To survive winter, when food and temperatures are low, animals can use strategies, such as hibernation or migration, to reduce the energy they burn. Animals of the same species generally use the same strategy for winter. However, the common blackbird (Turdus merula), found across Europe, can use different overwintering strategies; blackbirds from one population will occupy their summer breeding grounds, but some members of that population migrate to warmer locations in southern Europe for the winter whereas others stay at the breeding site for the entire winter. As little is known about the differences in the birds’ energy use across the two overwintering strategies, Nils Linek, Scott Yanco and colleagues from the Max Planck Institute of Animal Behavior (Germany), University of Konstanz (Germany) and Yale University (USA) set out to measure the energy burned by the common blackbirds as they flew south for the winter or stayed at their original breeding grounds.
Over three consecutive autumns, starting in 2016, the team captured 118 male and female blackbirds in southern Germany using mistnets. They subsequently transported the blackbirds in cloth cages to the Max Planck lab. Then they surgically implanted each bird with a heart rate and body temperature monitor and fitted them out with a mini backpack containing an external radio transmitter to track each bird's location. Afterwards, they released all of the birds in the area they were caught. Then, during each following spring, from 2017 to 2019, the researchers successfully relocated 83 of the 118 common blackbirds. The team removed the backpacks and the implants to collect the heart rate and body temperature information and re-released the blackbirds at the same capture site.
Back in the lab, the team used the radio transmitters to determine whether the birds were migrants (left the breeding grounds for winter) or residents (stayed at the breeding grounds during the winter) and the date that the birds either arrived at or departed from the breeding site. The researchers also obtained recordings of the environmental temperatures of 25 known wintering areas, which had been measured previously, to determine the weather conditions experienced by the birds. Then, by using the heart rate and body temperature measurements, along with the weather conditions, the researchers calculated the energy the blackbirds were using at different time points.
Linek, Yanco and colleagues discovered that the amount of energy the resident birds burned when staying at the colder overwintering grounds was similar to the energy that the migrant birds burned at the warmer, southern locations. This was surprising, as it is thought that birds in colder locations would need to spend more energy to stay warm. However, the migrating birds needed to spend a large amount of energy when completing their arduous flight – one that can be 793 km long on average. However, the researchers found that 28 days before the migrating blackbirds left their breeding grounds, the birds reduced their heart rate during the night. As the date of the autumn departure approached, the blackbirds further reduced their heart rate (up to 19.5%). Similarly, 17 days before the birds’ departure date, they also decreased their body temperature. The decreased heart rate and body temperature likely meant that the birds decreased the energy they burned, saving it for other uses, such as fuel storage and to build the flight muscles needed for migration.
Overall, the team identified that the birds’ overwintering strategy did not change the amount of energy burned during the winter. Even though migratory birds have to expend a lot of energy to complete their long flight, they can prepare by reducing the energy they use before they leave. No one knows why blackbirds choose one strategy over the other, but the results of this study indicate that birds of a feather may not always flock together.
