Head of a Macaw parrot bird.
It is a Green-winged Macaw, also called Red-and-green Macaw
This is one of two Macaw parrot birds that can fly free and are trained to come back to their owner at a voice signal. The birds owner often let them fly around Dolores Park in the afternoon, when it's not too windy. It's amazing to see those beautiful rainbow-colored pet birds fly around and come back to land on their master's shoulder.
For more information about Red-and-green Macaw birds, read en.wikipedia.org/wiki/Red-and-green_macaw.
Photo taken in Dolores Park (San Francisco)