Why are my robins laying on the ground?

Yesterday I saw my favourite baby robin that feeds from my feeder laying on its stomach on the ground, with its wings spread... this is really near a window and I feared it had been stunned... but i'd been in the room and heard nothing, and when another bird went nearby it jumped up, absoultely fine!

Today, after what looked like playing under the feeder between two baby robins and one of the parents, the parent and one baby flattened themselves to the ground and spread out their wings.

What are they doing? Are they sunning themselves? Or is it some sort of submissive behaviour (I'm sure I have more than 2 [adult] robins in my garden, they seem to be everywhere!) Maybe they're resting their tummies from all the delicious food they've eaten? Or is something not agreeing with them and they're getting sick?

I'm not sure what other information you might need, but if you ask i might have it somewhere in my brain!

Thanks! :)

"Watch with glittering eyes the world around you, because the best secrets are always hidden in the most unlikely places. Those who don't believe in magic will never find it."