 There is a specific individual Wood Thrush (Hylocichla mustelina) whose place in ornithological history ought to be assured. Although this particular bird is a member of a flock, she is also an individualist and a trailblazer. She may be the very first of her kind to have returned to the flock's breeding ground, in northwestern Pennsylvania, from their winter range in the Honduran and Nicaraguan coastal regions of the Yucatan Peninsula, by a route utterly different from every other member of her group. It is possible that other wood thrush have done this before, but this particular bird is the very first one known with absolute certainty to have done so.
The other birds in the flock flew straight across the Gulf of Mexico on the first leg of their springtime route northward. Our special wood thrush, by contrast, totally avoided over-water flight, circumnavigating the entire Gulf, and remaining well inland for the entire journey. The direct route back to Pennsylvania is 1560 air miles, which the others flew in an average of 14 days. The lone wood thrush flew 2875 miles in a span of 29 days, of which 13 were spent in stopovers. (Actually, make that nights, for the wood thrush migrates only at night.)
How did we get such detailed information? How much of it is new to the world of ornithology? What might be the significance of the "pioneer" thrush's different course? Let's start with the second question first. The main migratory pathways of most bird species have long been known. It's the specific data, the exact location and timing of individual bird flight, however, that is break-through discovery. The data are called "exciting" by Peter Marra of the Smithsonian Migratory Bird Center, quoted by Thomas Maugh II in the Los Angeles Times for February 13, 2009.
The new information resulted from a study published in the journal Science for the same date, by researchers in the Department of Biology at York University, Toronto. Their study began in 2007 and was led by professor Bridget Stutchbury. She and her team accomplished what was previously impossible—tracking individual songbirds all the way to their specific tropical wintering grounds, and back. Songbirds are far too small for satellite tracking, radar images cover only a brief interval, and Stutchbury says that apart from one exceptional, four-day tracking of radio-tagged songbirds in Europe by an aircraft, the data heretofore has been limited to "opportunistic" sightings and recaptures of banded birds.
The Stutchbury team used a combination of old-fashioned field work and a device known as a "geolocator," consisting of a light-sensor and a recording microchip. This was the first use on small songbirds of a recently miniaturized version, weighing 1.5 grams, a twentieth of an ounce. The team selected two groups of birds from their study areas in northwestern Pennsylvania: 14 adult Wood Thrush—seven of each gender, and 20 Purple Martins (Progne subis)—11 males and 9 females. Each bird in both groups was banded for individual identification, a blood sample drawn for genetic and metabolic study, and outfitted with a geolocator mounted in a "backpack" held in place by teflon ribbon leg-loops. The geolocator has no telemetry, so the team had to wait until late spring of 2008 to begin data collection.
I've neglected the Purple Martins so far, as their group lacked any outstanding individualist comparable to the Wood Thrush who refused to venture over the Gulf. (I would too, so I can't help anthropomorphically identifying with what I would call her wise prudence. Stutchbury's speculation on that bird's "deviant" behavior last spring is more scientific, and we'll get back to that a bit farther on). The Purple Martins are only slightly larger than Wood Thrush, but their winter range is much farther south, in the Amazon basin of Brazil, and they migrate during daylight. A female Purple Martin definitely wins the prize for speed. On her springtime return from winter range, she flew 4,688 miles in just 13 days, four of which were stop-overs. That works out to 520 miles per flight day. Ornithologists had previously greatly underestimated small birds' flight performance during migration.
The study revealed other unexpected results. The birds have greater fidelity to their specific summer and winter ranges, and the winter range of the Wood Thrush was much narrower, than previously thought. All the birds flew back from winter range to the serious business of nest building and breeding a whole lot faster than they flew south in the fall—from two to six times faster, Stutchbury reports. She emphasized the importance of tracking individual birds, writing: "Alarming long-term declines of migratory songbird species in North America and elsewhere heighten the urgency of mapping migration routes and wintering locations with far greater accuracy… [and is] essential for predicting the impact of tropical habitat loss and climate change."
Let's get back to that one individualistic Wood Thrush. The report labels her WOTH(920F), which I'll condense to "920." Why did 920 take that overland route? Maugh reports that Stutchbury ventured a speculation in their interview, that 920 may have sensed a lack of sufficient body fat to fuel the long over-water flight, signifying perhaps a shrinking food supply in the tropical Yucatan. The record shows that she did make it back to the breeding grounds, but three weeks later the others. She took a much longer route, of course, but also rested almost as many nights as she flew. Despite the delay, she found a mate, and laid her first egg of the season on June 8th. Has Wood Thrush 920 enjoyed a more nourishing 2009 winter down in the Yucatan? In a few short weeks she will presumably start back to her breeding area in Pennsylvania. Will she take the overland route again? The ornithologists are able to recapture only a fraction of the birds they outfit with the geolocators, so we can't be sure they will get a new report from 920. But let's keep a hopeful watch for it.
The Wood Thrush, Hylocichla mustelina, is a North American passerine bird. It is closely related to other thrushes such as the American Robin and is widely distributed across North America, wintering in Central America and southern Mexico. The Wood Thrush is the official bird of the District of Columbia. The Wood Thrush is a medium-sized thrush, with brown upper parts with mottled brown and white underparts. The male and female are similar in appearance. The male has one of the most beautiful songs of birds in North America. The Wood Thrush is an omnivore, and feeds preferentially on soil invertebrates and larvae, but will also eat fruits. In the summer, it feeds on insects continuously in order to meet daily metabolic needs. It is solitary, but sometimes form mixed-species flocks. The Wood Thrush defends a territory that ranges in size from 800 to 28000 square meters. The Wood Thrush is monogamous, and its breeding season begins in the spring; about 50 percent of all mated pairs are able to raise two broods, ranging in size from 2 to 4 chicks.
Trackback(0)
 |
TrackBack URI for this entry
Subscribe to this comment's feed
Show/hide comment form