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 supported by the fact that larger size may provide an advantage in physical defense against predators, making it more difficult for predators to capture and kill them compared to smaller birds. While it's possible that their size could also affect visibility to predators, the key factor here is their ability to escape predation based on their size relative to the attack abilities of the predators.