When mounted on a bird, a geolocator records light intensity data at intervals throughout each day. Retrieved later, the data can be downloaded and migration routes, pace, and destination can be determined. For species with broad breeding and wintering ranges, this can provide important information on exactly where particular breeding populations spend the winter. Given that we have long-term Breeding Bird Survey data indicating that populations of some species are declining in particular regions of North America, figuring out where these birds spend most of the year helps us identify what may be causing their declines, and helps us devise more effective conservation measures.
This fall, RRBO is placing geolocators on a sample of summer resident Gray Catbirds. This project is a joint effort between RRBO and Dr. Melissa Bowlin, who joined the UM-Dearborn faculty late last year and who is a research partner of the Environmental Interpretive Center. Dr. Bowlin's interest is in how physical traits of birds influence their migratory performance. In our catbird study, she will be looking at how the wing shape of individual birds impacts the pace of their migration. The geolocator data will allow us to determine the time it takes for each of the birds to reach their wintering destination, and hopefully where the birds stop each day, so we can calculate the rate of migration for each bird.
RRBO is interested in the approximate route followed by catbirds breeding in this area, as well as where they spend the winter. Previous data from other researchers have indicated that Midwestern catbirds winter in Central America, whereas catbirds Mid-Atlantic nesting catbirds winter in the southern U.S. and the West Indies. This information is based on small sample sizes: recoveries of 17 banded birds and 6 birds with geolocators. Our data will add to this knowledge.
The geolocators are tiny, and are carefully fitted on the birds with a flexible harness that loops around the bird's legs. The total weight of the geolocator plus the harness is less than a gram, or about 2.5% of the weight of an average catbird. The recommendation for any kind of radio transmitter or attachment to a bird is 4% or less. The geolocator rests just above the bird's rump. Dr. Bowlin has done careful studies, published in the journal Methods in Ecology and Evolution, determining the effect of drag and weight of various types and configurations of geolocators, using preserved bird bodies in a wind tunnel. The set-up we are using is the best available configuration to minimize these effects.
|A geolocator, attached with a leg harness, above the rump of a
catbird at RRBO. The little wand pointed toward the tail is
the part with the light sensor.
|When the feathers are smoothed out, only the light wand
As you can see, the potential for these little devices to unravel basic life history mysteries is huge. The pioneering study with bird and geolocators was done by Dr. Bridget Stuchbury and published in the journal Science. The study looked at Wood Thrushes and Purple Martins, and is summarized here. Geolocators are being used to study Bicknell's Thrushes, Bobolinks, Red Knots, Ruddy Turnstones, and Golden-crowned Sparrows, among other species. Much more information on this technology, as well as the importance of what we can discover is available at the web site of the Migratory Connectivity Project.
We plan to offer donors an opportunity to sponsor these catbirds. To be included in our fall campaign, add your name to the RRBO mailing list.