Researchers warn others in the scientific community that tracking ground-nesting grassland birds using radio transmitters – a common practice – can result in the bird’s entanglement, particularly for fledglings. The two biologists make suggestions for alterations to the apparatus to prevent this.
Radiotelemetry is the use of tiny radio transmitters, worn by animals, which emit radio waves that can be detected several hundred meters away. These transmitters help biologists locate and follow shy or cryptic animals. Radio tagging is frequently used by researchers to track animals and gain vital information about their behaviour and survival, over the changing seasons and over the life span. Although many studies have shown that radio transmitters are safe for birds to carry, a recent study found a surprising exception.
York University Masters Student Heidi van Vliet and Biology Professor Bridget Stutchbury, in the Faculty of Science, began a study on the effects of intensive agriculture on grassland birds, including how this affects the survival of young birds after they leave their parents’ nest. To do this, they launched a study that looked at radio tagged fledglings, or young, Savannah Sparrows (Latin name: Passerculus sandwichensis).
“Studying ﬂedgling mortality and dispersal are key to understanding the population dynamics of these birds, and birds in general. Few studies to date have reported any negative impacts on ﬂedglings wearing radio transmitters, but the research community would beneﬁt from this new information,” van Vliet explains. “It could lead to improved methods for new research and to harm fewer individuals in the future,” she adds.
This research was funded by the Natural Sciences and Engineering Research Council of Canada. The findings of this research were published in Ibis, the International Journal of Avian Science, in 2018.
The varied data that can be gained by radio tagging birds
There’s no doubt that radio tagging provides an invaluable glimpse into the whereabouts and behaviours of birds. It can track the following data:
- Geographical coordinates;
- Speed, altitude and pressure both barometric (air pressure) and hydrostatic (under water);
- Acceleration and angular speed – that is, the speed of rotational motion, which is measured using a gyroscope;
- Terrestrial magnetism or the magnetism of the Earth, measured using a magnetometer, which provides information about animal’s orientation; and
- Temperature and light intensity.
Stutchbury notes, however, that the tags she and van Vliet used in this study were tiny and could only emit radio waves for classic ground tracking, excluding the collection of more specific data on things like air pressure.
Researchers tagged 50 sparrows over two years
To undertake this research, van Vliet and Stutchbury deployed 94 radio tags on Savannah Sparrows over two years, from 2016 to 2017, in southwestern Ontario. Fifty birds were young/fledglings, while 44 were older/adult. The fledglings, expected to be mobile around at nine days of age (at this time, they cannot fly; they walk on the ground), were tagged using a tiny loop harness on their legs, around the age of seven days old as nestlings. The adult birds were tracked with an equivalent system.
The fledglings were tracked every three-to-five days, for three weeks. The adults were tracked at a minimum of one week and then one month after tagging to ensure that the tags were still on the birds before migration.
Stutchbury explains how the researchers accounted for ﬂedgling mortality, due to entanglement, for each individual bird: “The probability of entanglement during the three-week tracking period was calculated using the nest survival model, an algorithm that can estimate the probability of entangled vs. not entangled – or, simply put, alive or dead.”
Findings reveal young birds get caught right after learning how to fly
The researchers found that none of the adult birds were entangled in the study.
However, it was a different story for the younger birds, but only in the second year: In the first year (2016), no entanglements of the fledglings occurred. In the second year of the study, 23 per cent of the tags and antennas were entangling with vegetation, resulting in the ﬂedglings not being able to move. “Out of 30 tags deployed in 2017, seven tags were entangled in vegetation,” van Vliet says.
“The fact that there were no problems in the first year makes the entanglements in the second year all the more surprising,” adds Stutchbury.
Five of these seven fledglings were entangled by the antenna twisting in grass, while two were entangled by the transmitter (on the bird’s back) having been wedged under fallen vegetation. Four of the seven incidents happened while tracking was taking place, so the fledglings were untangled and released by the researchers – and they did not become entangled again.
Importantly, three of the seven became entangled right after leaving the nest, when they cannot yet fly. “These ﬂedglings were found less than one meter from the nest and their back ends were covered in faeces. Based on their known age at the time of discovery (11 to 12 days old), they would have left the nest within one to two days prior to being discovered and, presumably, the parents continued feeding them after entanglement,” van Vliet explains.
“They cannot fly when they first leave the nest – instead, they scurry around like mice,” Stutchbury explains.
Two of the three birds were rescued by the researchers, while one died. So the overall mortality caused by radio tracking was zero out of 20 in 2016, and one out of 30 in 2017. However, the researchers emphasize that mortality would have been much higher if the researchers had not been able to locate the injured birds and rescue them.
Take-away message for researchers and manufacturers
Since current guidelines for attaching transmitters to wild birds do not consider the possibility of entanglement, one solution might be to add a weak link to the apparatus that could break if the animal becomes entangled. The research duo also suggests that trimming the length of the antenna might help to avoid entanglement, or a wider, flatter transmitter that lies closer to the bird’s body.
By Megan Mueller, senior manager, research communications, Office of the Vice-President Research & Innovation, York University, firstname.lastname@example.org