Numerous species of birds, both common and rare, migrate, sometimes across great distances and continents. While we have had more data on larger migratory birds for decades, small birds such as warblers were less known as it there was no easy way to track them.
Now, with the aid of tiny radio trackers, GPS, and other devices, scientists can study complex migratory patterns of even the smallest birds.
Using Radio and Telemetry to Track the Migration of Small Birds
Over the last few years, even some of the smallest birds can now be tracked using radio trackers or automated radio telemetry devices that can be fitted on small birds without it hindering their normal routines.
Birds such as the Kirtland’s Warblers have now been shown to have much larger migratory patterns than previously believed, where these birds can travel over 1700 miles to reach their destinations between the Bahamas and northern Michigan.
What was surprising is not only the great distances such tiny birds flew but also the way in which they did it, often flying at night. Data showed that breeding pairs and non-breeders had very different ranges of travel.
The combination of a network of stations monitoring the same frequency allows scientists to capture data from tiny tags that enable such work to be possible.
Mapping Small Bird Migration
Such new insights are possible thanks to a new system of tracking stations that have been developed, called Motus Wildlife Tracking System, which is a global collaborative of automated radio telemetry arrays. Tracking data is collected and shared across the collaborative and even made public.
Development of the bird tracking system started in 2013 and covered Canada and New England states; today the network encompasses over 900 stations and 330-plus projects in 31 countries on four continents have been involved.
Stations can be set up by almost anyone. While they are not exactly cheap, they are also relatively cheap for major scientific gear, with a basic station setup costing between $5000-10,000.
The actual receivers tagged on birds weigh no more than 0.2 grams. Batteries can last between one month to three years.
With this cost-effective setup and tiny tags, scientists now have the capability to study almost any bird migration and not just the larger species.
Birds are an indicator species
While such results are fascinating for scientists, there are other concerns scientists have that make tracking bird migrations critical. With habitat loss, complex webs and food systems could begin to breakdown; changing bird migrations could be a signal that food webs are beginning to change or even breakdown.
Having data that can track large-scale migrations could also enable scientists to better known how habitat loss such as forests could be affecting birds.
Vesper Sparrows, for instance, are an endangered bird in Oregon, with perhaps no more than 3000 birds remaining. Scientists can now tag the birds to see if chicks can survive not only their first few months but also see if they can survive their first migration as their habitats have become increasingly threatened.
Analyzing changes in bird migration behavior with geospatial technologies
There have been notable discoveries using the Motus System. This includes demonstrating that Swainson’s Thrushes, which fly between the Andes and North America, including the US and Canada, have different migratory strategies depending on where they nested part of the year.
First, some of these birds have adapted to deforestation by using coffee plantations in the Andes for shelter. Additionally, some birds spend more time looking for food before then leaving on their migration, where the extra time feeding enables them to have more energy for a quicker flight than the ones leaving earlier.
Interestingly, birds living in forested areas, rather than coffee plantations, usually left later. Other birds take a strategy of slowly flying to their destination.
One bird tracked flew a total of 3,700 miles in 34 days and covered about 109 miles per day, which scientist found astonishing for such a small bird.
Overall, it seems that birds departed later from moister and high-quality forested areas, where presumably the better food enabled later migrations and those birds could catch-up with the earlier travelers.
Tracking bird migrations from space
International Cooperation for Animal Research Using Space
New initiatives are attempting to enable even better coverage of bird migrations. The International Cooperation for Animal Research Using Space (ICARUS) program is establishing a space-based system using tiny tags that can use the International Space Station (ISS) to receive data.
As the ISS travels about 200 miles above the Earth, it is much closer than some of the satellites used in traditional GPS trackers. This enables a stronger signal that means tags can become even smaller since they do not need to carry larger receivers.
This program is now monitoring many different types of animals and not just birds. As this initiative is still relatively new, other researchers have still been using satellite-based GPS receivers which have gotten smaller but are still mainly used for medium or larger birds.
The Argos satellite system is used and has been in operation since 1978. Seven satellites makeup the system and are dedicated to monitoring global bird migrations. Birds, such as nighthawks, can be tagged with GPS trackers. Data on nighthawks showed these birds migrate between Canada and Brazil, with birds often knowing within a half a mile where their previous nests were.
Tracking devices on birds are beginning to completely change our understanding of bird migrations, particularly for smaller species that scientists did not even know had traveled extremely far distances before.
This has also informed us why certain migrations are occurring and the potential threat habitat loss can have on bird migrations and the health of ecosystems.
With tracking devices becoming smaller and more complex, we can expect even more migratory details and information to shape our understanding of bird ecology and behavior in the near future.
 For more on how scientists are tracking even the smallest birds using radio telemetry , see: https://www.birdwatchingdaily.com/news/science/new-tracking-tools-reveal-bird-migration-secrets/.
 The Motus Wildlife Tracking System and data on bird migrations can be obtained here: https://motus.org/.
 For more on the Swaison’s Thrushes and their migration, see: González, A.M.; Bayly, N.J.; Hobson, K.A. Earlier and Slower or Later and Faster: Spring Migration Pace Linked to Departure Time in a Neotropical Migrant Songbird. Journal of Animal Ecology 2020, 89, 2840–2851, doi:10.1111/1365-2656.13359.
 More on the ICARUS program can be found here: https://www.icarus.mpg.de/en.
 For more on nighthawk migrations, see: https://www.ualberta.ca/science/news/2018/july/scientists-reveal-threatened-bird-migration-route.html.