By migrating out of the tropics and back to Europe to mate and raise their young, many birds species avoid having to deploy a hefty immune system.

When scientists analyzed the immune systems of sedentary birds species in Africa and Europe, as well as migratory birds that winter in Africa but return to Europe to breed and raise their young, they found birds living in the African tropics had more varied, robust immune systems.

Birds in Africa are exposed to a greater diversity of diseases, so a more capable immune system makes sense. But researchers also found migratory birds had the same simplified immune system as sedentary birds in Europe -- an unexpected discovery.

"What really surprises me is that the immune systems of migratory birds show a similarly low variation to that of European sedentary birds," Helena Westerdahl, a researcher at Lund University in Sweden, said in a news release. "After all, migratory birds don't only need to resist diseases in Europe, but also during their migration and in the tropics."

Researchers say their findings, published this week in the journal Nature Ecology & Evolution, suggest scientists have underestimated the drawbacks of a strong and complex immune system.

Hardier immune systems increase the risk of autoimmunity and other immune system disorders like chronic inflammation. Young birds, being exposed to diseases for the first time, are most at risk, but because migratory birds leave Africa to birth and raise their young, they're able to minimize risk without adopting a complex immune system.

"When the migratory birds breed, they have moved away from many diseases and therefore do not need an immune system that is equally varied," researcher Emily O'Connor said. "Another advantage is that the risk of damage caused by the immune system drops considerably if the immune system is less complex."

Because avian immune systems are built up similarly to human immune systems, the research could offer new ways of understanding the evolution of the human immune system.

To woo females, diving Costa's hummingbirds 'sing' with their tail feathers
Washington DC (UPI) Apr 12, 2018 - Male Costa's hummingbirds don't use their pipes to serenade would-be mates, they use their tail feathers. In a new study, scientists found male hummingbirds aim their dives to the sides of females in order to minimize the Doppler effect.

The vibrations created as air flows across the tail features of a diving male Costa's hummingbird produce a song of sorts. The musical dive is the male's way of showing off in front of females, a mating display.

New analysis of the display suggests males position their dives to the side so that females can't use the Doppler effect to determine the speed of their dives.

"Recent studies in birds and other animals suggest that females prefer higher speeds during male athletic displays," Christopher Clark, an assistant professor of biology at the University of California, Riverside, said in a news release. "By concealing their speed, males are not necessarily cheating, but instead have evolved this placement of trajectory out of female choice."

While diving to the side of females, males twist some of their tail feathers up to 90 degrees to aim their song at potential mates.

"We don't know why males twist only half of their tails toward the females, but it may be due to anatomical limitations that prevent them from twisting their whole tail around," Clark said.

Researchers recorded diving males in the wild and also tested the sounds produced by vibrating tail feathers in a wind tunnel. Clark and his colleagues had difficulty estimating the speed of the bird's dive using the recorded vibrations.

"Once I realized it wasn't trivial for a scientist to measure, I realized it wouldn't be trivial for a female to measure either," Clark said.

While many studies have focused on static mating displays, bright colors and uniquely shaped feathers, the nest study -- published this week in the journal Current Biology -- highlights the importance of dynamic, athletic mating displays.

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