The most credible explanation for the higher rate of survival of the large birds is:
The large birds may be the hardest for predators to attack.
This reasoning follows from the observation that larger animals often have physical advantages, such as greater strength or size that makes them more difficult for predators to handle or overpower. The other options either do not logically contribute to survival rates or contradict the established facts (e.g., the size of the group does not directly correlate with survival in this scenario).