The bird really isn't bad at all. Flight is great to have, and it actually has three primary natural attacks, which is actually very good. The base damage is actually superior to the small cat's (though small cat has trip, better str, and much better dex). The 4th level increase is teh suck, though, I would trade it out for something else at that point or at the very least by effective druid level 7, when all the animals have gotten their upgrade.
The Wolf starts off kinda bad, but with the level 7 changes it becomes a viable choice for a Spring Attack type attacker, and does well with Power Attack as well.
I still think I like Roc the best of all the Core Rule Book + Bestiary I options, though. And it's actually pretty dang good from the start, just sorta fragile with Con 9. Three good nat attacks and +5 nat armor aren't bad at all combined w/ flight, though.
[sblock]Rocs taken as animal companions by druids or rangers are typically newly hatched birds—a baby roc is the size of a person and ready for flight and hunting within minutes of hatching. Unfortunately for druids seeking animal companions of legendary size, an animal companion roc is limited to Large size—still large enough for a Medium druid or ranger to use the flying beast as a mount.
Starting Statistics: Size Medium; AC +5 natural armor; Speed 20 ft., fly 80 ft.; Attack 2 talons (1d4), bite (1d6); Ability Scores Str 12, Dex 19, Con 9, Int 2, Wis 13, Cha 11; Special Qualities low-light vision.
7th-Level Advancement: Size Large; AC +3 natural armor; Attack 2 talons (1d6 plus grab), bite (1d8); Ability Scores Str +8, Dex –2, Con +4.[/sblock]
I'm not sure if it applies w/ PF's grapple changes, but one fun thing to do in 3E with flight and Grab was to take the victim on a merry little ride up into the skies and...let falling damage do its thing.