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Location and Species Connections Maps

Connections maps are accessible on desktop and tablet.

As migratory birds travel between summer and winter ranges, they connect places across the globe. Researchers map these connections by recording when a bird originally seen in one location is subsequently re-encountered in another location. While banding is the oldest form of collecting re-encounter locations of individual birds, newer techniques such as automated radio telemetry, genetic markers, and tracking (e.g., satellite telemetry) are additional ways that scientists study bird movement.

Location Connections Maps allow you to choose any location across the hemisphere and see where else its migratory birds travel throughout their annual journeys, using millions of re-encounter observations. The maps display the number of individual birds that traveled between locations, across the entire timespan of the dataset. Species Connections Maps show where tagged birds of one species have been re-encountered across the hemisphere.The maps display the number of individuals that have traveled between locations.

Technologies for Collecting Re-encounter Data

Key sources of bird re-encounter data include banding data from the USGS Eastern Ecological Science Center Bird Banding Lab, automated radio telemetry data from the Motus wildlife network developed by Birds Canada, genetic connectivity studies shared by the Bird Genoscape Project, and relocation data from hundreds of tracking datasets shared by our partners and the research community on Movebank.

Banding. Banding is the oldest technique for marking birds to study migration. Bird banding (also known as ringing) uses metal or colored leg bands or colored wing tags with a unique number or alphacode as a method for documenting re-encounters of individual birds. Bands are inexpensive, dependable, and extremely lightweight, so they can be used on all sizes and species of birds, from hummingbirds to eagles. When a banded bird is recovered or resighted and reported to the USGS Eastern Ecological Science Center Bird Banding Lab (established in 1920), the location is recorded as a re-encounter. Although only a portion of marked birds are ever re-encountered, the high volume of tags and long-term data collection efforts have resulted in millions of re-encounter records that help scientists unravel the mysteries of bird migration. In the last 100 years, approximately 77 million bands have been deployed with over 5 million re-encounters reported.

Automated radio telemetry. Radio telemetry uses unique radio frequencies to locate and identify a radio-tagged individual. Radio telemetry can be automated through an established array of receiving towers that automatically record radio signals when a bird passes by. The Motus wildlife tracking system is the world’s largest collaborative automated radio telemetry array, including over 750 receiving stations across the Western Hemisphere. An advantage of radio tags is their relatively low cost and very small size—Motus tags are small enough to deploy on any species of bird.

Genetics. Researchers with the Bird Genoscape Project are using genetics to identify bird re-encounter locations. This uses a method similar to those used in ancestry DNA tests that are now widely available to people. Researchers begin by developing a map of genetic variation across the breeding range of a species – this map is referred to as the “genoscape.” Once this map is complete, they can collect feathers or blood samples from birds on the non-breeding grounds and use the genoscape to identify where those birds breed.

Tracking data. A number of technologies, including GPS satellite telemetry, Doppler satellite telemetry, and light-level geolocation are used to track individual birds for months or years. We used these tracking datasets to generate re-encounter data. Read about Species Migration to learn more about these tracking technologies.

Interpreting the Connections Maps

The Location and Species Connections Maps show the known number of birds and species that have traveled between binned locations based on the available re-encounter data.

Despite the rich data driving these maps, they are not a complete picture of all connections. Connections Maps in the Bird Migration Explorer are informational and subject to data gaps; these data have not been analyzed to identify priority areas or to estimate migratory connectivity. Sampling bias—meaning where and how many birds are tagged—skews our view and leaves gaps in our knowledge. For example, banded birds are more frequently recovered in places where there are lots of people; and in places where there are few people, there may be no recoveries, even if those places are well-connected by bird movements. The amount of re-encounter data also varies across species. For waterfowl, many re-encounter data have been generated by the return of bands recovered by hunters, but for most other species, the number of band recoveries is dramatically lower.

Spatial error in recorded locations varies depending on the tracking technology. While banding and some types of tracking data record locations at a much finer resolution than our 150-km hexagons, light-level geolocators and genetic data may sometimes have spatial uncertainty that exceeds this resolution.

Unlike tracking data that generally show a more complete pathway an individual bird followed over one year, re-encounter data may link places that are far apart in space or time. The quantity and location of tagging efforts is limited due to factors such as the accessibility of field sites and the cost of collecting data, among many others. As more data are collected, these maps will expand toward a more complete picture of which places around the world are linked through bird migration.

Mapping Methods

The Location and Species Connections Maps are based on millions of observations generously shared by a variety of data holders and partner organizations. To represent connections, bird movements were aggregated and summarized into 150-km hexagons covering the Americas. Before summarizing the tracking and re-encounter data, we developed an extensive process to flag and remove points with high locational error (i.e., outliers), such as points collected during the spring or fall equinox period for geolocator data, and incomplete or questionable banding and Motus records.

For each species, the remaining points were overlaid with the 150-km hexagon grid. For each individual bird that had points spanning more than one hexagon, we generated the unique set of hexagon pairs that were linked together through the bird’s movement, disregarding whether points originated in or ended at any given hexagon. We considered locations connected so long as the same individual occurs in both of them, whether those occurrences were within the same year or multiple years apart. We summarized linked hexagon pairs across individuals to generate species-level summaries used in Species Connections Maps, and across species to generate multi-species summaries used in Location Connections Maps.

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