Birds: What is the purpose of rump secretion?

In addition to hearing and sight, birds also use less intelligible senses, including smell. They lubricate their feathers several times a day with an aromatic secretion from their gland. The smell of this spanking oil indicates that it is not only used for practical feather care. In a meta-study, researchers have now examined in more detail what the scent of secretion might use.

Most bird species have a special gland at the base of their tail: the maidenhair gland. It secretes an oily secretion that birds spread with their beak on their feathers several times a day. This spanking oil is used, among other things, for the care of feathers, for their lubrication, and for making them water-repellent. But the smell of this secretion has long raised suspicions that rump oil could have other functions about which little is known.

To illustrate, Leanne Grieves of McMaster University in Canada and colleagues evaluated data from a total of 55 studies looking at rump secretion and composition. Above all, seasonal fluctuations in the chemical composition of Borzel oil should provide information about its function. Researchers’ idea: If rump secretion is more smelly during a bird’s breeding season, it may indicate that the oil is being used to camouflage young, unprotected birds from predators. The difference between the sexes in composition, especially during the mating season, would support the hypothesis of a communicative function of the scented oil.

rump secretion to camouflage the nest

Studies, which looked at seasonal variations in rump secretion composition, confirmed variability over time in 95 percent of the bird species studied. “Seasonal changes in the chemical composition of rump oil were nearly ubiquitous,” Greaves and colleagues report. Especially during the breeding season, the oil secretion of the birds’ parents contained much more of the diester organic molecule. The researchers suspect that this evaporates more slowly, alters the scent mark of the nest, and is therefore better able to hide it from predators.

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The research team also took a closer look at the link between rump oil and the location of a bird’s nest. In fact, seasonal variations occurred mainly in bottom-dwelling birds such as shorebirds. Co-author Mark Giles of Bielefeld University explains why this might happen: “Of course, ground-dwelling birds are particularly vulnerable to scent-dependent predators such as foxes because their nests are easily accessible. Olfactory camouflage means increased chances of survival. life for small animals. Also striking: in bird species in which both partners are interbreeding, the rump secretion contained the same amount of diaster in both sexes. On the other hand, if only the males or the females only cuddle, their rump oil smell is correspondingly stronger.

Partner recognition by Bürzel-Sekret

In general, gender differences were less common than seasonal fluctuations. Studies have shown differences between male and female birds in about 47 percent of the bird species examined. Here, too, the researchers suspect a link: “When we found differences between the sexes, these differences occurred mainly during the mating season,” according to Gill. Female secretion has usually been associated with a much greater abundance and diversity of chemical compounds.

They suspect this could be an expression of increased female acceptance. But a change in female rump oil can also serve as regional scent markings or identification of the mother and offspring. Indeed, in some studies, the chemical composition of the secretion is also related to the expression of the immune system. Birds can use their scent to see how closely they are related. This gives the birds clues about whether a potential mate is genetically compatible enough to produce healthy offspring.

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The data is still very inaccurate

Therefore, the results indicate that both hypotheses can be true. The smell of spanking oil can serve to protect the nest and chicks from predators and give information about the reproductive state of a particular species during the mating season. “Unfortunately, there is currently insufficient data to further test the hypotheses,” says senior researcher Barbara Kaspers from Bielefeld University. “However, additional information about the nature of the differences can help separate the two hypotheses.”

For example, it is still not clear to what extent birds and predators can smell rump oil. In addition, it would be important to include more bird orders in the studies: for example, shorebirds have been studied primarily for the olfactory camouflage hypothesis. “In this way, we will gain deeper insights into the role of chemical cloaking and chemical signaling in birds,” Kaspers says.

Source: Bielefeld University. Articles: Biological Reviews, doi: 10.1111/brv.12837

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