Your question is surprisingly relevant. According to the National
Audubon Society, the number of pigeons in Chicago rose 63% in 2003.
(http://www.bird-x.com/links/pigeonproblems.html) But back in the
1980s, there were a lot more.
Pigeons are also known as rock doves. According to a Field Museom
ornithologist quoted in the Chicago Tribune, pigeons originally came
from the cliffs and mountains of Europe, Africa, and Asia. They were
brought to the U.S. as pets about 400 years ago. Pigeons like downtown
areas because of the "cliffs" they find.
There are also a number of falcons and hawks downtown, but the number
of predators they face in the Loop is much lower than the number they
would face in the country.
* Feeding a pigeon in Chicago can get you a $500 fine.
* Rats eat pigeon guano, and feeding pigeons and thus helping to
propagate the species also helps the rat population.
* In 1945, Chicago considered killing its pigeons because they were
traced to some cases of pneumonia in humans.
* The National Science Foundation once gave a grant to Project
PigeonWatch, a program designed to bring parents and children together
to study the birds.
If you have a lot of free time, check out pigeons.com for stories
about blind pigeons, a campaign to end pigeon poisioning, and my
favorite, the "Pigeon Stories" page.
Search strategy: Google search for - pigeons Chicago