wcaptain

Interest to know whether these 3 Koels are brothers and sisters. Or they are from different parents.

It is supposed that the newly hatched Koel will quickly kick out eggs of their froster parents (whose eggs are smaller). In this case, they may be able to recognize eggs of their species and did not kick out these eggs.

Anyway, thanks for sharing this observation.

wcaptain

Perhaps the no of eggs laid by cuckoo relates to the size of froster parents.

For the common cuckoo, the size of froster parents are usually like "Common Tailorbird", so one egg is enough, otherwise the froster parents are too exhausted.

For Asian Koels in HK, the froster parents are starlings/myna, so more than one egg is still ok and would not make the froster parents too exhausted.