I never noticed the blue patches on the Mallards’ sides before. They are called the speculum. Their wings usually cover them up. This afternoon for some reason both the male and the female Mallards, while dabbling for their dinners, had the brilliant blue patches on their sides, toward the rear, exposed wide open, and as they turned in the sun, the patches lit up brightly. Then, after a while, they paddled off with their wings covering up this hidden feathery gem. Check out this short video on YouTube.