Mine don't sing. My rooster sings that exact same warning call when they lay though. He stands outside the coop ramp/door and does it over and over and over until they come out of the coop. I think he believes they are in danger. It's also the same sound he makes when you are chasing him trying to catch him, he sees a 'predator' (10lb almost blind white cat) or when his #2 lady jumps off the 5 gallon waterer, tips it over, crashes into the metal pan and he almost has a heart attack.
