How do budgies know that there eggs r no good?

Question

How do budgies know that there eggs r no good?

Answer 1

They don't!  They will incubate even plastic eggs of the appropriate size.  Eventually they'll quit setting if the eggs don't hatch.  You can "candle" the eggs to see if an embryo is developing in the egg.  Get a small bright penlight, turn out the lights and hold the budgies egg against the light.  If an embryo is growing, you'll be able to see blood vessels early and darkening of the contents as the embryo grows.

Answer 2

They don't. A broody ( ready to incubate Hen ) will sit on even rotten or fake eggs.  A nestbox is required as budgies are tree cavity nesters. Sometimes a hen won't be interested in sitting on her eggs but many are. Single ( without a mate females ) of course their eggs won't be fertile and you can  dispose of them. So good luck.