Magpie in the sunshine

I recorded a video of a magpie sunning itself.  It fluffed up its breast feathers and crouched down into the grass whilst spreading its wings wide.  This behaviour is well known but this is the first time I’ve seen it.  When the magpie gets up after a few seconds it walks off with its beak wide open.  It doesn’t appear to be panting so I’m not sure why it is adopting this posture.  If anyone has a theory as to the significance of this behaviour I’d love to hear it.