Does preen oil matter for incubation?

Preen oil: the Swiss Army knife of birds

When they preen, birds collect a waxy substance — called “preen oil” — from their uropygial gland (also called “preen gland”) and smear it onto their plumage. Preen oil is like a Swiss Army knife for birds, it serves multiple functions. While its primary role is feather maintenance and waterproofing, it may also serve as a protection against ectoparasites, an olfactory signal to attract mates or even a way to mask scent from predators (Figure 1). For decades, researchers have been trying to understand the function(s) of preen oil. But this is complex, especially because these functions are multiple and can differ between species, seasons and sexes.

Figure 1. Current evidence for the hypothesized functions of preen oil (Gilles 2024).

A function in incubation?

In a comparative study published in 2022, my colleagues and I had found that preen oil is expected to differ between sexes, especially in species with uniparental incubation (Grieves et al. 2022; summarised in a really nice BOU blog post). This suggests that preen oil has a role in incubation, such as protection against eggshell bacteria or olfactory camouflage against nest predators. Compelling results were notably found in shorebirds by Reneerkens and colleagues (Reneerkens 2007). They have shown that the composition of the preen oil of many shorebird species shifts from monoesters to diesters essentially during the period of incubation, and essentially in the sex that incubates. Diesters, which are produced during the period of incubation, are less volatile and less odorous. This shift may be a way to reduce the scent of the incubating parents and/or the nest, to reduce the detectability of the nest by foxes or other olfactorily searching nest predators (“olfactory crypsis”).

Figure 2. Kentish Plover © Marc Gilles.

Kentish plovers: no sex difference in preen oil

We put Kentish Plovers to the test (Figure 2). We hypothesized that female and male Kentish Plovers, a should secrete a similar preen oil during incubation, because it is a species with biparental incubation. Spoiler: this is exactly what we found! We sampled preen oil from Kentish Plovers in Samouco in Portugal, and we analysed its chemical composition using gas chromatography-mass spectrometry. We found no sex difference in chemical composition, neither in beta diversity nor in alpha diversity (the number and diversity of substances – Figure 3).

Figure 3. There were no sex differences in preen oil composition in incubating Kentish plovers. (a) Non-metric multidimensional scaling (NMDS) plot representing Bray–Curtis dissimilarity in chemical composition. The ellipses for each sex (95% confidence intervals assuming a multivariate t-distribution) overlap entirely, highlighting the absence of a sex difference in beta diversity. Besides, no sex difference was detected in alpha diversity, namely (b) chemical diversity (Shannon index) and (c) chemical richness (number of substances) of preen oil.

The mystery remains

What does this absence of sex difference in preen oil tell us? It tells us that Kentish Plovers follow the general rule: species with biparental incubation = no sex difference in preen oil during incubation. Does it tell us whether preen oil has a function in incubation, or which function it may have? No, it does not. The similar preen oil secreted by females and males during incubation may have a function for olfactory crypsis, as proposed for other shorebird species, but also for protection against ectoparasites and/or olfactory communication, but it may also have no incubation-related function at all. But it gives us hints.

Unlike other shorebirds

What was surprising was that, unlike other shorebirds, the preen oil of incubating Kentish Plovers contained mainly monoesters and no diester. Kentish Plovers thus differ from the other shorebirds studied by Reneerkens and colleagues, as their preen oil likely does not shift to diesters during incubation. Without diesters, the preen oil of incubating Kentish plovers likely does not have a role in olfactory crypsis as in other shorebirds. This warrants more research, ideally experimental studies, on the preen oil of Kentish Plovers.

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