A bird is perched on a line of some sort attached to a pole.
A bird is wallowing on a pole.
Being perched on a line is not the same as wallowing on a pole.
The bird is on a line attached to a pole, not on the pole itself. Sentence 1 doesn't indicate the attitude of the bird.
A bird perched on a line cannot wallow, because wallowing is a process done in dust or dirt.