When it comes to variety of color and pattern, no other free-living bird compares with the feral pigeon. Most or all of these plumages are represented in large city pigeon flocks. Feral pigeons often choose mates based on plumage color or pattern.
Studies show that ferals often choose mates based on their color patterns.
This type of breeding, known as assortative mating, varies by location, and has helped perpetuate the various color types commonly seen throughout cities. Since 1994, Cornell University's Laboratory of Ornithology has run a program called Project PigeonWatch, which aims to survey regional differences in the prevalence of different color types of North America's pigeons. Still in the data-collection phase, Project PigeonWatch enlists the help of school groups and other participants who monitor local pigeon flocks and submit tallies to Cornell twice per year. Their coverage provides data from all across the continent that would otherwise be nearly impossible for the ornithologists to collect by themselves. I joined Project PigeonWatch earlier this year, and, armed with my pigeon identification chart and tally sheet, casually surveyed local flocks.