Which One of the Following Was Not an Observation Made

Which one of the following was not an observation made in studies of the decline in populations of the American redstart,a migratory songbird?

A) Breeding bird studies show that the American redstart has declined by 50% in the decade of the 1970s.
B) American redstarts overwintering in mangrove and wetland forests of Central and South America accumulate little ¹³C in their tissues.
C) Dry scrub forest is substandard overwintering habitat for the American redstart in Central and South America.
D) American redstarts overwintering in mangrove and wetland forests are heavier as they begin their return migration north in the spring.
E) American redstarts with high levels of ¹³C in their tissues arrive earlier at their summer breeding grounds.