Donk-donks are funny that way! Our horses will nicker at us when we approach, most of the time. They only neigh at us when it's feeding time...
Now, when we had our ba-donk-a-donk, he would bray whenever he saw us, as well... And, he'd be headed our way when he did! He also brayed at intruders, visitors...
I have to admit I love that sound. Whenever he'd bray, I'd have to giggle.

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"Met a girl in a chair with wheels, but no one else would see her...
Met a girl in a chair with wheels...
Everyone was so afraid to even look down on her,
And she just spread her little wings and flew away..." - Big & Rich