Great tit chicks are vomiting things up

I have a bird box that has a brood of 8 great tit chicks in it. They are almost ready to fledge by the looks of things, but starting last night I noticed that every now and again one of them vomits up a 'package' of something. It looks like poop, but I dont know why it would be coming out of their mouths. I haven't seen any evidence of them swallowing poop that they later regurgitate (although I could have just missed it). 

I put a video clip of it happening here:

https://youtu.be/dGQLvgHPnp0

Does anyone know what it might be?

  • I'm guessing a bit here. It may simply be they are coughing up indigestible bits of what they are being fed - like the hard shells or wings of insects. We know owls cough up pellets of things like fur and bones which they can't digest.

    PS I'm pretty sure I've seen Robins coughing up pellets.

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    Regards,Tony

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