Deer oh Deer!

We have deer that live in the green belt behind and to the side of our property. They come into our yard when the weather warms up. Yesterday was the first day we've seen them in the yard this year. Aidan took this picture through the window. We've been working on animal sounds with Mason and when he saw the deer he started going "Mooo Moooo Moo" Ha ha - it was so funny - what sound does a deer make? We couldn't correct him because there's no sound we know of to teach him for a deer.