Ardea
Official journal of the Netherlands Ornithologists' Union

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Smeenk C. (1972) Ökologische Vergleiche zwischen Waldkauz Strix aluco und Waldohreule Asio otus. ARDEA 60 (1-2): 1-71
The Continental Field Vole Microtus arvalis is one of the principal prey animals of the European birds of prey and owls. The predominance of this rodent in the food of so many predators seems to contradict Gause's rule, viz. that species with similar ecological requirements cannot live side by side in the same area. It should be kept in mind, however, as pointed out by Lack (1946), that the various predatory species may take a common prey in different ways and in different habitats. Moreover, the prey species may be so numerous temporarily, that food competition between the birds of prey and owls living on it is out of the question. Assuming that the population density of birds of prey and owls principally depends on the amount of available prey one now has to study a situation in which the density of the common prey species, in our case the Continental Field Vole, is at a low level. It then might appear that the various predators turn to quite different alternative feeding methods, showing striking ecological differences, so that interspecific competition during food scarcity is avoided. The possible existence of such differences between the Tawny Owl and the Long-eared Owl, both regarded as typically rodent-eating birds, is examined in the present paper. The study was carried out during the spring of 1965 in Twente, the easternmost part of the Netherlands province of Overijssel (Fig. 1). A comparison of this kind between two species would provide the most reliable data when carried out in areas where both species live side by side. Such areas were, however, very difficult to find. The search for nest sites of Tawny and Long-eared Owls already revealed striking differences in habitat selection between these species. In Twente the Tawny Owl nests in woods, park landscape and tree-rich cultivated areas (Fig. 2-5), thus being a species of the more elevated grounds (loam, sand). The Long-eared Owl, on the contrary, here nests in small patches of woodland amongst open meadows and fields and in the very edges of larger woods (Fig. 6); it is a bird of the young meadow and pasture landscape on brook sediment soils. In those places where the Kestrel occurred also the nesting sites of Long-eared Owl and Kestrel lay in the direct vicinity of each other. In some localities Tawny and Long-eared Owl were found to nest in the same area; there too, the differences in habitat selection appeared very clearly (Fig. 7, 8). From the Dutch and foreign literature one may conclude that similar differences in habitat selection occur in other parts of Europe. Assuming that an owl's nesting site is within or in the immediate vicinity of the bird's hunting area, an examination of the relations between habitat selection and hunting methods is obviously necessary. The Tawny Owl seems to be a still hunter mainly, looking for prey from a perch and gliding down onto its victim in a short flight. This hunting method marks this owl as a woodland inhabiting species. The Long-eared Owl searches for its prey in a persistent hunting flight over open country. This type of terrain is always directly accessible from the owl's eyries. Both owl species hunt birds in a different way than terrestrial animals; birds are taken away from their sleeping perches. The recognition pattern 'bird' is quite different from the pattern 'mouse'; a ground animal must look like a naturally moving body with legs, a bird like a motionless perching animal with a characteristic bird silhouette (Raber 1950). The Tawny Owl often indulges in hunting insects, probably mostly by running over the ground. The following morphological characters are in close connection with these differences in hunting methods: the Tawny Owl has short, broad, rounded wings with short 1st and 2nd primaries, the 4th and 5th being longest (Fig. 9). The Long-eared Owl has long, relatively narrow, pointed wings with a long 1st primary, the 2nd and 3rd being longest (Fig. 10). The index of Kipp (1959) indicates the ratio between the length of the primaries and the whole wing; in the Tawny Owl this ratio is 31-32, in the Long-eared Owl 46-50. A wing structure like that of the Tawny Owl is characteristic for woodland birds; their wing beats are rapid and shallow, giving the bird a high degree of manoeuvrability. The Long-eared Owl's wing type is that of birds of open country (long-distance flyers); here the wing beats are slow and deep, but the Long-eared Owl obtains a higher speed than the Tawny Owl. The wing surface per gram of body weight (wing loading) of the Tawny Owl is 2.7 cm2; that of the Long-eared Owl 3.2 cm2 (Brnll 1964), in agreement with the above differences. A heavily dark pigmented feather is stronger than a light coloured one; the more is demanded from a wing quill, the darker it is pigmented. The heavier pigmentation in the Tawny Owl's primaries as compared with that in the Long-eared Owl consequently is in agreement with the more rapid wing beats of the former. Only the Tawny Owl was studied with respect to its feeding habits during our investigations. In Table 5 the results of the pellet analyses are given in two kinds of percentages; those according to prey items show the numerical relations between the various preys in the pellets, those according to weights the proportional share of each prey species based on the average values for the specific body weights. For calculating the latter percentages we made use of the concept of prey units according to Southern (1954). One prey unit has been fixed by Southern upon 20 grams, being about the average weight of a small vole. In Twente a change in the Tawny Owl's diet becomes apparent in the course of the season (Fig. 11, 12). From April onwards shrews and small rodents decrease, birds on the contrary strongly increase, as to a lesser extent do the somewhat larger mammals: Mole, Rabbit, Water Vole and Brown Rat if taken together. This change has probably to be explained by the increasing density of the ground cover and the unfolding of the undergrowth leaves in the woods. The small terrestrial animals become less accessible to the Tawny Owl, therefore. During the same period the need of food in a Tawny Owl's family increases because of the growth of the chicks. The Tawny Owl now avoids these difficulties by changing to other preys belonging to higher weight categories. A decreasing amount of small mammals in the Tawny Owl's food is recorded in England as well (Southern 1954), but here a substitute is provided by the relatively larger mammals only, not by the birds. Other authors demonstrate an increasing amount of birds in the Tawny Owl's pellets during the summer. The make-up of the Tawny Owl's diet is highly varied (Table 1): this owl hardly seems to depend on one special prey species. This broad variation in the food composition reflects the fauna of the Tawny Owl's hunting terrain. The Continental Field Vole, living in open country, contributes only little more than 7% according to prey items, less than 5% according to weights (Table 5). The relations in numbers between the three small vole species: Bank Vole, Short-tailed Field Vole and Continental Field Vole in the Tawny Owl's pellets are 1.3: 1.1 : 1.0. The Short-tailed Field Vole and the Bank Vole in particular, are species of thicker cover than the Continental Field Vole areas where all these rodents occur. The list of bird species preyed upon (Table 1), shows a marked diversity of woodland species and additionally those confined to human habitation. The Skylark, though very numerous in meadows and fields, is completely absent from the Tawny Owl's food. Almost exclusively those species are taken that live high up the trees;' thrushes, tits, Starlings and sparrows are the most important birds for the Tawny Owl. Beetles too increase in the pellets from April onwards. Data from the literature were used to compare the food of the Tawny Owl with that of the Long-eared Owl. In Table 7 the pellet analyses of the Long-eared Owl from The Netherlands, arranged according to the various geographical areas, are summarized. Table 8 gives a comparison of the food of the Long-eared Owl in Twente with that of the Tawny Owl from the same area as shown in Table 5. The Continental Field Vole is the predominating prey species of the Long-eared Owl, as could be expected from this owl's hunting terrain. Shrews are rejected by the Long-eared Owl in the first instance, but can be taken as emergency food according to some authors. Larger mammal prey is rarely recorded for the Long-eared Owl. Bank Vole and Short-tailed Field Vole occur in very low numbers in the pellets. The Long-tailed Field Mouse, a species living in a broad variety of habitats, occurs in about the same numbers in the food of both owl species. Birds are eaten to a lesser extent than: by the Tawny Owl; among these House Sparrows predominate. Frogs do not occur in the Long-eared Owl's food, insects only very infrequently. When expressed in percentages according to weights, the differences in nutrition between both owl species are still more marked. Striking differences in the diet of the Long-eared Owl exist between the various areas in the Netherlands. This demonstrates the variation in landscape from place to place. In the remaining parts of Europe similar differences in food selection between both owl species occur. This becomes clear from the data of O. Uttend÷rfer (1939, 1952). These differences are especially obvious in places where food analyses from both species can be compared (Table 9). Short-tailed Field Voles may be taken more frequently by Long-eared Owls breeding in more wooded areas (Tables. 9, 10). The food showed striking similarities with that of the Tawny Owl in one particular locality, where a pair of Long-eared Owls resided in a for this species very abnormal, 'Tawny Owl like' habitat and where probably no breeding occurred (Table 9). This strengthens the view that the choice of hunting terrain is responsible for the differences in diet between Tawny Owl and Long-eared Owl. From the literature one may conclude that striking differences in the population dynamics of the Tawny Owl and the Long-eared Owl occur in close connection with the above differences in feeding habits. The Long-eared Owl shows marked oscillations in density of breeding population and average clutch size, depending on the population density of the Continental Field Vole. Where this vole species is lacking, other rodents of open country constitute the bulk of the diet of the Long-eared Owl. Among them are the Short-tailed Field Vole and the Long-tailed Field Mouse in Scandinavia and the British Isles. Bank Voles always are taken in only very small numbers even in such situations. During the winter large concentrations of Long-eared Owls often occur in those areas where the Continental Field Vole is abundant. Other concentrations are found near towns, where House Sparrows are hunted. By this more or less nomadic behaviour the Long-eared Owl is able to build up rapidly a breeding population in areas rich in Continental Field Voles. After the sudden break-down of the vole population many Long-eared Owls, unable to reach areas richer in food, are found dead. In Scandinavia the Long-eared Owl almost exclusively winters in favourable years, being a migratory bird otherwise; in other parts of its distribution area it is a more or less erratic bird. The Marsh Owl shows an even more strikingly erratic occurrence, depending on populations of the Continental Field Vole or other open country inhabiting rodents. The Long-eared Owl occupies a somewhat intermediate ecological position between the Marsh Owl and the Tawny Owl by its habitat selection. On the contrary, the Tawny Owl hardly or not shows these population oscillations, parallel to those of the Continental Field Vole. Although the Continental Field Vole when superabundant during plague years certainly is present in great numbers in the Tawny Owl's pellets, and although under these conditions the owl sometimes even produces a larger clutch than normally, this, however, by no means implies that the Tawny Owl gets into trouble in a period of scarcity of the Continental Field Vole. Throughout its distribution area it is a sedentary bird and mass mortality after a vole maximum does not occur. The average clutch size seems to be rather constant and increases from the south to the north; in Switzerland it is 3.2-3.3, in Denmark and Finland 3.6-3.7 and in Scandinavia 3.8-4.0. The annual mortality within a Tawny Owl population also increases in this direction. Among the first-year Tawny Owls in all parts of Europe mortality is greatest in summer and autumn, immediately after the time the chicks have become independent. Among the full-grown birds in Scandinavia and Finland the greatest mortality occurs in winter, in Switzerland in early spring. In Scandinavia and Finland winter mortality is the higher as the winter is more severe or snow-rich. We know nothing about the causes of the higher degree of spring mortality in Switzerland. The higher degree of winter mortality in Scandinavia as compared to that in more southern areas is not likely to be directly related to temperature, but to more extreme food conditions during such circumstances in the north. In England the average clutch size is very low, viz. 2.5. Winter mortality is very low here, probably due to favourable food conditions in the mild winters in the oceanic climate. In this country the reproduction rate of the Tawny Owl shows oscillations, correlated with those of the small rodents, especially the Bank Vole: in. bad mouse years a considerable number of pairs make no breeding attempts. In Finland a similar dependence of the Tawny Owl on the Short-tailed Field Vole exists. Another factor also determining the reproduction rate of the Tawny Owl is the population density of the owls themselves. Southern (1959) and Lack (1%6) described the developments for many years within a Tawny Owl population in England. After a break-down of the population in a given area during a very severe winter the number of resident pairs increased gradually. This means a decrease of the average territory size per pair. Simultaneously however, the average clutch size and the number of off-spring per pair decreased; the number of pairs making no breeding attempts in bad mouse years increased. This possibly means that in small territories the Tawny Owl becomes more sensitive to even small oscillations in the density of its prey animals; the risks for the Tawny Owl become more pronounced by the decreasing buffer effect of the hunting territory with its fauna. Andersen (1961) studied a population of Tawny Owls in Denmark, in an area markedly similar in landscape to that of the English terrain of study. He found the actual numbers of Tawny Owls, present in July on one unit of area, to be about equal to those in England. Territory size and reproductive rate are higher in Denmark than in England. Possibly the changes in the reproduction mortality equilibrium within the English population can be considered as a model for the phenomena taking place in this respect within the whole Tawny Owl population in Europe at different latitudes, and probably changes in territory size play a role here. In fact, the territory size in Finland is much larger than elsewhere in Europe. Summarizing it appears that the Tawny and Long-eared Owl, which are known as typically mouse-eating birds, show striking differences with respect to their habitat selection, hunting methods, morphology, feeding habits, movements outside the breeding season, and population dynamics, or briefly, have a completely different biology. The main prey animals in common are the Continental Field Vole, the Short-tailed Field Vole and the Long-tailed Field Mouse. However, these either show such diverse ecological qualities that each owl species obtains them in its own way and its own habitat, or they are so numerous that food competition between the predators is out of the question. Tawny Owl and Long-eared Owl both react quite differently to scarcity of a common prey species.


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