The alpha is used feather’s png alpha channel.
and it is 2 UV channel for the feather.
The feather - Color :
uv1_golden eagle_color.png “use uvmap”
The feather - Normal :
uv2_golden eagle_normal.png “use uvmap.001”
The feather - Alpha :
uv2_golden eagle_normal.png( alpha channel ) “use uvmap.001”