Purple Martins gather before heading south.
Photo by nature artist Toni Kelly
I spent a few hours last week with Tom Somerville of the state Department of Environmental Conservation looking over a Niagara County meadow for short-eared owls. We saw no owls that evening, but the project Somerville is working on with Chuck Rosenburg has found and mounted communication backpacks on over a dozen birds of this endangered species. The satellite-bounced transmissions from those backpacks will provide more information about the migration of these rare birds.
During the previous winter just one owl was similarly armed and its beamed messages showed that it wandered southwest to Ohio before heading back north to its breeding grounds in Labrador. With the information gained this year, we should learn if other owls follow a similar path.
The communication devices mounted on the owls weigh only a few ounces and don't hinder the birds' flight in any way as last year's odyssey confirms. They are another result of the remarkable miniaturization of our modern technology, but these devices appear to be only a step along this progress. Professor Bridget Stutchbury, a biologist at York University in Toronto, has attached still smaller devices to the backs of two species of songbirds and recorded their migration histories.
The light-level geolocators Stutchbury uses weigh only about one-twentieth of an ounce and are approximately the size of the nail on your little finger. Resting on birds' backs above their hips, the sensors are held in position by loops running around each leg. Unlike the transmitters attached to the owls, these devices accumulate information based on light collection that is only interpreted when (and if) the bird returns and is recaptured.
Until now information about the migration of individual birds was only obtained through banding. Audubon began this practice by attaching tiny wires to the legs of a pair of phoebes. When he recaptured them the following year, he proved that the birds returned to their previous home.
Today hundreds of bird banders like Betsy Brooks, Jerry Farrell, Paul Fehringer, Dave Junkin, Tom LeBlanc and Bob McKinney contribute to our knowledge of birds by trapping them, attaching the small metal bands to their legs and releasing them. Initial captures not only tell us about occurrence of the birds but also provide information about their condition. Recaptures add information about longevity and migration.
But the number of banding returns is very small. Bird mortality is exceptionally high. Even if they avoid hawks, do not succumb to disease, and don't fly into buildings during migration, they only live a few years. For these and many other reasons the return rate is far less than one percent.
Even with the returns accumulated over time, banding records only give us isolated bits of information. A bird banded near Buffalo and recaptured a year later near Binghamton tells us nothing about where that bird was between those two dates.
Stutchbury's monitoring gives us exactly that kind of information. In 2007, she and her colleagues attached geolocators to 14 wood thrushes and 20 purple martins in northern Pennsylvania. Then in 2008 the team recaptured 5 thrushes and 2 martins.
In her Science report, Stutchbury tells us: "Rapid long-distance movement occurred in both species, and prolonged stopovers were common during fall migration. Both purple martins flew south to the Yucatan Peninsula in 5 days (300 miles/day) and then had a stopover of 3 to 4 weeks in the region. Four wood thrushes spent 1 to 2 weeks in the southeastern United States before crossing the Gulf of Mexico, and two stopped on the Yucatan Peninsula for 2 to 4 weeks before continuing.
"Overall migration rate was 2 to 6 times as rapid in spring than in fall. One female martin left the Amazon basin and flew about 4700 miles in 13 days (359 miles/day). Nine days involved migration flights and 4 days were spent on stopover. Most wood thrushes returned to their breeding grounds in only 13 to 15 days (145 to 168 miles/day). One wood thrush did not cross the Gulf of Mexico on spring migration and took 29 days to complete the 2900-mile overland route. Previous studies appear to greatly underestimate the true flight performance of migrating songbirds because spring migration speed has typically been estimated at under 95 miles per day."
Of what value are these studies? Stutchbury responds: "Alarming long-term declines of migratory songbird species heighten the urgency of mapping migration routes and wintering locations with greater accuracy than is currently possible with stable isotope analysis.* Tracking individuals to their wintering areas is essential for predicting the impact of tropical habitat loss and climate change. Survival estimates can now be obtained from regions where individuals from a specific breeding population overwinter, improving our understanding of how wintering versus breeding threats drive population fluctuations of migratory songbirds.-- Gerry Rising"
* For information about the use of stable isotope analysis for ecological studies, see this U.S. Geological Survey site.