The Northern Oriole is better known to most people as the Baltimore Oriole, especially, no doubt, in Maryland, where it is the state bird and provides the name for the baseball team. Authorities have differed on whether it is the same species as the more western Bullock’s Oriole. The high point of unifying the two as one species may have been reached in the 1990s when hybridization in the plains states was thought to be common, but by the time of publication of the Audubon guide in the late 1990s, thought began to swing back the other way (1) and then seemed to be pretty well decided a few years later by publication of another good reference, namely, National Audubon Society: The Sibley Guide to Birds (2) by David Allen Sibley.
Here Sibley illustrated an example of a hybrid, although hybrids were considered less common than ten years earlier. The males of the two species differ in that the Baltimore develops an entirely black head, whereas Bullock’s has only a black throat and eye line. Both have bright orange breasts. The females are not as bright in appearance. The songs of both are whistled and being pidoo tewdi tewdi yewdi tew tidew in the former and goo gidoo goo peeka peeka in the latter.
The whole question of what constitutes a species is not settled although most biologists since the time of Darwin’s work in the mid-1800s have tended toward the “biological” species concept which emphasizes reproductive isolation as the main criterion. If the species hybridize extensively then they are not separate species, but, if they don’t, they are separate species. The two populations have historically had very different nesting ranges between east and west, but introduction of shade trees to farms in the plains states brought the two together, providing a test of whether speciation occurred or not. The consensus is swinging back to thinking it is not common enough to bring about extensive genetic mixing. For birds, plumage color and song are the communications most likely to keep them from mating. I will consider both species to make up the single species Northern Oriole for the rest of my discussion because the two have more in common than their few differences.
Another surprising fact about the Northern Oriole is that it belongs to the American blackbird family. This is one of those confusing things about common names. Europeans named birds in the New World according to, often superficial, resemblances to Old World birds. The Northern Oriole reminded them of the European orioles because of the orange breast, even though they are not closely related. On the other hand, the Northern Oriole has strong affinities with New World blackbirds, while the Old World blackbird is a thrush. Our Oriole then is placed among the Red-winged Blackbirds, Grackles, and the Cowbirds with the last mentioned having an unsavory reputation for laying its eggs in the nests of other species, a phenomenon called nest parasitism.
That is enough about confusing issues. Now, let’s get on to what we do know and admire about the Northern Oriole. Perhaps most notably, the nest is large and pendulous, carefully woven together and hanging from a tree branch. Few birds go to so much effort. The pair watch over 4-6 eggs with irregular dark markings on a gray background. Orioles feed on a surprising variety of unpopular insects including tent caterpillars and fall webworms, in spite of the strong, unsightly silk around the larvae. Orioles, however, do damage some fruit crops. Another species is known as the Orchard Oriole. Both species winter in the tropics; we welcome the beautiful and industrious orioles back again to nest in the spring.
- National Audubon Society: Field Guide to North American Birds, Alfred A. Knopf, Inc., New York, 1998, by John Bull and John Farrand, Jr.
- National Audubon Society: The Sibley Guide to Birds (Alfred A. Knopf, Inc., New York, 2000)