The Eurasian Collared Dove were thick as flies over in the Netherlands when I lived there (95-99). Used to tick me off as I couldn't whack them. First ones I saw here was I think was in 2005-6.

Up here in NE, those doves are in places thicker than Mourning doves. But their niche is slightly different. Seems the collared doves like the same as what pigeons prefer. I could shoot a limit at the edge of Fremont anytime...