Ive mentioned it on the big GTA thread on the BB.

You get the 2 in the storymode (the one you save from the fire, and the double dildo casino bird, as catalina dont count as one on the game for some reason), but the other 2 i just found...

One in the driving school (i remember she was there 1st time out but told me to feck off as i was too skinny- ie no fat, full muscle. But i went back as a big lard ass n shes lovin it, lol)

One in a village called San Quadros or summat, top left of map there the lil town with the marina, the village to the right of that, she outside the sherrifs office. She also likes fat dudes it seems

There 1 or 2 more birds but i not found em yet.