Tree Martins

I am almost certain that these tiny birds, seen way up high on a dead tree in the middle of Lake Nuga Nuga, are Tree Martins (Petrochelidon nigricans). However, the specimens in my pictures have yellow beaks, while all other pictures of Tree Martins I can find (including my reference books) show them with dark beaks. Thanks to JeffCrocombe for confirming the ID as juvenile tree martins.