TENGMALM’S OWL (Aegolius funereus) - Chouette de Tengmalm
Summary
A fairly uncommon bird of forests between 1000 and 2000m asl in Switzerland. Its calls are a series of rather monotone “poops”:
This owl is found throughout the boreal forest zone and is typical of that habitat. “Boreal” means "northern", and these forests are found between 50 deg and 60 deg latitude in a region which has short, moist, warm summers and long, cold, dry winters; 65% of such forests are found in Siberia, and the rest in Scandinavia, Canada and Alaska. In many places this bird is called the Boreal Owl (for obvious reasons). The European name “Tengmalm” comes from a Swedish medical doctor and scientist Peter Gustav Tengmalm (1754-1803) who formally identified the species in Sweden, and its original scientific name was Strix tengmalmi which has now changed but that original vernacular name stayed with it.
Across its range there were originally seven sub-species, but recent DNA analysis has made an argument for only 5, and also that the North American sub-species may be a true separate species - so watch this space ! One feature affecting this is that whilst the males are reasonably sedentary, juveniles and females may travel long distances in search of the small mammals that are their staple diet (Korpimäki & Hakkarinan 2012). This means there is much potential for genetic mixing across their range, and so clouding the species relationships. For a fuller account of the zoogeography and origins of this genus see Robb etal 2015.
The population in Switzerland is part of an isolated branch (see map), found in that zone where boreal conditions are created by altitude. It is found in the sub-alpine forests of the Jura and Alps between 1000m - 2000m. Like the Pygmy Owl they nest in old woodpecker nest holes, but being almost twice the size of that distant relative it is the old holes of Black Woodpeckers which Tengmalm's chooses. But even though it is bigger than a Pygmy Owl this is still a small owl, weighing between 100g and 180g only (females are larger than males). Its main diet is small mammals, mainly voles, although small birds are also taken sometimes.
Like other owls it hunts using sound as its main cue, and to help this it has evolved very special ear structures: the ear-slots occupy almost the full length of the skull and the right ear slot is 6.5mm higher than the left and differently shaped. The difference we hear between our two ears is important in locating sounds, so this effect is enhanced in Tengmalm's Owl, and by processing in the brain it is thought to make it a very accurate locating system (Norberg 1978). It also has internal ear structures that make it more sensitive to low frequency sounds like a small mammal rustling on the ground and it is said to be able to dive through shallow snow and catch voles which are tunnelling beneath based entirely on sound localisation (Voous and Cameron 1988). Such adaptations help it to hunt in those long dark winters.
Tengmalm Owl skull showing the asymmetry which enhances the accuracy of hearing
Eight vocalisations have been recorded for this species, the most commonly heard is the primary advertising and territorial song which is a series of rather hollow and rapidly repeated "poop" notes (cow bells and voices from a distant farm in the background):
The notes come in strophes varying from about 5-10 notes, followed by a gap of just a few seconds. Each strophe starts at about 700 Hz rising to about 850Hz, it also gets louder as it rises and then stops suddenly. See the sonogram above and watch it play below:
This is the principle song of the Tengmalm’s Owl, and it can be quite variable between individuals, some sing more notes in each strophe, or with different timings between notes and strophes, or perhaps small frequency shifts. The mixture of all these is probably how individuals can be identified, but there do not appear to be any regional dialects (Korpimäki & Hakkarinan 2012) . This individual identification is probably important because the males will attempt to become polygamous. They choose a primary nest hole and use the song to attract a female. Then, when the pair bond is secure, and the female in occupation of the nest hole, he may go to a second hole and try again for a second female and so on. These secondary nest holes can be as much as 3 km from the primary hole (Carlsson 1991). The success of such promiscuity is probably dependent upon food availability in any particular year (Robb 2015)
Once he has a female interested, the male can change his song and make what Robb (2015) calls “engagement hooting” this is a long sequence of notes not broken into strophes. This one I found also did not have a rising crescendo but was stable at around 700kHz, and so seems to be of a lower tone than the rising advertising song.
There are a variety of other calls used when the young hatch or when food is delivered, these are well-described in Robb (2015). But there is one call frequently heard which is a fairly loud short call, Robb calls it a “tsyuck” call, Korpimäki & Hakkarinan (2012) call it a “skiew” call. You decide which you prefer. It sounds very variable depending upon how close you are to the caller, or the purpose of the call. The above authors refer to it being used in aggression, defence, anxiety, excitement, or even just contact. Some of the variation is undoubtedly to do with the circumstances, and clearly there is still a lot to be learned here. I post below 4 cuts of this call: (1) the first cut has the call repeated 5 times, (2) the second cut is a single call made in October during the Red Deer rut, which you can hear clearly in the background (NB: it is not wise to wander in forest in the dark when testosterone loaded 200kg stags with antlers are looking for mates!) (3) is two calls from quite close by and (4) is a single call made in June when the cows were grazing in a nearby alpage.
You can appreciate the variability in the style of delivery of these calls. If we look at the sonogram of cut (3) we can see some detail of the call and how the explosive nature of it is clear when close enough to hear the higher frequencies:
All these recordings were made during the hours of darkness, but they will call at dawn or soon after as the light is breaking. Here is one during the dawn chorus it starts with the rather sonorous "chack-chack" alarm call of a Ring Ouzel which may have seen the owl, with Song Thrush, MistleThrush, Blackbird and Robin among the background singers.
This bird has been labelled in an unfortunate way: Aegolius comes from the Greek for “bird of ill omen” whilst funereus is “funereal” - the poor thing is the “funereal bird of ill omen” - I am sure it must star in some Scandinavian folk tales (Jobling 2011).
