This is anti-supported. We know the smaller blackbirds were generally younger than the larger ones, and the smaller blackbirds were more likely to survive. That suggests that there is a relationship between size and age among the surviving birds. The smaller ones tend to be younger.

The author does not suggest that larger blackbirds of the same age as smaller ones are more likely to survive. Only that the smaller ones are not more likely to survive. But the chances of survival among larger and smaller birds of the same age could be equal.

The author suggests that among blackbirds of the SAME AGE, size wouldn’t indicate a difference in survival. But this does not imply that among blackbirds of the SAME SIZE, age doesn’t indicate a survival difference. In fact, the author suggests younger birds are more likely to survive than older ones.

This is strongly supported, because the author says size is not a determinant of a blackbird’s chances of survival over a month-long period. In other words, you can’t assume that a smaller bird automatically has a lower chance of survival than a larger bird simply due to its size.

The stimulus doesn’t suggest what would happen with a larger sample of blackbirds. If we know that a larger sample of blackbirds would involve smaller birds that are more similar in age to larger birds, then something like (E) would be supported. But we don’t know that a larger sample of birds would make the ages more similar between smaller and larger birds.