Wired Science has a post on a program that has citizen science recording night time bird migrations using home-made monitors.
William Evans, the ornithologist who developed the set-up describes it:
"It's one resistor, two capacitors, a 9-volt battery, a microphone element, a dinner plate, some Saran wrap and a flower pot...It costs less than thirty bucks, and you can build it in a couple hours. You can potentially monitor thousands of birds in a night. And that's something we've never been able to do in the field."
You can find instruciton on Evans' web site.