Um …? I think birds survived because they could FLY to where the food was!
And possibly because they could hide from a burning sky.
Pretty much every land animal that survived the Chicxulub impact was a burrow dweller/nester.
Nothing much larger than a house cat made it through.
Given that, I’d guess the surviving early birds were like modern puffins, or burrowing owls, to name only two of the numerous modern examples.
I think the complexity of 3 dimensional travel vs. 2 D. resulted in advanced brain development as the 3 D travel became more developed, a feedback loop situation.