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 is based on the idea that larger size can confer physical advantages, making it more difficult for predators to catch or overpower them compared to smaller birds. The other options, such as being harder to see or differences in population size, do not directly explain the observed difference in survival rates as effectively as the physical size advantage does.