Tracking devices decrease survival of migratory birds

Tracking devices could impact the survival of migratory birds, especially if they wear the geolocators for multiple years.

These new research findings have implications for the way researchers conduct many modern bird tracking studies — especially with smaller-sized birds.

“Researchers using geolocators should be cautious,” said Veli-Matti Pakanen, a postdoctoral researcher at the University of Gothenburg in Sweden and the lead author of the study published recently in the Journal of Avian Biology.

Dunlins (Calidris alpina) are small wading birds that breed across the circumpolar north. A subspecies called southern dunlins (Calidris alpina schinzii) spend their summers in the Baltic region, Greenland, Iceland and the British Isles. Researchers have been closely tracking a population considered endangered in the countries that it breeds (Finland, Germany, Denmark, Sweden and Russia) in order to improve their conservation.

Geolocator devices are tiny, and attached to one of the upper legs of the birds. Credit: Kari Koivula

The population has been rapidly declining in some of these countries due breeding habitat loss, low reproduction and nest and chick predation. But little had been known about their migratory routes and destination during the winter months. Researchers fitted tiny geolocating tracking devices to the birds to determine that they ended up along the coast of Mauritania in West Africa after flying from Finland down to the German coast on the Baltic Sea.

“These are species that always follow the coastlines,” Pakanen said.

But Pakanen and his co-authors wanted to see whether the devices themselves had an impact on the survival of the birds they were hoping to conserve. They examined capture-recapture data taken from 2002 to 2018, including the history of 338 adult dunlins, 53 of which were fitted with tracking devices in 2013 and 2014. Some of these geolocators were removed within a year, while others were never removed as researchers didn’t manage to recapture the dunlins.

The researchers plugged bird survival data into models, separating the birds into categories of those that never had devices, those that had them for one year, and those that had them for two or more years. Survival of the birds is usually about 80% every year, and dunlins typically live about 7 years on average, though the oldest bird Pakanen and his colleagues have tracked has lived a robust 17 and may still be alive.

Survival of the dunlins decreased the longer they carried the devices. Credit: Veli-Matti Pakanen

The researchers found that even though the tiny devices weigh only 0.65 grams, they had an effect on survival. Those that wore the devices for one year had similar survival rates as the dunlins without geolocatoers. But those that had them on for two or more years only had a 60% survival rate every year.

“That model suggested that there’s a decline in survival the longer you carry that geolocator,” Pakanen said.

Females that carried the devices also had lower survival than males with geolocators. Pakanen said this is an odd result given that the females are a little larger than males and carry egg clutches that weigh more than half their body weight. He said that this data may be impacted by the relatively short duration of the monitoring program and flooding that occurred in 2015 at their breeding sites.

Nonetheless, Pakanen said, the take-home message is that researchers need to consider potential impacts that tracking devices themselves can have on the survival of their subjects when conducting survival studies — especially those on migration.

Header Image: Southern dunlins are declining swiftly in many parts of their range. Credit: Veli-Matti Pakanen