PULLMAN, Wash. — The birds of a feather not only flock together, but also seem to settle together.
A study of nomadic pine siskins, a type of finch often seen at backyard feeders, found that when male migrating siskins were paired with a sedentary male bird in captivity, they also began to show signs of ending migration. They stopped flying without rest and lost body mass, fat stores and muscle size, compared to a control group placed in solo pens.
“The presence of another bird that is not migratory seems to be a very strong signal to stop the migration,” said Heather Watts, behavioral ecologist at Washington State University and corresponding author of the study. published in Biology Letters. “We saw changes in their behavior and changes in their physiology associated with the energetics of migration.”
Why pine siskins decide to stop migrating can also give clues to the nomadic behavior of other animals.
“It has long been thought that social cues or information could be important not only for nomadic birds, but also for other nomadic animals,” she said. “We suspect this is probably a more general phenomenon, that other animals’ attention to social cues may be important in migratory decisions for a variety of species.”
The migration of birds and other animals impacts ecosystems around the world, with potential effects on agriculture and disease transmission, Watts said. Some well-known migratory animals have predictable patterns, such as geese and monarch butterflies that travel to the same breeding and wintering areas each year. Not much is known about how unpredictable nomads, such as pine siskins, choose their destinations.
Still, just because pine siskins are unpredictable doesn’t mean they don’t travel far.
“A bird that was banded in Pennsylvania could be found in Washington State next year, so they are traveling on the order from the mainland,” Watts said.
Studies of migratory pine siskins in captivity have shown that they exhibit “nocturnal migratory restlessness”. This means the birds tend to move a lot at night – jumping, flying, flapping their wings rapidly – at a time when they would normally be resting. Birds also gain muscle and accumulate large fat deposits to use as fuel in flight, increasing their body mass in preparation for migration.
In this study, researchers captured a sample of 44 male Pine Siskin birds from the western United States. Towards the end of the spring migration season, the researchers divided the birds into migratory and non-migratory birds, placing half of each group into migratory bird pairs. and non-migratory male birds and keeping the other half as controls. The researchers paired birds of the same sex to eliminate the potential influence of breeding cues on the decision to end the migration.
Migratory birds placed with more sedentary birds lost much of their nocturnal restlessness and body mass, indicating that they were responding to cues to stop migrating. Solitary migratory birds in the control group continued to display behaviors and physical signs related to migration.
Many nomadic animals depend on variable food sources, which may explain some of their unpredictable migration patterns, Watts said. For example, owls are known to follow the boom and bust cycles of rodents. Birds that depend on conifer seed such as pine siskins likely follow the variable “poles” or seed flowers of conifers since these trees do not produce seed every year. They may also look to other animals to determine where to land.
“We think these animals are likely relying on local cues to assess whether an area is potentially a good site or not,” Watts said. “One type of signal comes from other individuals – if these birds enter an area where there are already settled birds, it could tell them that this is a good place to stop.”
It may also be why birdwatchers are noticing pine siskins suddenly swarming or disappearing from their feeders altogether, she said.