I've done some research...

...and I've decided that the birds at Matt's house are brown headed cowbirds. I haven't actually gotten close enough to tell if I'm right-I'll look tonight perhaps, but it's the only thing that makes sense. Brown headed cowbirds don't make nests-they lay eggs in other nests and leave them for other birds to raise. Then, they all flock to bamboo groves at night.

Source

If you find this interesting, there is a more in depth article here.