means. (Half an hour of this gets you into the Nature Army!)
My current obsession at the moment is NestWatch. If you see a bird's nest, report what kind of bird it was and where the nest is. Putting up a birdhouse is a great way to attract birds and establish your position in the Nature Army, but you should completely disregard what I said earlier in Ruling Places for the Royal Avian Family (first post) that dynamic, colorful birdhouses will still attract the braver birds. My two crazy birdhouses have so far attracted nothing, but I got a family of bluebirds in my boring cedar birdhouse a couple weeks after I put it up. As for nests not in birdhouses, (as they'll tell you at Cornell) finding them is a mix of knowing what to look for and luck. This mourning dove nest was luck.
The mother sat on her fledgling for so long, I thought she was still incubating her eggs. Then, one day when I checked on her, she flew off and where she had been sitting was this enormous chick, three fourths her size! It was like the Ford Anglia in Harry Potter when Mr. Weasley secretly enlarges the car from the inside but its outside size stays the same.
Anyway, these scientists know what they're doing. Google them, grab a field guide, and get outside!