I have both of the birds now. Been in 8 raids in total, only caught each one once.
It is good fun getting together with randoms and playing Raid Bingo:
Older Guy looking Sheepish
Chinese Guy on a Bike
Fat, Hairy Late-Teen Who Run to The Raid
Lad, mid/late 20's, Ashamed and Trying to Hide He's Playing
Chunky Lesbian Who Knows It All
Kids on Scooters Who are Really Excited About It
Fat Male Virgin Wearing Video Games Apparel
Bloke in Mid late 30's/early 40's Playing It For His Kid
From my survey after two days playing it, raids for the Ice Bird pull in less people than the raid for the Bird That Sounds Like Luigi. So yeah, catch them soon because once everybody has a couple of them, the raids will be tougher because hardly anybody will care again. Apart from me and Chris, of course.