How do you catch only a yellow and black finch?

I am a bird researcher and I love birds I need to catch a yellow and black finch only. I have a bird cage so I can look at them. I tied a string on the door of the bird cage and put the string over a tree branch and pulled it and it opened they go around it but not in it. also when I let go of the string the cage door closes how else can I catch a bird without spending money?