Eurasian Eagle-owl

A male Eurasian Eagle-Owl Bubo bubo plays the landscape like a musical instrument. Crags of a certain shape and orientation to wind, water and forest allow him to project his hoots as far as possible. Each one has a sweet spot, ideal for targeting those listeners that matter the most. ‘Floating’ males must be repelled at all costs, neighbours should be kept exactly where they were yesterday, and a female’s demands must be met. 

A disused quarry with a tiny lake inside makes a fine bass drum. This one is in Extremadura, Spain. Normally the resident male plays it from the highest point on its rim. The quarry is tilted slightly towards the east, where neighbours on the cliffs of a gorge will hear him better than most. For listeners inside the quarry, he resonates long and powerfully. I’m sure his mate enjoys the sensation every bit as much as I do. 

Tonight there is no wind and the temperature is well below freezing. The roosting Griffon Vultures Gyps fulvus might as well be rocks. It is easy to forget a huge dormitory of White Wagtails Motacilla alba until the female flies too close and wafts them like dust off a shelf. A great roar of tiny wings tails slowly away as the wagtails return nervously to their perches.

Some time later the male hoots again while taking off and flying across the quarry (CD3-27). When he passes just a few metres away, the movement of his echoes reveals the shape of the quarry walls. Each hoot consists of two notes slurred together, the first higher and louder than the second. The female responds with gruff rrrèh calls from somewhere down on the right. 

160 km to the south, another pair breeds on a rocky headland overlooking the controversial Alqueva reservoir. Submerged below them, Aldeia da Luz (‘village of the light’) was sacrificed to the gods of progress when the Guadiana River was dammed in 2002. The owls also lost one of their old homes, a nest that disappeared below the surface. At least the water is good for one thing. It carries the male’s hooting over very long distances. In Sweden, Håkan once heard one hooting from an island that turned out to be 5 km away when he measured this on a map.

A huge, well-worn boulder forms the highest point of the headland. With such an obvious songpost, all I need to do is shelter my microphones from the wind, retreat some 500 m and wait. The male calls twice from this boulder during the night. CD3-28 must be more or less how a male Eurasian Eagle-Owl hears himself. He is much closer to the microphones than his echoes, so the recording lacks the aura of the ones I made in the quarry. In the background, his mate gives rrrèh calls from a lower cliff some 250 m away. Earlier in the night, she gave the same calls from the big boulder (CD3-29). Håkan has noticed that during the courtship period, these soliciting calls of the female become smoother and more inviting while still retaining a dangerous tone of voice. They remind him of a female spider, as if she were saying, “come to me… and I will eat you.”

During the autumn and early winter, females are more likely to hoot than to rrrèh. In October 2002, Arnoud and I recorded a pair in South Korea, belonging to the far eastern subspecies kiautschensis. The hooting started as we were returning from a small wetland, under a bright moon. The first owl in CD3-30 is a female, and her hooting is much higher-pitched than that of her partner. Now listen to a male and a female hooting in Bulgaria, on the shores of the Black Sea (CD3-31). Females not only tend to sound higher-pitched than males; they also have a smaller pitch interval between the first and second notes. In other words, hooting of females descends less than that of males. 

In December 2009 I visited the Sierra Norte near Seville, Spain, where Vincenzo Penteriani and colleagues have been studying Eurasian Eagle-Owls for many years. Much of their research has focussed on visible aspects of hooting. Watch an eagle-owl hooting by day, at dusk or under a full moon, and you may notice that with every hoot, a white and more or less rectangular patch or ‘badge’ appears on its throat. This becomes brighter during the territorial-mating period, and females have brighter badges than males (Penteriani et al 2006). Badges are honest signals of condition: research has shown that males with brighter ones breed more successfully (Penteriani et al 2007). By hooting most often at twilight, eagle-owls can flash their badges when they contrast most strongly with the background (Penteriani & Delgado 2009). For the same reason, they call more often and move to higher, more conspicuous songposts when there is strong moonlight (Penteriani et al 2010).

At dusk when I recorded CD3-32, I was amazed to hear up to four males and one very distant female calling in the space of just two minutes. The nearest two males were only about 500 m apart. At one particular spot, Vincenzo has heard up to eight eagle-owls at dusk, four males and four females. Amazingly, the closest pairs nest only 250 m apart. The area has high densities of European Rabbits Oryctolagus cuniculus, no doubt explaining why it is able to support around 40 pairs of eagle-owls in only 100 km2 (Delgado et al 2013). Vincenzo and colleagues have recently discovered hierarchies within this rich social fabric. It seems that in clusters of neighbouring territories, the males hoot in a particular order. The male that hoots first occupies the best territory and raises the most young, and also has the brightest badge and the highest percentage of red blood cells. The other males apparently hold him in such esteem that if he fails to hoot during the evening chorus, they remain silent too (Penteriani et al 2014).

Male hooting peaks during the territorial and courtship phases of the annual cycle, before gradually becoming less frequent through the rest of the breeding season (Delgado & Penteriani 2006). From late August, juveniles start to disperse: they become ‘floaters’. From roughly September to December, territorial males direct their hooting mainly at male floaters, which helps to prevent potentially deadly confrontations. They also hoot to maintain stable relations with their neighbours. Then in January and February, they hoot for the female too. During the courtship period, male songposts are often lower than during the territorial phase, because the female tends to stay close to the nest (Penteriani 2002).

As with most owls, male Eurasian Eagle-Owls propose nest sites to females who then make the final decision. In CD3-33, a male is proposing a nest in the resonant quarry with the Griffon Vulture roost. At the beginning, we can hear his wingbeats as he flies rapidly to a high perch on the left where the female calls rrrèh. Both owls hoot, then the male starts adding a few short, extra notes. As he flies down to a proposed nest on the right (from 0:30), he gives a continuous, rather breathless hooting with higher-pitched chitters interspersed. The female does not follow him this time, but comes a little closer without visiting the nest. 

Sounds associated with copulation are loud and carry far. CD3- 34 starts with the male hooting on the right and the female giving rrrèh calls on the left. The male then flies across to the female, still hooting. Then he erupts into an excited, rapid-fire hooting that has a chimpanzee-like quality. A moment later the female gives a high-pitched whine: her copulation call (0:32). She follows this with one hoot of her own, perhaps a hint of aggression. Then with a series of harsh, impatient rrrèh calls, she sends the male off hunting again.

If you go close to an eagle-owl nest without knowing it, you may get the fright of your life. ‘Devil’s cackles’ are excitement calls the owls use not only in this context but throughout the year, and especially during the pre-laying period. Most often, devil’s cackles precede or follow pair contact (Delgado & Penteriani 2006). CD3-35 from the Sierra Norte illustrates them at their most diabolic. CD3-36 from southern Finland shows how loud and resonant they can be, alongside the hooting of a male. 

At times the individual cackles can become slurred together as shown by a female in CD3-37, reminding me of the way a dog sometimes slurs its barks. When recording autumn migration I sometimes pick up very distant devil’s cackles of eagle-owls from my back yard, where lots of houses have the effect of dispersing the sound and making it sound muffled. There are many dogs in the neighbourhood, so it is easy to miss a distant cackle. CD3-38 is an autumn example with some more impressive wild mammals: rutting Red Deer Cervus elaphus. The owl is clearly a male, because of the low pitch of his devil’s cackles. The closer male in CD3-39 was a Spanish second-winter that had already completed his first breeding season as a territory holding ‘adult’ (mated to the female in CD3-35). 

Dick has often visited nests to ring the young. Besides devil’s cackles, he tells me that some angry females may give a loud, monosyllabic ‘crack’, a sign that they are ‘losing it completely’. Nevertheless, this is a fairly shy species that rarely attacks humans. Many other creatures are less fortunate. As opportunistic hunters, Eurasian Eagle-Owls will take mammals up to the size of a Brown Hare Lepus europaeus or even a young Red Fox Vulpes vulpes (eg, Olsson 1979). They often take other owls and raptors, with Common Buzzard Buteo buteo and Long-eared Owl Asio otus among their favourites (Mikkola 1983). It all depends on what is plentiful and easy to catch in their territory. During peak vole years in northern Europe, their diet may be almost identical to that of the Eurasian Pygmy Owl Glaucidium passerinum, which is 30 times smaller by weight.

Being close to the top of the food chain, it is perhaps surprising that eagle-owls have a well-developed distraction display. They only use it as a last resort when all other attempts to warn enemies away from large young have failed. Exposed and fluffed up, a desperate adult looks the intruder in the eye before throwing itself to the ground, dragging its wings and looking quite bedraggled, like a wader feigning injury near its nest. The irregular stream of high-pitched humming and piping that accompanies this display reminds Håkan of a Brown Rat Rattus norvegicus being tortured, or perhaps trodden on. 

Patrick Franke witnessed a distraction display in June 2013 while taking part in a ringing session in Germany. As the ringer got close to the juvenile, the presumed adult female flew off the nest, land-ed on the floor of the quarry and feigned injury. Despite a strong wind, Patrick managed to record the owl’s weird calls (CD3-40). 

Nestlings also have some special defensive tactics that they use when they feel threatened. From about 15 days of age, they will hiss and bill-snap in response to any disturbance (CD3-41). Some nestlings may also raise their wings in a ‘forward-threat posture’ common to many owls (Desfayes & Géroudet 1949), which has the effect of greatly increasing their apparent size. Half-grown young may also lie on their backs and raise their claws in threat (Schnurre 1936). 

Juvenile Eurasian Eagle-Owls beg with a loud, slightly harsh, short hiss. It may carry as far as 1 km (Frey 1973), while seeming to be much closer. This can sometimes lead to confusion with young owls of other species that have much weaker voices and may indeed be much closer to the observer. The two juveniles in CD3-42, recorded in Finland in late June, were already very large, and they called virtually all night long. 

As in other owl species, there is a chitter call used mainly as a signal of discomfort. According to Scherzinger (1974a), chicks start using this a day before hatching when the eggs are turned, then after they hatch it becomes gradually louder. The nestlings in CD3-43 were seven weeks old.

Karla Bloem of the International Owl Center has been studying the vocal development of another eagle-owl, the Great Horned Owl B virginianus of the Americas. She is breeding wild injured adults and observing them and their offspring with remote cameras and microphones. On 8 October 2013, she wrote to me: “Very interestingly, the owlets were able to produce ‘hoots’ of the proper rhythm at just over two weeks of age, albeit in very squeaky little voices. These little hoots only continued for three weeks. The owlets, when separated from their parents at 5 months of age, began hooting again, but their voice quality was like a teenage boy whose voice was changing. All three owlets finally attained their adult voice quality and produced territorial-style hoots indistinguishable from adults when they were between 6.5 and 7 months of age. In all instances, there was no practicing. They simply produced hoots of the proper rhythm in the proper context from the beginning. Even at 2 to 5 weeks of age their heads lowered forward when hooting and their featherless tail stubs tilted upward, although not as much as adults.”

Sounds of owls are thought to be ‘hard-wired’, not learned. Owls are unable to add attractive new sounds to their repertoire or imitate other individuals. The fixed nature of their vocal repertoire is of crucial importance in taxonomy. Owl sounds evolve through natural selection or drift, and this is a slower kind of change than can be achieved through learning. When two populations sound different, it strongly suggests that they are separately evolving lineages, which have not been in contact for a long time.

One population still included in Eurasian Eagle-Owl does seem to sound slightly different. Rothschild & Hartert (1910) described B b interpositus or ‘Byzantine Eagle-Owl’ based on a specimen obtained at Ereğli in southern Turkey, and that happens to be where I first recorded it (CD3-45). In the far east of Turkey, Roy Slaterus and I came across another pair a year later (CD3-46). Additional recordings from Georgia, Israel and Turkey made by other people gave seven adult interpositus, of which four males and two females were hooting. Compared to other Eurasian Eagle-Owls, the lower second note follows the higher first one slightly more quickly in Byzantine. Eurasian can vary quite extensively, so there is almost certainly some overlap in this character.

Such a subtle difference based on a tiny sample would hardly be worthy of comment, were it not for a surprise genetic result. According to Wink et al (2009), a single individual of interpositus from Israel was more closely related to Pharaoh Eagle-Owl B ascalaphus than to Eurasian Eagle-Owl. Its mtDNA differed from Pharaoh by 2%, suggesting that interpositus may even be a separate species.

Other subspecies of Eurasian Eagle-Owl are much easier to identify than interpositus, such as the very pale sibiricus, breed-ing in northern Russia from the Urals to the river Ob. Up on the Arctic tundra there is an eagle-owl that can be completely white, but that is a very different creature altogether.

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