When flying thousands of miles, birds often make pitstops to recharge on food and energy.
New research shows that the birds making these twice-yearly journeys may be doing more than filling their bellies and resting their wings: Their immune systems may need a boost to keep the birds from succumbing to disease or infection.
“These birds basically run 100 marathons — they are super athletes,” says Cas Eikenaar, an ecologist at the Institute of Avian Research in northern Germany. “They make stopovers sometimes to recover, and not just to refuel.”
Researchers have been studying various components about migration for years, including the amount of fitness and fuel it takes to fly so far.
In some cases, researchers don’t even know basic facts such as where species spend winter or summer every year, or the trajectory they take to make these journeys.
Eikenaar studies the physiological components of bird migration. Curious feats of strength allow birds to make these epic journeys, such as performing at a rate 20 times above their base metabolic rate.
Most mammals could never accomplish this level of physical activity without dying due to a breakdown in basic physiological functions, he says.
Added Toll to Immune System
Previous research had shown that the immune systems of birds may suffer as birds put all their focus on their intense athletic feat.
“They downregulate their investment in their immune system,” Eikenaar says.
The trouble is, migration itself likely exposes birds to new kinds of pressure from pathogens.
Individuals often cluster together in concentrated groups during stopovers — so, social distancing is hardly an option.
There are also often multiple species that share stopovers, giving harmful microbes a greater chance of jumping to new vectors. This has occurred recently in the worldwide spread of the highly pathogenic avian influenza that has killed large numbers of birds across species.
Catching Birds to Sample
Since birds have evolved to migrate for eons, Eikenaar speculated that the birds may use these breaks to recharge their immune systems while they rest.
In a study published in 2020, the researchers had taken blood samples from northern wheatears, which are long-distance migrants, captured during their stopover at a long-term bird monitoring station on the German island of Heligoland in the North Sea. They kept these birds in cages for a few days, then sampled them again before release.
More recently, in a study published in Biology Letters, Eikenaar and his colleagues trapped common redstarts, which migrate from Europe down across the Sahara Desert in Africa every year. They also sampled dunnocks and chaffinches, 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 kind of natural, genetic resistance an individual is born with, while acquired is the built-up resistance that an individual has developed over its life of fighting pathogens and harmful microbes in the body.
The researchers measured the innate immunity by introducing Escherichia coli bacteria into the blood samples and seeing how well it began to spread.
“If your immune system is poor, the E. coli will spread through your petri dish like mad,” Eikenaar says.
In the other test, the team introduced rabbit blood cells to the bird blood.
In healthy systems, the immune cells will clump together the foreign cells, such as rabbit blood, then begin to break them apart.
Tests from both of the above samples showed that the birds that had been on the stopover site for several days had better immunity by both measures — innate and acquired. And the patterns of recovered immunity were similar for all the species they tested.
Eikenaar says that this research further highlights how important it is to protect and conserve stopover locations.
Migrating birds today are facing a number of challenges during migration due to climate change, which is delaying or hastening the window for their journeys.
If birds have less time to recharge their immune system at stopovers, they may be more susceptible to disease. The finding also highlights the necessity for humans and other animals to avoid disturbing birds at stopover sites.