Fierce Featherweights
Posted on December 16, 2024 by Steve Millburg
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 鈥渃ar鈥� 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 P茅rez, an assistant professor of biology at South, 鈥渢hey actually have to spend a couple of days rebuilding it when they get to stopover sites to be able to feed.鈥�
Longer-term modi铿乧ations allow birds to channel as much energy as possible to migration. For example, reproductive organs contract during times of the year when they鈥檙e not needed.
鈥淢ost if not all seasonal breeders basically go to an infantile state in terms of their reproductive system outside the breeding period,鈥� says P茅rez, who researches bird migration and the timing of their reproductive activity. 鈥淭hey regress to a prepuberty stage.鈥�
In some birds, the pectoralis muscles 鈥� the main engines of 铿俰ght 鈥� grow 40% larger just before migration, says Emma Rhodes 鈥�17. She鈥檚 a South biological sciences graduate and Auburn University Ph.D. candidate who鈥檚 preparing her dissertation on migration physiology.
The birds don鈥檛 work out in some avian gym. Their bodies just change 鈥� triggered, scientists have found, by seasonal changes in daylight. 鈥淭hink about that in terms of an athlete,鈥� Rhodes says. 鈥淭o be able to increase your muscle mass by 40% within a few weeks would be pretty spectacular.鈥�
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. 鈥淭o put that into perspective, a nickel is 5 grams. Prior to jumping across the Gulf, they will weigh upwards of 6 grams. So they鈥檙e doubling their body weight.鈥�
During each spring and fall migration season, Rhodes says, approximately 1 billion birds pass through the northern Gulf Coast. 鈥淥ne night when we were banding, upwards of 10 million birds were coming through the Mobile-Baldwin County area.鈥�
She knows that thanks to a research consortium called BirdCast. It uses weather radar
to track 铿俹cks and estimate numbers. Its website, birdcast.info, includes continually
updated migration maps that show the number of birds currently in 铿俰ght.
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 鈥� and, like Fred Flintstone, supplying all of your car鈥檚 power yourself.
The name of P茅rez鈥檚 lab at South reveals the focus of his research. It鈥檚 called Bird Brain鈥檇. 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, P茅rez says. That tells the body to prepare for migration. The cryostat helps him study those receptors.
What determines the exact departure date? 鈥淚t seems to be mostly weather,鈥� he says. Songbirds can detect incoming storms and move out to avoid the turbulence. Among waterfowl, 铿俹cks of geese will take test 铿俰ghts to assess wind conditions. 鈥淭hey go up, circle around, come back down, go up, circle around, come back down 鈥� for days. And then one day they just go up and are gone.鈥�
P茅rez 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鈥� migration. 鈥淚鈥檓 fairly convinced that at least a good chunk of our bluebird population is resident year-round, even though, officially, the population is migratory.鈥� 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鈥檚 leaders, Martha and the late Bob Sargent, became mentors.
Today, as co-founder of a nonpro铿乼, all-volunteer organization called Banding Coalition of the Americas (banding coalition. org), Rhodes carries on the Sargents鈥� 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 铿乼ted 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鈥檚 铿乺st Motus receiving station.
Mostly, though, the coalition uses a distinctly low-tech tracking system. It attaches a featherweight, numbered aluminum band to a bird鈥檚 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 鈥� P茅rez is one of them 鈥� gently remove chirping, squawking, 铿倁ttering 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 铿倅 500 miles nonstop across the Gulf of Mexico, besting storms, headwinds and predators.
鈥淚t鈥檚 a really special experience,鈥� Rhodes says, 鈥渁nd one that we really believe helps make people advocates for the wildlife.鈥�