First, a disclaimer: I'm not an expert in birds by any stretch, but I believe it has to do with air pressure.
Pressure drops in advance of an approaching storm. Lower pressure also means the air is thinner, making it more difficult for the birds to create lift with their wings -- in essence, it's harder for them to fly. So they stay closer to the ground and hang out on perches instead of expending energy flying.
If a bird expert out there has a better explanation, please send us an e-mail at email@example.com and we'll bring it up again in a future column.