I don't understand the question

7 pigeons sitting on the branches,  5 pigeons under the tree

=12 in total

Step-by-step explanation:

Upper branch has x pigeons, lower has y.

Top branch said: "If one of you flies up to us our number will be double yours":

Well, if one of the bottom branch flies up, the top branch will have x+1 birds and the bottom will have (y−1). The top will have double the bottom, so:

x+1=2(y−1)

Top branch said: "If one of us flies down to you, our numbers will be equal":

Now, the top branch will have x−1 and the bottom y+1. The numbers should be equal:

x−1=y+1