When flying thousands of miles, birds often stop to refuel with food and energy.
New research suggests that birds that make these trips twice a year may be doing more than filling their bellies and giving their wings a rest: their immune systems may need a boost to keep the birds from succumbing to disease or infection.
“These birds are basically running 100 marathons – they’re super athletes,” says Cass Eikenar, an ecologist at Bird Research Institute in northern Germany. “Sometimes they make stopovers to recover, not just to refuel.”
Extreme physiology
Researchers have been studying various components of migration for years, including the amount of fitness and fuel needed to fly so far.
In some cases, researchers don’t even know basic facts, such as where species spend the winter or summer each year or the trajectory they take to make those journeys.
Eikenaar studied the physiological components of bird migration. Curious feats of strength enable birds to make these epic journeys, such as running at 20 times their basal metabolic rate.
Most mammals could never achieve this level of physical activity without dying due to a breakdown in basic physiological functions, he says.
Read more: Birds travel thousands of miles to conserve energy
Added charge to the immune system
Previous research has shown that the immune system of birds can suffer as the birds put all their focus on their intense athletic performance.
“They reduce their investment in their immune system,” Eikenar says.
The problem is that migration itself likely exposes birds to new pathogen pressures.
Individuals often gather together in concentrated groups during layovers – so social distancing is hardly an option.
There are also often multiple species that share stopovers, giving harmful microbes a greater chance to move to new vectors. This happened recently in the worldwide distribution of highly pathogenic avian influenza which has killed a large number of birds of various species.
Read more: What is bird flu and why was it so bad in 2022?
Trapping birds for sampling
Because birds have evolved to migrate for centuries, Eikenaar hypothesized that birds may use these breaks to recharge their immune systems while they rest.
IN study published in 2020, researchers took blood samples from northern barnacles, which are long-distance migrants, captured during their stopover at a long-term bird-watching station on the German North Sea island of Heligoland. They kept these birds in cages for several days, then sampled them again before releasing them.
More recently, in a study published in Biology Letters, Eikenaar and his colleagues trapped common redfins that migrate from Europe across the Sahara desert in Africa every year. They also sampled loons and finches, shorter-distance migrants, at the same location a few days after their arrival.
E. Coli and rabbit blood immunity tests
They analyzed them for two measures of immunity – innate and acquired.
Innate is the type of natural, genetic resistance that an individual is born with, while acquired is the built-up resistance that an individual has developed over a lifetime to fight pathogens and harmful microbes in the body.
The researchers measured innate immunity by induction E. coli bacteria in the blood samples and see how well it started to spread.
“If your immune system is weak, E. coli it’s going to spread around your petri dish like crazy,” Eikenar says.
In the other test, the team introduced rabbit blood cells into the bird’s blood.
In healthy systems, immune cells will stick the foreign cells together, like rabbit blood, then begin to break them down.
Convincing results
Tests from the two above-mentioned samples showed that birds that had been at the landing site for several days had better immunity in both innate and acquired indicators. And the patterns of recovered immunity were similar for all the species they tested.
Eikenaar says this research further highlights the importance of protecting and preserving stopovers.
Migratory birds today face a number of challenges during migration due to climate change, which slows or speeds up the window for their travels.
If birds have less time to recharge their immune systems when they are shut down, they may be more susceptible to disease. The finding also highlights the need for humans and other animals to avoid disturbing birds at staging areas.
Read more: Scientists can now predict bird migration with radar and weather data