Migration is the fun time of year to bird. Because the birds come to us, we can fill out our lists with species that we could not afford to go track down during the breeding season or winter. But something else gets me out and keeps me going out each fall. When I see a bird during migration, I can’t help but wonder where that particular bird spent the summer and where that particular bird will travel next. Will the bird have a safe journey and find a good territory in Mississippi, Panama, Argentina or wherever it is bound? Will that bird make it home again, nest, and contribute to the age-old gene pool of migratory birds?
Distribution maps can provide us with the geography of migration at the impersonal level of the species. But for two centuries, ornithologists have been driven to extremes to track the movement of birds, from their often surprisingly long daily foraging expeditions, to forays for assignations on their neighbor’s territories, all the way to journeys between hemispheres.
The time-honored method of banding and recapturing birds has worked well for tracking the migrations of certain types of birds—most notably waterfowl. It is a particularly effective approach for game birds because a high proportion of the populations are taken by hunters and bands are recovered at checkout stations. Furthermore, the movements of larger, more conspicuous birds, such as colonial waterbirds, can often be mapped by dying or otherwise conspicuously marking birds in a way that easily indicates locality of origin.
But for most birds, the resighting and recaptures are few and far between and the number of banding stations that handle large numbers of birds are insufficient to map migration routes comprehensively. And the areas in Central and South America where so many of our migratory birds winter have virtually no banding coverage
For smaller birds in particular, the search for ways of tracking individuals or at least connecting birds from particular breeding and winter populations, has taken a turn to the high-tech. These high tech approaches have taken advantage of three revolutions in twentieth century science: communications, nuclear physics, and molecular genetics.
Radio transmitters can be attached to a bird, with a harness or glue, and the location of the signal tracked from a distance with one or more receivers. Telemetry on small birds is limited because, although the miniaturization of electronic devices has allowed for decreasingly tiny transmitters, these tiny devices produce weak signals with a short battery life. Weaker and more ephemeral transmitters mean that you can only stay with a bird through a short period of travel and then only if you stay close. It is no wonder that telemetry has been used for daily movements over short distances for most bird studies, and for migration tracking for only a few large species of special concern, such as cranes and raptors.
The search for smaller and more powerful is ongoing, and researchers are even experimenting with adapting cell phone technology to keep track of the lives of birds. Let’s hope that migrating with a cell phone is not as dangerous as driving with one.
However, it is space exploration that brought use the next frontier for tracking migratory birds. Transmitters are now being used that can be tracked by satellites. Tracking raptors, such as Swainson’s Hawks and Peregrines between continents, and sea ducks from remote taiga breeding grounds to even more remote coastlines, have been some of the successes so far with this new generation of transmitters. Hopefully a future generation of transmitters may allow us to track even small birds over great distances.
For many birds, the research goal has shifted to mapping the connections between populations at different times of year. We may not know where bird x from Alaska is headed, but we can begin to know where it and its Alaskan brethren tend to go. This linking of populations across seasons has been deemed “connectivity” and the use of molecules and atoms has provided intriguing clues.
Molecular geneticists have been coming the genome of migratory birds for population specific markers to help map the connections. With the increasing ease by which the nucleotides in DNA can be sequenced, mutations at the level of single base substitutions can be detected. Alternatively variation can be found in the presence or absence of particular chemically-snipped fragments of DNA. Using these methods, local markers can sometimes be detected in breeding populations and their occurrence in migrating or wintering birds provide an insight about the birds region of origin. This approach has met with some success, particularly in shorebirds. For songbird populations, which expanded rapidly over the past 10 thousand years, the genomes are remarkably free of clues that could help map birds from particular areas.
Still, look at rapidly evolving makers has allowed us to determine some relatively broad patterns—such as the division of west Mexico and the Caribbean slope as wintering areas for populations of Wilson’s Warblers from Western and Eastern North America. However, more fine-scale mapping has proven impossible so far for most species.
This is where the use of the very expensive, but also very promising technique of stable isotope analysis comes in. Many common elements contain atoms that have different numbers of neutrons in their nucleus. For example, some Hydrogen atoms, known as Deuterium or heavy water, have two instead of the more normal one neutron.
The portion of Deuterium in rain water varies consistently with latitude and season and birds incorporate the local hydrogen into their feather tissue when they drink or eat local food items. So Hydrogen isotope ratios may tell us something about when and where the bird molted. Similarly, the isotope ratio of carbon tends to vary with habitat. With some knowledge of where birds molt different feathers (some are molted on the breeding ground and others in the winter), it is possible to develop at least a general picture of where birds might have come from and where they are headed. Use of the Hydrogen isotopes has been particularly useful in determining the latitude where a bird molted in its feathers—usually right after breeding. The fact that genetic markers seem to provide an east -west signal and isotopes a north-south signal has led some to explore the use of both approaches to hone in on a population.
Other ways of tracking bird movements loom on the horizon. Clever scientists have been looking at identity of parasites that birds carry with them. Others have been matching climate variation patterns on the wintering grounds with year to year variation of return rates on the breeding grounds as a clue to the connections between populations. It is likely that there will be no single silver bullet for tracking birds, but rather some or all of the innovative techniques mentioned here may be summonsed as evidence in this ongoing search. But as we search for the answers, one thing remains clear. It takes all of the imagination and skill that ornithologists muster to answer that simple question, where do birds go?
This article first appeared in the Naturalist News, a publication of the Audubon Naturalist Society.