On a hot (35 degrees C), stuffy, windless day, I saw a phenomenon that I had never seen before. The birds sat in silence with a wide open beak, sometimes slightly raising their wings (a female sparrow, a robin and a crow in the photo). The crow rested in this way for about a half an hour after a long gnawing of the bone, the sparrow parents did this after each arrival with food for about a half a minute, and the robin and the starling flew in from somewhere and sat in the shade also for a while. I think that birds did this for thermoregulation, in order to save themselves from overheating, just like dogs do in hot weather, when they open their mouths and stick out their tongues. There was COOL to see the same behavior in different birds on the same day.
top: male and female sparrow;
bottom: robin, crow and starling.

Nikon Coolpix P900

Cropped, combined