Frederick II, the Holy Roman Emperor from 1220-1250, knew his birds. His ornithological magnum opus—De Arte Venandi Cum Avibus, in English The Art of Hunting with Birds—was probably the first treatise devoted to the study of birds. Despite his title, based on Frederick’s love of falconry, he describes many aspects of the behaviour, morphology, and ecology of all kinds of birds.
Frederick devoted an entire short chapter to what we now more often call the preen gland or uropygial gland. Here is his whole chapter, translated from the Latin (Wood & Fyfe 1943):
CHAPTER XXXI – OF THE OIL GLAND (PERUNCTUM)
This is a peculiar structure which lies above the tail. It consists of a double gland in the center of which (toward its end) rises a compact, stout elevation resembling a brush. This gland receives fluid oil from the body, which the bird squeezes out with its mandibles, collects, and then conveys to both feathers and talons, in consequence of which they are able better to resist moisture. Rain affects the oiled parts very little but runs off them completely and swiftly. Feathers and claws are thus preserved in good condition. Talons of birds of prey, owing to the noxious character of this oil, inflict more deadly injuries upon and bring about quicker death of their quarry because the wounds they make are toxic.
Birds differ in the amount of glandular secretion produced. Aquatic species as a rule have a larger gland and oil in greater profusion than either neutral or terrestrial birds.
Figure 1. Frederick II and Eagle.
Frederick is remarkably and characteristically perceptive and accurate here. His only mistake is his suggestion that the preen oil of birds of prey is toxic This was eventually corrected in 1678 by the equally perceptive John Ray and Francis Willughby in their monumental treatise on ornithology, The Ornithology of Francis Willughby (Ray 1678; Birkhead 2020). As Schöpffer (1896; cited in Wood & Fyfe 1943) pointed out, two centuries later, a toxic preen oil would probably have killed the very raptors who put it on their talons.
The study of preen glands and preen oils has been quite episodic. In the late 19th and early 20th centuries, there was a flurry of interest in the histology and embryology of the preen gland, and how that varied among species (see Hou 1928ab for comprehensive review). Several authors suggested, for example, that the structure of the preen gland might be a useful taxonomic criterion, particularly for defining genera and families.
Surprisingly, there was also a vigorous debate about the water-repelling properties of the preen oil. Charles Waterton [1782-1865] started this controversy with a series of articles claiming that he had “proof positive that the plumage of the bird has not been lubricated with oil from the tail gland.” and that birds preened only to remove lice from their feathers (e.g. Waterton 1932). Although several ornithologists soon provided evidence that preen oil was needed for water repellency (e.g. Matthews 1861), William Pycraft (1910) disagreed and instead felt that the uropygial was a scent gland, analogous to the scent gland of mammals. A century later we now know that he was insightful if not entirely correct.
Figure 2. White-winged Crossbill extracting preen oil from its uropygial gland © Darroch Whitaker BY SA 3.0 Wikimedia Commons.
Despite some convincing evidence of the waterproofing provided by preen oils (Hou 1928ab), the controversy was rekindled by Eugene Law (1929) and Harry Madsen (1941) both of whom conducted some simple experiments claiming to show that the feather structure alone provided the waterproofing and not the preen oil.
The matter was finally laid to rest in 1954 by William H. Elder [1913-2006], professor of wildlife management at the University of Missouri. In 1947 and 1951, he conducted experimental studies on the oil gland of ducks at the Delta Waterfowl Research Station in Manitoba, Canada. Among other things he showed that the preen oil of ducks (i) contains fatty acids and waxes, (ii) “maintains the water-repellent quality of feathers either directly or by preserving their physical structure”, (iii) is needed to maintain the structure of feathers between moults, (iv) helps to preserve bill condition, and (v) is essential for the survival of free-living ducks. He was uncertain about the role of the uropygial as a scent gland.
The study of uropygial glands lay relatively dormant for the next half century, but in the last 20 years there have been at least 70 publications on preen glands and oils. This renewed interest may have been re-awakened by discoveries about the olfactory capabilities of birds and the influence of olfaction on social behaviours, but also by technological advances in the analysis of preen oils and feather structures. We now know that preen oils have odours that other birds can detect (Grieves et al. 2019) and that they vary among individuals, across seasons, between and among mated pairs (Gilles et al. 2024), and with health and condition. Intriguingly, the preen oils of high latitude nesting sandpipers are less volatile during incubation when they might be detected by mammalian predators (Reneerkens et al. 2002). One thing we also now know for certain is that they are largely responsible for the shedding of water off a duck’s back as Frederick II observed almost eight centuries ago.
Further Reading
Birkhead, T.R. 2023. The Wonderful Mr Willughby: The first true ornithologist. Bloomsbury:London
Elder, W.H. 1954. The oil gland of birds. Condor 66. VIEW.
Gilles, M., Fokkema, R.W., Korsten, P., Caspers, B.A., Schmoll, T. 2024. Preen oil composition of Pied Flycatchers is similar between partners but differs between sexes and breeding stages. IBIS 166. VIEW.
Grieves, L.A., Bernards, M.A., MacDougall-Shackleton, E.A. 2019. Behavioural responses of songbirds to preen oil odour cues of sex and species. Animal Behaviour 156. VIEW.
Hou, H-C. 1928. Studies on the glandula uropygialis of birds. Chinese Journal of Physiology 2.
Law, J.E. 1929. The function of the oil gland. Condor 31.
Madsen, H. 1941. What makes bird waterproof?. Foren. Tidsskr. 35.
Matthews, H.S.R. 1861. Oil-gland in birds. Zoologist 19.
Pycraft, W.P. 1910. A history of birds. Meuthen & Co.:London.
Ray, J. 1678. The ornithology of Francis Willughby. John Martyn:London.
Reneerkens, J., Piersma, T., Sinningh-Damste, J.S. 2002. Sandpipers (Scolopacidae) switch from monoester to diester preen waxes during courtship and incubation, but why?. Proceedings of the Royal Society B 269. VIEW.
Schoepffer, C. 1896. Des Hohenstaufen-Kaisers Friedrich II. Bücher von der Natur der Vögel und der Falknerei, mit den Zusätzen des Königs Manfred. Aus dem Lateinischen übersetzt und versehen mit Originalzeichnungen, sowie einem Wörterbuch der Falknereisprache von H. Schöpffer. [The Hohenstaufen Emperor Frederick II’s books on the nature of birds and falconry, with the additions of King Manfred. Translated from the Latin and provided with original drawings, as well as a dictionary of falconry language by H. Schöpffer]. Paul Parey:Berlin.
Waterton, C. 1832. On birds using oil from glands “for the purpose of lubricating the surface of their plumage”. Magazine of Natural History 5.
Image credits
Top right: Mallard shedding water © George Thomas CC BY NC ND 2.0 Flickr
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