How do you catch starly in Pokemon HeartGold?

you can catch starly in pewter city in a certain tree by using headbutt after obtaining the national dex
In Pewter City you have to use headbutt on a certain tree. To get to this tree go down to the 'city limits' and around the fence on the left then headbutt the tree 2nd from top. It might take a couple of tries but you should get it after a while.