I have seen them in different tall trees around the garden before — the last time in a pair. The males and females look exactly alike.
Today, this one was making only a noisy, repetitive croak, which drew me to look for it. But, previously, I have also heard them make a more melodious call, which I remember as sort of a loud cooing sound (a contradictory description, I know).
My soul into the boughs does glide;
There like a bird it sits and sings,
Then whets, and combs its silver wings;
And, till prepar’d for longer flight,
Waves in its plumes the various light.
— Andrew Marvell, from “The Garden“