re: What type of ducks are these?

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Aren't they invasive and take over wood duck boxes?

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I don't think these are cavity nesters. Not these in the pics anyway. Not sure about the other tree duck though (I think those are called Fulvous Tree Ducks). About the only other duck that I have seen take over a Woodrow box is a Hooded Merganser.

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Aren't they invasive

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They are invasive in the sense that they are expanding their range (i.e. natural invaders). They aren't an invasive that was released by man, whether intentionally or unintentionally, like many other detrimental animals and plants.

Edit: Inca Doves, White-winged Doves, and Cattle Egrets have all invaded naturally, to my knowledge. The two dove species are exhibiting the same sort of eastern expansion as Black-bellied Whistling Ducks are showing.