Raven describes the legs in such different ways in the two quotations to manipulate the perceptions of the birds he is speaking to.
In the first quotation, when he refers to Gull as a "noble bird" with leg characteristics implied to be more impressive or dignified, he is flattering Gull to gain his trust and provoke jealousy against Heron. This is part of his scheme to incite conflict.
In the second quotation, he describes Heron’s legs as “scrawny,” which belittles her and heightens Gull's sense of superiority, hence encouraging Heron to be angry and defensive about her position.
By contrasting the descriptions, Raven effectively stirs up animosity between Heron and Gull, which allows him to enact his plan of stealing the fish while they are distracted by their argument.
Thus, the correct answer is:
Raven is describing Gull’s legs in one and Heron’s in the other.