In Yasukawa' s month-long study of blackbirds, the percentage of smaller birds that survived the duration of the study exceeded the percentage of larger birds that survived. However, Yasukawa' s conclusion that size is a determinant of a blackbird' s chances of survival over a month-long period is probably mistaken, since smaller blackbirds are generally younger than larger ones.
The statement "Among blackbirds of the same age, a difference in size may not indicate a difference in chances of survival over a month-long period" is supported by the statements above.