More than maps – extracting demographic information from bird tracking data

Submitted by Michi on 27 October 2024.

Photo above: red kite (Milvus milvus). © Patrick Scherler.

Text by Steffen Oppel

Technical progress is constantly improving the way we study wildlife. For the last 2 decades devices to track the movements of animals have become smaller and more powerful, and vastly increased our understanding when, where and how individuals move through their environment. But besides knowledge on the movements of animals, new analyses can also extract demographic information from such data that help us to understand why populations are increasing or decreasing.

                                                         
Photo above: A red kite equipped with a solar-powered GPS transmitter. © Patrick Scherler

Wildlife populations are generally regulated by how many individuals survive from one year to the next, and by how many offspring are produced each year. To estimate the productivity of a bird population, researchers often spend many weeks in the field, trying to find nests, and then monitoring the broods until the young birds fledge. This can be very labour-intensive and may require climbing to nests that are located on trees or on cliffs.

However, if individuals have been equipped with a tracking device that records their location several times a day, then the movements of the birds themselves can be used to estimate whether they breed and raise offspring, and how they may respond to environmental factors.

                                          A researcher climbing to a red kite nest to assess productivity © Lukas Linder

In a new paper published in Journal of Avian Biology, a team from the Swiss Ornithological Institute provided a tool to estimate whether Red Kites raised offspring based on the information contained in the tracking data. Most bird species frequently return to their nest over the course of a breeding season, and those movement patterns can be diagnosed in tracking data of individuals (originally described by Picardi et al in 2020).

The new ‘NestTool’ allows researchers to estimate whether individuals settled in a home range, started breeding, and raised offspring based on their tracking data. The tool also provides a graphical interface to inspect cases that cannot be reliably classified, so that users can manually annotate uncertain cases.

                                        Graphical user interface of the NestTool to assist researchers in deciding whether a tracked individual initiated a nesting attempt and raised offspring.

An accompanying field app that is based on the same algorithm allows researchers to find nests during the breeding season using their most recent tracking data. This app is, however, only useful for researchers who manage their data on Movebank, because of the risk that this tool might be abused for raptor persecution.

                                                    A field app that allows researchers to point to a likely nest site of an individual bird given that this bird has transmitted its locations to the researcher’s Movebank study.

In another new paper published in the journal Ibis, the team used the same tracking data to detect locations where Red Kites are likely being fed by humans. Red Kites are very popular birds, and many people enjoy feeding them to view these stunning creatures from their garden. Using a similar concept as for the detection of nests, birds that attend a feeding site frequently return to the same location, and areas where multiple individuals exhibited this behaviour likely represent locations where people provided food for Red Kites.

These two new tools together now facilitate investigations whether the frequent use of anthropogenic food subsidies increases the productivity of Red Kites or not. Future investigations will combine these approaches with the flexible migration strategy of Red Kites in Switzerland, to examine whether the recent increase of the Swiss Red Kite population may have been caused by people feeding kites, by climatic changes, or a combination of several factors. You can find more information on the project at the Swiss Ornithological Institute.

Comments