Migration and Moulting Affect How Birds Change Their Colors: ScienceDaily



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In late summer and autumn, millions of birds fly overhead, often at night, flying to their wintering grounds.

Before the trip, many birds mutate their bright feathers, replacing them with a more muted palette. Observing this moult has led scientists to wonder how feather color changes are related to the migrations many birds undertake twice a year. Moulting is important, not only because replacement of worn feathers is necessary for flight, but because moulting is the catalyst for plumage changes that affect whether birds find mates and reproduce.

“We’re really lucky here, as nature lovers and birdwatchers, that we have many species of warblers here, which come in blue, green, red and yellow,” said Jared Wolfe, assistant professor at Michigan Technological University’s College of Forest Resources. and Environmental Science and one of the founders of the Biodiversity Initiative. “These brightly colored birds migrate and nest here and then leave for the winter. Everyone is so focused on coloring, but the mechanism of color change is the process of molting, replacing the feathers.”

While migration distances vary, many species fly thousands of miles each year, chasing summer as the planet leans towards and away from winter. These long journeys tend to wear out the feathers. In research published in the journal Ecology and Evolution, Wolfe and co-authors analyzed the variation in distances traveled with respect to the extent of the moult in a particular species. “Birds that go farther replace more feathers,” Wolfe said.

“The sun is the main reason feathers degrade and environments harsh,” he said. “In summer, in the northern latitudes, there is sun all day. As the birds move south, following the sun, they expose themselves to the maximum sun all year round.”

Feathers need to be replaced due to wear; what is the meaning of brightly colored plumage? Wouldn’t black be more protective against sunburn, or white better at deflecting heat?

For birds, like many animals, an attention-grabbing physical appearance plays a crucial role in attracting a mate. As trendy haircuts and makeup are for humans, beautiful feathers are for birds. But even spectacular plumage is pragmatic; it conveys age and health, which determine who should mate and who shouldn’t.

“Bright plumages are signs of habitat quality in the tropics,” Wolfe said. “Companion acquisition is based on a habitat quality signal from the wintering grounds. Undergoing a second moult on the wintering grounds before migrating north allows the birds to become colorful. Color is a signal for potentials. mates in places like the Midwest what jungle wintering habitats are like. “

Experiences during the winter months affect how birds become colorful, which affects their success in finding mates and breeding in North America. Scientists call these carryover effects. “It’s so elegant, but we’re only now starting to understand it,” Wolfe said.

Growing vibrant feathers is a physically strenuous activity, and the more likely a bird is to have it during the winter, the more colorful their plumage is during the summer. This makes food quality and availability, sheltering places and safety from predators important components of a wintering habitat.

Like humans looking for coveted places to live, birds flock to the best habitats. In both cases, resources are limited. What may have been an ideal wintering ground one year may be depleted of food sources or other important attributes the next.

“Better habitats offer resource stability over time, compared to lower-quality habitats that are variable month to month, year to year,” he said.

But what about birds that do not migrate, preferring to spend their lives within a single habitat? For them, it turns out that wetsuit is comparable to changing their clothes on a regular basis rather than changing out appearances to impress someone. Moulting and reproduction are limited by multiple factors: seasons, food abundance and the size of the home range play an important role in plumage and feather replacement.

“The birds here in the temperate zones are limited when they can breed and undergo their annual moult by winter,” Wolfe said. “In the tropics, there are wet and dry seasons, but there is less constraint from a real absence of food sources. Moulting is a calorie-costly process; birds need a lot of food while they moult.”

Wolfe and his collaborators have found that adjusting the time it takes for birds of the Amazon to complete their annual moult affects how they earn a living. For example, ant-tracking birds in Brazil eat insects that try to outrun the army ants. A tiny species, the white-feathered anthill, opportunistically dart past ants – not your garden ant variety but a species that can overwhelm and eat lizards, birds and small mammals as well as insects – to take advantage of a mobile feast.

“Her moult is very slow; it takes a full year,” Wolfe said, noting that the bird essentially lives in a constant state of moulting, dropping one feather at a time.

Obligate anteaters have huge home areas that overlap multiple army ant colonies, meaning they spend much of their day flying through the jungle in search of army ants. The bird’s long commute is a problem when it sheds its wing feathers, which creates gaps in their wings and impairs their ability to fly. How do they solve this problem? A very slow moult.

“A single feather at a time to minimize gaps, thereby improving their ability to fly and maintain large home ranges,” Wolfe said. “This unique adaptation made the white-feathered anthill the slowest moulting songbird on Earth.”

Despite migrant birds’ predilection for returning to the same nesting territory year after year, Wolfe and collaborators note that not all birds return to the same moulting locations. This finding confuses the home field advantage hypothesis, where birds benefit from completing their annual moult in a familiar location. But there doesn’t seem to be much relationship between the wetsuit activity and what Wolfe calls “site loyalty”.

“Until our research, it remained a mystery whether or not migratory songbirds returned to the same site to moult,” Wolfe said. “This is an important question because there is growing evidence that post-breeding season accumulated mortality – during periods of moulting, migration and wintering – is responsible for the continued loss of migratory songbirds. Indeed, bird abundance has decreased. 29% since 1970. Understanding where and why birds moult is an important step towards protecting vulnerable songbird populations. “

Wolfe and colleagues used 31 years of bird belt data from northern California and southern Oregon to measure the site fidelity of 16 songbird species during moulting. While the researchers found that breeding activity strongly correlated with site fidelity, moulting did not appear to influence a bird’s decision to return to a particular location or not. It appears that birds, like humans, tend to indulge in the luxury of fine feathers and then return home to show them off.

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