why are baby birds yellow

Starling chicks apply their preening oil as a lipstick to get more food from their parents

Spotless starling chicks make quite a sight when they’re hungry. Tucked inside their nest, the gray baby birds stretch their necks, stick their little faces up in the air, open their beaks wide and cry out insistently. Like many bird parents, all the adult starlings see when they look down at their chicks is a cluster of circular yellow mouths, each vying for a larger share of food. Now scientists know the color of those mouths results from a surprising trick that helps the chicks catch their parents’ attention: they make a bright yellow lipstick that shows off their immune health.

A team of ecologists in Spain found that the color of the chicks’ preen oil, which they take from a gland and apply to the edges of their beaks, influences how much food their parents give them. The birds with the most intense yellow and ultraviolet-colored mouths—an indicator of good immune health—get more worms. This finding, published in a recent study in Behavioral Ecology, represents one of the first known examples of birds using cosmetics to communicate between parents and offspring.

“Cosmetic coloration in birds mostly has been studied and understood or interpreted as serving as a signaling function, a sexual signaling function, like something that birds would do to attract other mates,” says Liliana DAlba, an evolutionary biologist at the Naturalis Biodiversity Center who was not involved in this study. This research “provides good evidence that this can also be a very important part of the communication between parents and their offspring.”

Juan José Soler, lead author of the study and evolutionary ecologist at the Estacio?n Experimental de Zonas A?ridas, explains in an email that this research was actually kicked off by an accidental discovery. Birds have a specialized gland that secretes the oil they use for preening their feathers, and Soler’s team suspected that the oil might also have beneficial bacteria living in it. His team had been working with spotless starlings for years, but it was only when they started collecting samples for their microbe study that they noticed the conspicuous yellow color of the chicks’ preen oil. The researchers found that the bright yellow preen oil produced by baby starlings eventually pales to a light beige by adulthood. Because only chicks have this yellow oil, they wondered if the youngsters use it to catch their parents’ attention.

The scientists suspected that the yellow color of the oil might be due to carotenoid pigments. In addition to providing color, carotenoids also function as antioxidants. Soler writes that having lots of carotenoid pigments indicates that a bird has a generally healthy immune system. Since adult birds will often give more food to their healthiest offspring when resources are limited, the researchers thought that chicks with yellower mouths might get fed more by their parents.

To test this hypothesis, the scientists monitored the 94 nest boxes at their field site in southern Spain at the beginning of the starling breeding season in March 2019. They placed video cameras inside the nest boxes to verify that nestlings actually spread their preen oil on their mouth. They then tracked how much food the parents gave the chicks. At ten days old, the researchers took color measurements of the chicks’ mouth and preen oil. At 14 days, the scientists measured the carotenoid levels in the chicks’ blood.

The video recordings verified that spotless starling chicks actively collect oil from their preen gland and spread it over their body. When the researchers rubbed cotton swabs over the chicks’ mouths, the cotton turned a bright yellow from the oils. The scientists also found that parents gave more food to chicks whose preen oil was warmer-colored overall and whose mouths had more yellow, orange, or red and more ultraviolet coloration—a wavelength beyond the visible spectrum for humans. Chicks with more ultraviolet preen oil also tended to have higher carotenoid levels in their blood. The oil’s yellow color was not directly associated with carotenoid levels, but the researchers note that the complex associations between the various color traits made it difficult to untangle how each one related to the chicks’ health.

In another part of the experiment, the researchers switched two chicks between different nests so they would be raised by different parents. At ten days old, the warm colors as well as the hue of the preen oil were more similar among biological siblings, even if they were raised by different parents, than among unrelated nestmates, which suggests that these characteristics have a genetic basis. But other color characteristics were more influenced by where the chicks grew up, so the coloration of the preen oil seems to be due to both nature and nurture.

Based on all these findings, the scientists concluded that spotless starling chicks use their preen oil to alter the coloration of their mouth, which advertises their health to their parents. The adults then feed the chicks with yellower and more ultraviolet-colored mouths more often, possibly because those chicks have the healthiest immune systems.

D’Alba suspects that this use of natural cosmetics by chicks may be found in other bird species as well. “I think it might be very common, especially in those birds that have what we call altricial chicks, those chicks that when they come out of their eggs are completely naked, and they have to spend a long time in their nest,” she says. “There has to be something about this individual chick that makes the parent feed them more. So I think, in any of these species where we see this case, then I think its very likely that cosmetic coloration could be important.”

More research is needed to fully understand how and why the spotless starling chicks use their preen oil as makeup. Philipp Heeb, a behavioral ecologist at the Université Paul Sabatier who was not involved in this study, asks “why would the birds put coloration in the preen gland instead of placing it directly in the mouth and the gapes? You know, why would they evolve this new system?”

Soler says that his team is excited to study these birds more so they can piece together a clearer picture of the evolutionary origin and function of this fascinating behavior.

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Wild ducklings, like these baby Mallard ducks, are in fact typically only partly yellow:

Photo by TheBrockenInaGlory via Wikimedia Commons, used under the CC-By-SA 3.0 license.

Though I’m not an expert, I would assume that the juveniles’ mottled yellow-brown coloring is, at least in part, protective coloring, similar to the adult females’ feathers’ somewhat similar pattern. In their natural habitat, ducks will frequently seek cover among reeds and other vegetation, where the erratic patterns of light and shadow would provide an excellent disguise for the ducklings, even though they may look noticeable on open water.

As for the all-yellow ducklings of domestic ducks, these presumably arose via elimination of the darker parts of the coloring as a result of selective breeding, perhaps as a side effect of artificial selection for the white adult plumage found in many domestic ducks today.

Specifically, according to a recent study of domestic duck genetics, it appears that the all-white plumage of domestic Pekin ducks is caused by a single recessive mutation to the MITF gene, which regulates melanin production. The mutation, when homozygous, causes the normal melanin production pathway in the skin to be almost entirely shut off, so that the adult plumage is pure white regardless of what other plumage color genes the bird may be carrying (a fact apparently known for a long time from cross-breeding experiments, even though the specific mutation behind this effect was not determined until recently).

However, it appears that the MITF mutation has no effect on the yellow base pigmentation in young ducklings, as it persists even in those who are homozygous for it. This is most likely because a different pathway, active in young ducks but inactive as they grow into adult plumage, produces the yellow color. However, because MITF controls the brown pigmentation pattern overlaid on top of it in ducklings of the wild type, it does not appear in ducklings carrying two copies of the mutated gene, resulting in pure yellow juvenile plumage.

To the best of my knowledge, the precise cause of ducklings’ yellow pigmentation not being regulated by the MITF gene is still unknown. Alternatively, if it is known, my Google search abilities were insufficient to locate it.

Juan José Soler, lead author of the study and evolutionary ecologist at the Estacio?n Experimental de Zonas A?ridas, explains in an email that this research was actually kicked off by an accidental discovery. Birds have a specialized gland that secretes the oil they use for preening their feathers, and Soler’s team suspected that the oil might also have beneficial bacteria living in it. His team had been working with spotless starlings for years, but it was only when they started collecting samples for their microbe study that they noticed the conspicuous yellow color of the chicks’ preen oil. The researchers found that the bright yellow preen oil produced by baby starlings eventually pales to a light beige by adulthood. Because only chicks have this yellow oil, they wondered if the youngsters use it to catch their parents’ attention.

“Cosmetic coloration in birds mostly has been studied and understood or interpreted as serving as a signaling function, a sexual signaling function, like something that birds would do to attract other mates,” says Liliana DAlba, an evolutionary biologist at the Naturalis Biodiversity Center who was not involved in this study. This research “provides good evidence that this can also be a very important part of the communication between parents and their offspring.”

The video recordings confirmed that the preen glands of spotless starling chicks actively collect and distribute oil throughout their bodies. The cotton turned a vivid yellow color due to the oils when the researchers rubbed cotton swabs over the chicks’ mouths. Additionally, the scientists discovered that parents fed more to chicks with preen oil that was generally warmer in color, mouths that were more yellow, orange, or red, and ultraviolet coloration—a wavelength that is outside of the human visible spectrum. Higher blood levels of carotenoid were also observed in chicks with greater UV preen oil. Although carotenoid levels were not directly correlated with the yellow color of the oil, the researchers point out that it was challenging to separate the intricate relationships between the different color characteristics and how each one affected the health of the chicks.

Spotless starling chicks make quite a sight when they’re hungry. Nestled within their nest, the gray chicks extend their necks, raise their tiny heads, spread their beaks, and let out a persistent cry. Like many other bird parents, the adult starlings only see a group of round yellow mouths competing with one another for the larger portion of food when they look down at their chicks. Scientists now know that the reason those chicks’ mouths are that color is because they use a clever trick to attract their parents’ attention: they apply a bright yellow lipstick that demonstrates the strength of their immune systems.

More research is needed to fully understand how and why the spotless starling chicks use their preen oil as makeup. Philipp Heeb, a behavioral ecologist at the Université Paul Sabatier who was not involved in this study, asks “why would the birds put coloration in the preen gland instead of placing it directly in the mouth and the gapes? You know, why would they evolve this new system?”

FAQ

Are the babies yellow in chickens?

Not all baby chicks are yellow: it depends on the breed. But, they start to lose their fuzz, growing real feathers before they’re a week old. These feathers are in the colors they will have as adults.

What is the color of a baby chicken?

Is a baby chick yellow because the egg yolk is yellow? No, all egg yolks are yellow, chicks come in all colors. Yellow chicks grow up to be white adults. Yellow duckings grow up to be white ducks.

How long does it take for a chick to get its feathers?

Chickens will usually be fully feathered by 5- to 6-weeks of age. Their wattles and combs should also start growing larger and becoming a deeper red.

Are geese yellow?

Adult Canada geese have grayish brown wings, backs, sides, and breasts; black tails, feet, legs, bills, and heads; and long black necks with distinctive white cheek patches. Males tend to be larger than females, though their coloration is identical. Goslings are light yellow with greenish-gray heads when they hatch.