They must grow larger, for once to support a larger brain, though avian brains are said to be somewhat better per unit weight.
Perhaps island gigantism would do, but it'd still need a strong evolutionary pressure favoring (only!) intelligence for a very long time, which is unlikely. They also lack both arms and hands, and they're exceedingly unlikely to re-develop them.
IN fact, our own species were probably on the brink of extinction several times as we lived in a dangerous savannah-niche lacking the strength and agility to compete with our hairier cousins while also lacking the speed and endurance of typical Savannah animals. Simple evolutionary pressure at that point dictated that our only path were to further develop our tool usage, if we gained muscle (strength and speed) we would lose stamina, if we gained stamina, we'd lose strength.
This is going to be tricky.