Trackers used to follow animals are often permitted to weigh no more than 5 percent of the animal’s total body weight. Researchers show how body weight can fluctuate throughout the day – and how this affects both the animal and the research.
When researchers track animals using a transmitter, they need to account for changes in an animal’s weight over the course of the day, argue researchers in an article published on 30 September in Royal Society Open Science. Failing to do so means the weight of the tracker can compromise animal welfare – and makes the results less reliable.
When a bird or mammal is fitted with a tracker, consideration is given to what the weight of that tracker means for the animal. A tracker that is too heavy makes it harder for a bird to fly, or to care for its young. These changes in behaviour mean that research results are also less representative of the natural situation. Trackers are often limited to 1, 3 or 5 percent of total body weight. But body weight can change throughout the day. A group of researchers from several countries wanted to find out what these fluctuations mean for the fitting of trackers and for research into animal behaviour.
The researchers carried out years of fieldwork to record daily weight changes in Arabian babblers (songbirds) and dwarf mongooses (small mammals). To do this, they trained the wild animals to step onto a scale in the evening and morning in exchange for a small amount of food.
The fieldwork made clear that weight can fluctuate considerably over the course of a day. This must be taken into account during the design and manufacture of trackers, the researchers argue. Rosemarie Kentie, an ecologist at the Royal Netherlands Institute for Sea Research, who was not involved in the study, believes it is important that findings of this kind are documented, ‘so that researchers remain aware of the impact they have on the animals they study’.
The authors recommend calculating the maximum weight of a tracker based on the lightest weight an animal reaches during the day. Kentie finds it striking that the researchers themselves did not examine the effect of the trackers. ‘The 3 or 5 percent is a guideline, but that does not mean that fitting trackers is harmless for every species,’ says Kentie. ‘You need to keep monitoring throughout the study. In my view, that aftercare is an obligation.’ In addition to weight, the shape of the tracker and the way it is attached also play a role. ‘It is important to tailor the tracker to the question you want to answer.’
Kentie herself conducts research into the flight behaviour of lesser black-backed gulls and herring gulls near wind turbines. For this she uses trackers weighing no more than 3 percent of the birds’ body weight. ‘We are very conscious of this and adapt our methods to minimise any impact on the animals.’ Kentie has been following the gulls for years, yet remains fascinated by them. The birds have just begun their southward migration – ‘every day I check where they are.’