Migratory Species Lighter?

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A summary of recent research claims that “Migratory birds undertake long and challenging journeys that have selected for a suite of adaptations from sensory mechanisms that facilitate orientation to extreme feats of endurance that push physiological limits. Recent work on two distantly related species revealed that migrating individuals increase their flight altitude dramatically during the day compared to at night. These studies suggested that the phenomenon is driven by thermoregulation: the ascent to cooler heights during the day may offset heat generated by absorption of solar radiation. If thermoregulation is an important selective force on migratory species, migrants should have evolved lighter, more reflective plumage to avoid overheating. Here we show, across the entire avian radiation, that migratory species are indeed lighter colored.”

Quickly moving flock of birds | Scene from Palo Alto Bayland… | Flickr
Short-billed Dowitchers

The researchers analyzed roughly 20,000 scientific illustrations of over 10,000 species of birds—almost every species. The team, led by an ornithologist at the Max Planck Institute for Ornithology , ranked how light in color each species’ plumage is and compared the color to the distances the birds fly on migration. On average, the farther the migratory distances the lighter the plumage color, but there were many exceptions.

This is an interesting idea but I question a few things, especially that color was measured on illustrations rather than live birds or museum specimens. The Handbook of Birds of the World, from which the researchers extracted the colors, is certainly a professional publication and I would surmise that the colors depicted in the illustrations are a pretty good representation of reality. But there are lots of variables such as light, reflection and refraction values, iridescence, etc. compared to a live bird that cannot be accounted for. But let’s say the colors reflected reality.

Do lighter colors offset heat generated by solar radiation absorption? When birds are migrating, it’s mostly the tops of their wings and back that are exposed to the sun. There is very little muscle in the outstretched wings and even less on the back under a very thin skin. What about the cooling effect of moving through the air at 30 mph or so? It may be that the lightness of the plumage has to do more with what happens on the ground. Solar radiation, camouflage, sexual selection, territorial defense, and other color characters factor in to direct the evolution of color, light or dark.

On the other hand, the researchers found that diurnal migrants flew higher than nocturnal ones, taking advantage of the cooler temperatures at higher altitudes. And their findings show that bird species get increasingly lighter with greater migratory distances: non-migrant (resident) birds tend to be darker than short-distance migrants and short-distance migrants darker than bird longer distance migrants. “One of the biggest surprises was how consistent the effect was across different types of birds. We saw the same pattern in birds large and small. The same held true in waterbirds and land-dwelling birds, too,” the researchers said.

This research brings up the question of whether migratory birds, given the warming earth, will evolve lighter plumage, shorten their migratory paths, fly higher, or switch to nocturnal migration. 

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