https://phys.org/news/2018-08-crows-winter.html

This information can serve as an important baseline for tracking how crows' migratory behavior is affected by factors including climate change and urbanization. Urban "heat islands," as well as general warming trends, could lead more birds to shorten their migration and spend the winter closer to their breeding territory. "If you live in a place, usually a city, with a huge winter flock of crows, you are seeing migratory birds that came south for the winter as well as your local, year-round crows," says Townsend. "Personally, I find the sight of an 8000-crow roost exhilarating, but if they or their feces are driving you crazy, you can at least take comfort in knowing that most of them will disappear in early March."

"It is surprising how much remains unknown about the seasonal movements of most partial migrant species, and this is especially true for variability among populations," adds the Smithsonian Migratory Bird Center's Emily Cohen, an expert on migration patterns who was not involved with the study. "This kind of information about populations-specific annual movements is not trivial to collect, but is fundamental to understanding most aspects of the evolution and ecology of species."

                                                            
                                    
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