The one simple observation that can be made in nature relating to brain size is that brain size tends to matter in terms of exhibited intelligence. Except crows are an excellent counter example.
Or maybe not. Their intelligence can also be explained as an example of a hive mind sharing and correlating information, just as a flock of starlings wonderfully synchronize. Importantly the flock processes the information and then directs individuals to react.
They have been shown to solve difficult problems and to react as a team. We do call the flock a murder of crows for some reason and this may well have inspired Hitchcock's 'the Birds'..