My atiel has escaped! What can I do?

My 2 atiels have sat on the window ledge in my lounge for years with window usually slightly opened, somehow one sqeezed out, He flew about in circles for while but soon disappeared, can't see or hear him anywhere what can I do?? I'm in Kirkcaldy, Scotland, He is white Lutino, yellow head, orange ears. Myself and other bird are going crazy with worry, they have not been apart for 10years, he doesn't like it