Fierce Featherweights
Posted on December 16, 2024 by Steve Millburg
Imagine being able to increase your muscle mass by 40% within a few weeks. Welcome to a bird’s life.
When you prepare for a long trip, you get the car fueled and ready. You plan your route, including stops along the way. You pack snacks and other necessities for the journey. You time your departure to reach your destination on schedule.
When they migrate, birds do much the same. Except that, for a bird, its “car” is its own body.
That body undergoes astounding changes, bulking up enormously with both muscle and fat. Some birds also shrink their gastrointestinal tract. In extreme cases, says Dr. Jonathan Pérez, an assistant professor of biology at South, “they actually have to spend a couple of days rebuilding it when they get to stopover sites to be able to feed.”
Longer-term modifications allow birds to channel as much energy as possible to migration. For example, reproductive organs contract during times of the year when they’re not needed.
“Most if not all seasonal breeders basically go to an infantile state in terms of their reproductive system outside the breeding period,” says Pérez, who researches bird migration and the timing of their reproductive activity. “They regress to a prepuberty stage.”
In some birds, the pectoralis muscles — the main engines of flight — grow 40% larger just before migration, says Emma Rhodes ’17. She’s a South biological sciences graduate and Auburn University Ph.D. candidate who’s preparing her dissertation on migration physiology.
The birds don’t work out in some avian gym. Their bodies just change — triggered, scientists have found, by seasonal changes in daylight. “Think about that in terms of an athlete,” Rhodes says. “To be able to increase your muscle mass by 40% within a few weeks would be pretty spectacular.”
To fuel up for the trip, birds go on feeding frenzies, packing on huge amounts of body fat that will serve as snacks along the way.
Ruby-throated hummingbirds, which can migrate across the Gulf of Mexico, normally weigh 2.5 to 3 grams, Rhodes says. “To put that into perspective, a nickel is 5 grams. Prior to jumping across the Gulf, they will weigh upwards of 6 grams. So they’re doubling their body weight.”
During each spring and fall migration season, Rhodes says, approximately 1 billion birds pass through the northern Gulf Coast. “One night when we were banding, upwards of 10 million birds were coming through the Mobile-Baldwin County area.”
She knows that thanks to a research consortium called BirdCast. It uses weather radar
to track flocks and estimate numbers. Its website, birdcast.info, includes continually
updated migration maps that show the number of birds currently in flight.
Most are traveling between breeding grounds up north, sometimes as far as the Arctic, and winter feeding grounds in warmer regions that provide plenty of year-round food. Some journeys last thousands of miles and include long segments over water.
Ruby-throated hummingbirds, for example, sometimes make the 500-mile Gulf of Mexico crossing in 18 to 24 hours. Imagine driving all day with no food, no beverages and no rest stops — and, like Fred Flintstone, supplying all of your car’s power yourself.
The name of Pérez’s lab at South reveals the focus of his research. It’s called Bird Brain’d. Last year, he won a $478,878 National Science Foundation grant to explore how songbirds use environmental cues to time their migrations. Part of the money went to purchase a cryostat machine, which preserves tissue samples by deep-freezing and allows them to be studied in their cryogenically frozen state.
Most birds apparently sense changes in daylight through photoreceptors in the brain itself, Pérez says. That tells the body to prepare for migration. The cryostat helps him study those receptors.
What determines the exact departure date? “It seems to be mostly weather,” he says. Songbirds can detect incoming storms and move out to avoid the turbulence. Among waterfowl, flocks of geese will take test flights to assess wind conditions. “They go up, circle around, come back down, go up, circle around, come back down — for days. And then one day they just go up and are gone.”
Pérez has set up 60 bluebird houses at South and 32 more at the Mobile Botanical Gardens just east of campus. He eventually wants to test a hypothesis about the birds’ migration. “I’m fairly convinced that at least a good chunk of our bluebird population is resident year-round, even though, officially, the population is migratory.” For now, the houses give students experience at gathering data (currently for reproductive studies) and working with wild birds.
As a child, Rhodes volunteered with a longtime bird banding group during spring and fall migration at Fort Morgan, on a peninsula at the mouth of Mobile Bay. The group’s leaders, Martha and the late Bob Sargent, became mentors.
Today, as co-founder of a nonprofit, all-volunteer organization called Banding Coalition of the Americas (bandingcoalition. org), Rhodes carries on the Sargents’ legacy.
Technology is helping to solve migration mysteries. For example, a conservation organization
called Birds Canada has set up the Motus Wildlife Tracking System. It uses radio telemetry
to trace birds that have been fitted with tiny radio transmitters. Last year, Banding
Coalition of the Americas worked with the South-affiliated Dauphin Island
Sea Lab and the Cornell Lab of Ornithology at Cornell University to install, at the
Sea Lab, coastal Alabama’s first Motus receiving station.
Mostly, though, the coalition uses a distinctly low-tech tracking system. It attaches a featherweight, numbered aluminum band to a bird’s leg. If the bird is recaptured, the number helps researchers track its travels.
The coalition leads multiday public banding events every fall at Fort Morgan and every spring at nearby Dauphin Island. Visitors watch, spellbound, as trained volunteers — Pérez is one of them — gently remove chirping, squawking, fluttering birds caught in nearly invisible mist nets (mostly songbirds), band them, quickly examine them and then safely release them.
During the fall sessions, 500 to 600 birds from 45 to 55 species get bands. To come so close to these wary, wild creatures, to sense their spirit and determination, to get a brief, intimate glimpse into a life so different from your own can be life-changing.
Songbirds seem so tiny, so delicate. And yet they can fly 500 miles nonstop across the Gulf of Mexico, besting storms, headwinds and predators.
“It’s a really special experience,” Rhodes says, “and one that we really believe helps make people advocates for the wildlife.”