How climate change may be pushing the Orchard Oriole farther north
Published: June 25, 2010
|In the spring of 2004, I surveyed breeding birds, including Orchard Orioles, in Fond du Lac County in south-central Wisconsin. After checking range maps and other data, it occurred to me that the seven territorial males my colleagues and I had found in a relatively small area might have been on the leading edge of a range expansion induced by climate change. |
The oriole breeds east of the Rockies and south into central Mexico in gardens, orchards, suburbs, farms, and other spots with scattered trees. At one time, its stronghold had been in the south, including Arkansas, Mississippi, and Louisiana.
Lately, the long-running Breeding Bird Survey, in which participants tally birds along established routes, has shown that the Orchard Oriole is becoming more abundant in the northern and western portions of its range: Montana, Colorado, North Dakota, southern Manitoba, Wisconsin, southern Ontario, and Vermont. Furthermore, the data indicate that local populations in Texas, Oklahoma, and other southern states have declined in recent years.
The oriole is not the only bird whose range is shifting as the changing climate pushes acceptable habitat to more northerly latitudes. Blue-gray Gnatcatcher, Hooded Warbler, and other birds are nesting farther north. (For more examples, see “Climate change and birds,” June 2007.)
What does an expanding breeding range mean for migrations? First and foremost, birds must fly farther than ever.
Orchard Orioles winter from central Mexico to northern Colombia and western Venezuela. Now that nesting occurs in southern Manitoba and central Wisconsin, it’s likely that a portion of the population is migrating 100-300 miles farther than birds did 30 years ago. That’s roughly a 10 percent increase in distance. To you and me, that may not sound like a lot, but to a bird it means it must add 10 percent more fat reserves for the journey. To deposit the extra fat, an oriole requires more feeding time — from several days to as much as two weeks.
In search of good habitat
We do not know the mechanism by which northward range expansion occurs, but several scenarios are possible. Because fledglings disperse outward from their natal sites, young birds may sample habitats north of the place they hatched. They might then return to areas where they found acceptable habitat and available territories in which to nest.
Another possibility is that second-year birds return in spring to natal sites and disperse outward until they find suitable nesting habitat. In a single year, the change might not be noticeable, but if it’s repeated over several years, the northward range expansion would become obvious to birders.
Orchard Orioles are more social than most Neotropical migrants. They are observed in loose flocks during migration and winter. (Whether they migrate in flocks at night is unknown.) And although it’s most common for orioles to select widely scattered nesting sites, the species also nests close together at times. Observers have reported four, six, and even 20 nests in a single tree. Their social tendencies remind me of their relatives the Bobolink and blackbirds.
The oriole’s social disposition and the natural wanderings of young birds may promote the colonization of new areas. In fact, observers of pioneering populations in Vermont and Manitoba have said that younger birds accounted for about half of all new nestings.
One of the more distinguishing aspects of the Orchard Oriole’s annual cycle is that it migrates later in spring and earlier in fall than most other songbirds, including other orioles. Birds begin to leave their wintering sites in March, although some remain as late as May. They arrive on territories in southern Missouri in the third week of April and in South Dakota in mid- to late May.
The young fledge by mid-July, and males begin to leave by late July and early August. Few males remain after mid-August. They have been reported in central Mexico as early as mid-July. In contrast, Baltimore Orioles fly south about three or more weeks later. Female Orchards and fledglings gather in flocks of up to 25-30 birds and remain together in the breeding area for an additional four to six weeks before starting their southward migration.
Until recently, scientists believed that the birds simply flew to their wintering range. Last October, however, Sievert Rohwer, professor emeritus of biology at the University of Washington, and other researchers added a new twist to our understanding of migration. They found that Orchard Orioles and other songbirds squeeze in a second breeding season during a stopover in western Mexico.
The birds nested in thorn forests of coastal Sinaloa and Baja California Sur. Rohwer noted that Orchard Orioles might raise a first brood in the midwestern and south-central U.S. and a second on Mexico’s western coast, yet both sets of offspring find the same wintering area in Central America. The question is how both groups find the right place, since they must travel in different directions.
Rohwer’s discovery of migratory double-breeding in southbound birds shows that we still have much to learn about birds. In the face of climate change, in particular, it’s important for students of bird migration to ask good questions.
I’d start with two: Will birds be able to adapt to changing conditions fast enough? And can they maintain their populations as climate change accelerates?