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 response aligns with the idea that larger size may provide a physical advantage against predators, making it more difficult for them to successfully capture and kill the larger birds compared to their smaller counterparts. While visibility can also play a role, in this context, the physical size likely contributes more significantly to their survival advantage. The options regarding population size do not directly address the observed predation dynamics.