I was getting mad anxiety watching that egg balance over the edge like that

1:07 mother looking at the camera like you see what i have to deal with here?

For a second I thought she realized it was an imposter and was gonna kick it out saving her actual babies and egg, but sadly not this time

This is first time I see birds do something about cuckoos behavior and not only watching it.

that was most likely a cuckoo baby with his instinct to back-push out other eggs and baby birds..

All she's done is postponed the tragedy for another day.

The cuckoo bird’s instinct to dispose of a rival is so strong that it will eventually succeed to push out that egg when the parent isn’t there.  After all, it was doing that while the parent was present!

Wow rare documentary footage of my sister in law in action. Didn't know it could be caught on film like that 👌🏽

Cuckoo chicks usually hatch before the mother birds original eggs with the purpose to grow quicker and stronger to push the eggs or chicks out. On top of this a baby cuckoo usually has a brighter and larger mouth to cause the mother bird to feed it more

Luckily the siblings are seemingly older and stronger than the parasite cuckoo. Hopefully the mother is already giving less food to the alien chick. The rest egg seems to be never born now.

And he still working on get the darn egg out 😂

Also sad that she fed the invasive one which was more insistent about asking for food than her own chicks.  Nature's rough!

For a second there is almost looked like Mom was thinking to herself that something is off. Maybe wondering why one of her babies looks different

The Cuckoo will try again tomorrow.

I hope we get updates on this. Echoing other commenters I think that's a cuckoo and it will probably continue trying to get rid of the other chicks/egg.

That's the cuckoo chick, a parasitic bird.  You see how it looks subtly different from other chicks.  Its behavior is to try to get rid of the host's eggs and new hatchlings to eliminate food competition.

She keeps looking at the camera, obviously concern of what it is.

Wow, what a close call.  Mom grabbed the bum of that cuckoo just in time.  But she still feeds him!

Mom : save the egg...
Cuckoo baby : not for long mom...🤣🤣

The chick that tried to push the egg out of the nest was a cuckoo, 100% sure.