Survival of the fittest has to do with ability to reproduce, not the biggest and strongest.
See video at Why our understanding of 'Survival of the Fittest' is wrong.
See also Why Birds Survived, and Dinosaurs Went Extinct, After an Asteroid Hit Earth
...and thus reproduce.Not that beaks guaranteed survival of the impact event. The duck-like bird Vegavis lived at the end of the Cretaceous and had a beak, yet there’s no indication that this avian survived. “Just having a beak was not enough,” Tucker says. Rather, it’s that birds with beaks and powerful gizzards capable of crushing tough seeds had an unexpected advantage that increased their chances of survival.