Dear student
When birds are sitting on a single line, they are safe. Even if the line is a high voltage one, there won’t be any potential difference between the line and the bird, and current cannot flow through the bird’s body. But, if the bird is sitting on a line and touches the other line, the bird will get fried. This is because, in this case, there is a potential difference between the line on which the bird is sitting and the other line which it touches.
Regards