From Northern Oriole to Baltimore and Bullock's
A Split Decision
Dr. James D. Rising is a professor in the Department of Zoology at the University of Toronto. He is an authority on Baltimore and Bullock's Orioles, and co-author of the accounts for both species for the most
important current work on American species, The
Birds of North America. Dr. Rising has conducted careful studies of how several
related species pairs of eastern and western birds hybridize on the Great Plains, and specifically studied orioles
there during a 30-year research project. He currently serves on the American Ornithologists' Union Committee on
Classification and Nomenclature. Because he is the recognized authority on the subject, we asked him to explain
to us at Journey North exact ly why ornithologists used to think Bullock's and Baltimore Orioles were the same species,
called the "Northern Oriole," and why they changed their mind. Dr. Rising explains:
First things first: What Is a Species?
"To answer your question we first must ask, "What is a species?" The simple answer is "Two
kinds of plants or animals that are different." But, of course, it really isn't as easy as that. You might
ask, "What do you mean by different?" and that would be a good question. Usually it means that they look
different. Eastern Bluebirds and American Robins look different, and they are different species. However, male
and female Cardinals also look different, but they are not different species. They are simply different sexes of
the same species. So there must be more to it than that. Biologists, therefore, often define a species as a group
of individuals who cannot mate with individuals of another group (species) and produce healthy young that will
be able to reproduce when they are adults. For example, the Hooded Merganser and Common Goldeneye (two kinds of
ducks) sometimes hybridize, but the young that they produce almost certainly cannot breed and produce young. Also
matings between these two ducks are very rare. Thus, they are not considered to be the same species."
Back to the Oriole Question
Audubon's Oriole Paintings
Dr. Rising continues, "Why have scientists' ideas on whether orioles are one or two species changed over time?
About 50 years ago people found that in the western prairies there was a zone where both Baltimore and Bullock's
Orioles are found. Many individuals had one parent that was a Baltimore Oriole and one that was a Bullock's Oriole,
that is to say, they were hybrids. These hybrids appeared to be perfectly healthy and they themselves mated
with other orioles and produced healthy young. The scientists reasoned that because of this hybridization in time
the two kinds of orioles would blend into one single kind of oriole. Therefore, they combined them into a single
species, and named it the Northern Oriole.
When John James Audubon painted these birds in 1836 and 1825, he considered
them to be two different species. For about three decades starting in the late 1960s, many ornithologists thought
they were the same species and "lumped them" into the Northern Oriole. Now they've "split"
them back the way Audubon thought they were all along.
Bullock's Oriole male
Baltimore Oriole male
"However, other scientists revisited this "hybrid zone" in the prairies over a period of 30 years,
and most recently in the 1990's. What they found was that yes, indeed, there are still hybrids there, but at all
localities there were also individuals of the pure Baltimore and Bullock's Oriole types. In other words, there
was a good reason to believe that hybrids existed only in a limited geographic area, and that the two kinds of
orioles were not going to blend into a single type of oriole. Therefore, they decided to recognize both species
Additional Supporting Information
Dr. Rising's fascinating oriole research centered on the "zone of hybridization," and his work there
formed much of the basis for the American Ornithologists' Union's new changes. Other ornithologists also studying
orioles have contributed additional information supporting the AOU "split."
- Molt Timing: Baltimore Orioles typically molt their plumage in summer after breeding but before migrating.
Bullock's Orioles molt during migration and on their wintering grounds. One hybrid was found on its wintering grounds
undergoing a molt, as Bullock's normally do, but it was molting out of brand new feathers! It was molting a second
time after apparently already molting on its summer grounds. Molting takes a LOT of energy. Ornithologists believe
that perhaps one reason why oriole hybrids don't increase is because they don't survive or reproduce as well as
pure Baltimore and Bullock's Orioles do thanks to that wasted energy molting twice a year
- DNA: Recent studies of mitochondrial DNA (the DNA in the mitochondria of cells, which many scientists
use to show relationships among various animals) show the Baltimore and Bullock's Orioles are not even very closely
related! The Baltimore Oriole shows more genetic similarities to the Altamira Oriole (which lives in Mexico, northern
Central America, and a tiny part of Texas) and especially to the Black-backed Oriole (from Mexico). Bullock's Oriole
is more closely related to the Streak-backed Oriole (from Central America, Mexico, and rarely southern Arizona and
Copyright 2001 Journey North. All Rights Reserved. Please send all questions, comments, and
suggestions to our feedback form