The team found that closely related birds that share the same habitat tend to look and sound different.
This evolutionary rule of thumb seems to help birds to identify members of their own species.
Dr Paul Martin presented the findings at the First Joint Congress for Evolutionary Biology in Ottawa.
He and his team studied 250 bird species throughout the world. With the help of the Macaulay Library of birdsong, they were able to compile a database of where the birds lived and what they looked and sounded like.
"We found, repeatedly, that birds that [live in the same location] with a close relative have more divergent songs and colour patterns," he told BBC Nature.
"So it looks like costs of sharing a location with a relative drives some of the amazing variation we see."