The most credible explanation for the higher rate of survival in 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 may provide a physical advantage in avoiding predation, as larger birds may be more difficult for predators to capture and handle compared to their smaller counterparts. While visibility could be a factor, the ability to escape or resist attack due to size is a more direct and plausible explanation for their survival.