In the hood people use the 'brid clap' to create a signal that the cops are about, as the birds (normally pigeons) fly into the air and make a clicking noise. This indication gives people time to stop doing illegal activities, and if they're dealing they can go hide their product and/or money.
This can be seen in Training Day starring Denzel Washington, and even on hip-hop videos such as The Game's "Hate It or Love It" video.
"Aiyyo, you see the pigs man... do a bird clap so I don't get knocked."
Guy 1: "Five-0! Five-0 just entered the block man!"
Guy 2: *starts to bird clap* "brrr-brrr-brrrbr"
Guy 3: "Oh shit, I gotta bounce! I ain't gettin' sent down today!"
Cops: "What the fuck are the birds doing?"