In addition to hearing and sight, birds also use less obvious senses, including smell. They lubricate their plumage several times a day with a fragrant secretion from their preen gland. The scent of this rump oil suggests that it is not only used for practical plumage care. In a meta-study, researchers have now examined in more detail what the smell of the secretion could be used for.
Most bird species have a special gland at the base of their tail: the preen gland. It secretes an oily secretion that the birds spread over their feathers with their beak several times a day. This rump oil is used, among other things, to care for the plumage, to lubricate it and make it water-repellent. But the smell of this secretion has long aroused the suspicion that rump oil could have other functions that little is known about.
To clarify this, Leanne Grieves from McMaster University in Canada and her colleagues evaluated the data from a total of 55 studies dealing with the rump secretion and its composition. Above all, seasonal fluctuations in the chemical composition of the Bürzel oil should provide information about its function. The researchers’ idea: If the rump secretion is more odorous during the birds’ breeding season, this could indicate that the oil is used to camouflage the unprotected young birds from predators. A gender-specific difference in the composition, especially during the mating season, would support the hypothesis of the communicative function of the scented oil.
Rump secretion to camouflage the nest
The studies, which looked at seasonal variations in the composition of rump secretion, confirmed a variation over time in 95 percent of the bird species studied. “Seasonal changes in the chemical composition of rump oil were almost ubiquitous,” report Grieves and her colleagues. Especially during the breeding season, the oil secretion of the parents of the birds contained significantly more of the organic molecule diester. This evaporates more slowly, changes the scent mark of the nest and is therefore better able to hide it from predators, the researchers suspect.
The research team also took a closer look at the link between rump oil and bird nest location. In fact, seasonal differences occurred primarily in bottom-nesting birds such as shorebirds. Co-author Marc Gilles from Bielefeld University explains why this could be the case: “Of course, ground-breeders are particularly endangered by predators such as foxes, who use their scent for orientation, because their nest is so easily accessible. An olfactory camouflage would mean higher chances of survival for the young animals”. Also striking: In the bird species in which both partners breed, the rump secretion contained the same amount of diesters in both sexes. On the other hand, if only the male or only the female incubated, their rump oil smelled correspondingly stronger.
Recognition of partners by Bürzel-Sekret
Overall, however, gender-specific differences were much less common than seasonal fluctuations. The studies showed differences between male and female birds in around 47 percent of the bird species examined. Here, too, the researchers suspect a connection: “When we found differences between the sexes, these occurred primarily during the mating season,” reports Gilles. The secretion of females was usually associated with a significantly greater abundance and variety of chemical compounds.
They suspect that this could be an expression of increased female receptivity. But the change in female rump oil could also serve territorial scent marking or recognition between mother and offspring. In fact, in some studies, the chemical composition of the secretion is also associated with the expression of the immune system. Birds could use their smell to tell how closely related they are. This gives the birds clues as to whether a potential mate is genetically compatible enough to produce healthy offspring.
Data still too imprecise
The results therefore indicate that both hypotheses can be correct. The odor of the rump oil could serve both to protect the nest and the chicks from predators and to give information about the reproductive status of conspecifics during the mating season. “Unfortunately, there is currently not enough data to further test the hypotheses,” says senior author Barbara Caspers from Bielefeld University. “However, additional information about the nature of the differences could help to separate the two hypotheses.”
For example, it is still unclear to what extent the birds and their predators can even smell the odor of the rump oil. In addition, it would be important to include more bird orders in the studies: for example, coastal birds have been primarily studied for the hypothesis of olfactory camouflage. “In this way, we would gain deeper insights into the role of chemical masking and chemical signaling in birds,” says Caspers.
Source: Bielefeld University; Articles: Biological Reviews, doi: 10.1111/brv.12837