There was a good article in the Denver Post this morning on how Eurasian Collared Doves have moved into every county in Colorado. 
Although they make a nasal honking sound, Collared Doves are beautiful birds. The problem is that they seem to be competing with native Mourning Doves. (By the way, in Natural Resources/Ecology we don’t refer to animals as “good” or “bad” as the Denver Post link does. We may or may not like what they do to an ecosystem, but that doesn’t make a species inherently positive or negative. It just is.)
Collared Doves can be white to tan to grey, but all have a dark ring around their necks.
Mourning Doves are large grey-tan birds, with three dots on their wings.
Here is a picture of a Collared Dove on the left and two Mourning Doves on the right. In this picture, they are trying to ignore each other, but I have seen the Collared Doves chase the Mourning Doves around the yard.
The two species don’t have exactly the same niche. We’ve had Eurasian Collared Doves in the backyard all winter. In contrast, the Mourning Doves just returned on Saturday. It will be interesting to see what other differences in niche emerge.
The Denver Post article does make a nice point at the end that Cooper’s hawks are increasing in numbers and will certainly take advantage of these large meals on wings. https://coloradogeography.wordpress.com/2013/10/28/some-loose-some-win/