Shadow has started frothing on walks this spring. It's something Willow has always done, but I've very rarely seen Shadow do it before. There are obviously exciting smells around at the moment
I'm pretty sure it's not that, because there's no frantic sniffing in the same spot, and he's ambivalent about walking right by a local bitch who is currently in season. He doesn't even seem to be sniffing any more than usual. We are doing quite a bit of hunting with him at the moment, though, trying to get that nose working. Compared to his sister, he's a bit rubbish at scenting things. I swear Willow is part bloodhound.
This is the same with Twiglet and her brother, Rossi. Twiglet's nose can find anything, anywhere - Rossi's sense of smell seems very poor in comparison.
Charlie does this all the time - he often can't take a treat when he does this and his jaw is still juddering (lovely description!). It's obviously a very intense experience!