Migration and moulting affect how birds change colors

Before traveling, many birds shed their bright feathers and replace them with a muted palette. Watching this moult led scientists to wonder how changes in feather color affect the migrations many birds make twice a year. Moulting is important – not only because worn feathers need to be replaced for flight, but also because molting is the catalyst for plumage changes that affect how birds find mates and reproduce.

“We are really blessed as nature lovers and bird watchers here to have many types of warblers here that come in blue, green, red, and yellow,” said Jared Wolfe, assistant professor at Michigan Technological University’s College of Forest Resources and Environmental Sciences one of the founders of the biodiversity initiative. “These brightly colored birds migrate and nest here and then move into winter. Everyone is so focused on the coloration, but the mechanism of changing the coloration is the process of molting, replacing feathers. “

While migration distances vary, many species fly thousands of miles each year and chase summer as the planet leans towards and away from winter. These long trips tend to wear out springs. In a study published in the journal Ecology and Evolution, Wolfe and co-authors analyzed the variation in distance traveled versus the amount of molt in a given species. “Birds farther away replace more feathers,” Wolfe said.

“Sun is the main reason feathers deteriorate and harsh environments,” he said. “In the northern latitudes it is sunny all day in summer. When the birds move south and follow the sun, they expose themselves to maximum sun all year round. “

Prothonotary Warbler, Copyright Frank Mantlik, from the Surfbirds Galleries

Springs must be replaced due to wear; What is the meaning of colored plumage? Wouldn’t black be better protected against sunburn or white better against heat?

For birds, like many animals, an attention-grabbing physical appearance plays a crucial role in attracting a mate. Just as stylish hairstyles and makeup are for people, so are beautiful feathers for birds. However, spectacular plumage is also pragmatic; It sends age and health determining who is allowed to mate and who is not.

“Bright plumage is a sign of habitat quality in the tropics,” said Wolfe. “The acquisition of partners is based on a signal of the quality of the habitat from the wintering areas. A second moult on the wintering area before migrating north will make the birds colorful. Color is a signal to potential partners in places like the Midwest what wintry habitats look like in the jungle. “

Experiences during the winter months affect how colorful birds become, which affects how successful they are in finding mates and breeding in North America. Scientists call these carry-over effects. “It’s so elegant, but we’re only just beginning to understand it,” said Wolfe.

Growing vivid feathers is a physically demanding activity. The easier it is for a bird in winter, the more colorful its plumage is in summer. This makes food quality and availability, shelter, and predator safety an important part of a wintering habitat.

Like people looking for desirable places to live, birds flock to the best of habitats. In both cases the resources are limited. What might have been an ideal wintering location one year may lose food sources or other important properties the next year.

“The best habitats offer resource stability over time compared to poorer quality habitats that vary from month to month and year to year,” he said.

But what about birds that do not migrate and prefer to live their lives in a single home area? For them, it turns out that molting is akin to changing clothes regularly, rather than changing looks in order to impress someone. Skinning and breeding are constrained by several factors: the seasons, abundance of food and the size of the home range play an important role in the exchange of plumage and feathers.

“Birds here in the temperate zones are restricted if they can breed through winter and go through their annual molt,” Wolfe said. “There are rainy and dry seasons in the tropics, but there are fewer restrictions from the actual lack of food sources. Molt is an expensive process in terms of calories; Birds need a lot of food while they molt. “

Wolfe and his coworkers found that adjusting the time it takes Amazon birds to complete their annual molt affects their way of life. For example, ant birds in Brazil eat insects trying to escape ants from the army. A tiny species, the ant bird with white plums, opportunistically shoots in front of the ants – not your garden ant, but a species that can overwhelm and eat lizards, birds, and small mammals in addition to insects – to benefit from a moving feast.

“His moult is slowly going crazy; It takes a whole year, ”said Wolfe, noting that the bird essentially lives in a constant state of molting, dropping one feather at a time.

Mandatory ant birds have huge home ranges that intersect with several army ant colonies, meaning they spend much of their day flying through the jungle in search of army ants. The bird’s long daily commute is a problem when shedding its wing feathers, creating gaps in its wings and affecting its ability to fly. How do you work around this problem? A very slow molt.

“A single spring at a time to minimize gaps and thereby improve your ability to fly and maintain long ranges,” said Wolfe. “This unique adaptation has made the white bird ant bird the slowest molting songbird on earth.”

Despite the preference of migratory birds to return to the same breeding area year after year, Wolfe et al. Note that not all birds return to the same molting area. This finding confuses the assumption of a home advantage in which birds benefit from having their annual moult in a familiar location. However, it appears that there isn’t much of a correlation between molting activity and what Wolfe calls “site fidelity”.

“Until our investigation, it was a mystery whether or not migratory songbirds return to the same place for molting,” said Wolfe. “This is an important question as there is growing evidence that post-breeding deaths during the molting, migratory and hibernation periods are responsible for the continued loss of migratory songbirds. In fact, bird frequencies have decreased by 29% since 1970. Understanding where and why birds molt is an important step in protecting endangered songbird populations. “

Wolfe and colleagues used 31 years of birding data from northern California and southern Oregon to measure the fidelity of 16 species of songbirds during molting. While the researchers found that breeding activity was strongly correlated with fidelity, molting did not appear to affect a bird’s decision whether or not to return to a particular location. It seems like birds, like humans, have a tendency to indulge themselves on fine feathers – and then go home to show them off.


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