Prey spectrum

Worldwide, a total of 477 prey species of the Longeared Owl were found (Table 2). Prey reflected 180 mammal, 191 bird, 15 reptile, 7 amphibian, and one fish species. Many invertebrate prey remains could not be classified to the species level, rather they were assigned to 83 different invertebrate groupings. The vast majority (99.6%) of identified prey items were vertebrates (Table 1). Among the vertebrates, mammals made up the majority of the prey, on average 93.5%. Birds were the second largest group with 6.2% (Table 3). The remaining vertebrate classes summed to a fraction of a percentage. In southern Europe 1.8 invertebrates could be found per 100 vertebrates. By contrast, in African prey lists, there were 26.9 invertebrates per 100 vertebrates. When related to the body mass of the prey species, however, invertebrates accounted for only 1% of the biomass.

The broadest prey spectrum (195 species) was found in Central Europe (Table 2). Prey spectrums from the other regions were: southern Europe (168 species), North America (133 species), south-eastern Europe (102 species), the British Isles (74 species), northern Europe (59 species), Africa/Near East (59 species) and Asia (45 species). However, only a few studies were available from these latter two regions.

Body mass of adult prey items ranged from less than 1 g (invertebrates) to over 500 g, with the majority of prey consisting of species ≤ 50 g (Fig. 1). When preying on heavier species, mostly juveniles were taken. The heaviest verified prey seized by a Long-eared Owl were two Ruffed Grouse Bonasia umbellus (Sutton 1926); female grouse average 575 g (Rusch et al. 2000) in their first winter, while males and older females are heavier.

Table 1.

Number of prey lists and prey items of the Long-eared Owl grouped by regions.

Figure 1.

The global avian and mammal prey species of the Long-eared Owl, arranged by adult mass of the prey species. Black portion of bars reflect avian species, grey portions of bars, mammal species.

Table 2.

Number of prey species of Long-eared Owl grouped by systematic class and region.

Table 3.

Number of prey items of Long-eared Owl grouped by class and region.

Diet composition in major regions

Frequency of Prey. Overall, regional differences in the frequencies of vertebrate prey were small (Table 3). The percentages of mammals in the diet for the various regions were: North America (98.1%), northern Europe (96.6%), Central Europe (94.2%), south-eastern Europe (93.0%), southern Europe (89.0%), the British Isles (87.9%), Africa/Middle East (86.4%), and Asia (80.9%). The percentages of birds in the diet for the regions were: North America (1.8%), northern Europe (3.4%), Central Europe (5.8%), south-eastern Europe (11.0%), southern Europe (5.9%), the British Isles (12.0%), Africa/Middle East (6.8%), and Asia (19.1%). The African prey lists contained a relatively large number of reptiles (6.8%) (Table 3).

The largest prey class among the mammals were rodents which made up 91.7% of the vertebrates (lowest 79.5% in Asia, highest 93.3% in northern Europe). The next common mammalian group were insectivores, reflecting 1.2% of the vertebrates. The insectivores were relatively scarce in prey lists from Africa/Middle East, Asia, and Central Europe (<0.7%), but slightly more frequent in North America (4.1%), northern Europe (3.2%) and the British Isles (3.5%).

At the taxonomic level of family, the pattern in the prey lists is less uniform. In Central Europe, Arvicolinae dominate with 80.1% of the vertebrate prey. In the rest of Europe, in North America and Asia, this family was also predominate, although the proportions were markedly lower (minimum 43.1%). The Murinae were second in Europe and the dominant family in Africa/Middle East (71.5%) and Asia (59.8%) while they are nearly absent in North America. In North America, the Cricetinae were the second largest family (51.0%).

Even more pronounced differences were apparent at the genus level. The genus Microtus was the most frequent prey in five regions: M. arvalis was the most common species in Central Europe and in south-eastern Europe (72.5%, 46.7%, respectively); M. agrestis in northern Europe and British Islands (48.8% and 37.0%, respectively); and M. pennsylvanicus in North America (23.8%). In south-eastern Europe, Apodemus was the most numerous genus accounting for 21.0% of the prey frequency, while in the British Islands, northern Europe, and south-eastern Europe the same species accounted for 35.6, 24.7 and 22.1%, respectively. In Africa, 67.3% of the diet was comprised of the species Mus.

Table 4.

Maximum proportion of prey species in Long-eared Owl diet. Prey species which reached at least 50% by frequency in at least one list were included. The number of lists in which a certain prey species is dominant is given between parentheses.

Worldwide, there are at least 23 species which appear as the most frequent prey in at least one list (Table 4). In Central Europe Micotus arvalis is the most common prey species in 81.6% prey lists. In other Central European lists, Apodemus sp. (8.5%), birds (7.2%), Arvicola terrestris, Microtus agrestis, M. oeconomus and Clethrionomys glareolus predominate.

Biomass of Prey. The 394 vertebrate and 83 invertebrate groups were organized by categories, based on the mass of the adult for each species/group. Weight categories for bird and mammal species are shown in Fig. 1, as these were the major prey. The majority (66.5%) of the vertebrate species were <50 g. Overall, the percentages by biomass correspond largely to the percentages of prey items. Even at the level of prey types, the differences between percentages of prey items and biomass are usually small. In half the cases, this difference is below 3.5% and in 90% of cases it is less than 11.1% (Fig. 2). The only exception to this pattern is found in prey lists in which the genus Rattus is frequent. The largest differences between the percentages of biomass and prey items at the class level occur in Asia. Mammalia account for 73.1% of the biomass but make up 80.9% of the prey items; birds account for 26.5% of biomass and 19.1% of prey items.

Figure 2.

Percentages of prey items compared to percentages of prey biomass. Triangles = Rattus spec. n = 6072 prey items from 798 prey lists of the Long-eared Owl.

Representation of diet studies

Compared to the estimated global distribution of Long-eared Owls, diet studies are strongly reported in some areas, and under-represented in others. Data from Central Europe (especially Germany) is most strongly represented. There are only relatively few lists with quantitative data from North America. Diet lists from Africa and Asia are limited and likely under-represent actual prey arrays (however, lists from Asian areas exist but were logistically difficult to obtain, and language translation was not available to this researcher). Diet lists within a certain region are not necessarily representative either, with some papers reporting dietary exceptions, e.g. data from severe winters or special habitats such as islands.

Proportions and biomass

In this study, I refer to proportions of prey items because uncertainties are involved in the calculation of body mass proportions. In many prey species, body mass varies considerably with geographical location, sex and individual traits. Often body mass of the lightest adults is only half of that of the heaviest. Also, it has been shown that Long-eared Owls prefer smaller items of the larger prey species. Further publications containing data on sex, age classes and a species mean (and range) of mass for prey would be very helpful for biomass calculations (Tome 2000, Pirovano et al. 2000).

The composition of the Long-eared Owls' diet depends on an array of interacting factors (Fig. 3). Season and geographical location of the diet samples, which are usually indicated in the publications, both have an effect on prey diversity. Some papers mention various other factors. Usually, the most abundant species available are preyed upon most often (Canova 1989, Guidoni et al. 1999). The proportion of Microtus in the prey lists increases with increasing density of this species (Schmidt 1971, Veiga 1981, Wijnandts 1984, Korpimäki 1992, Tome 1994, 2003).

Aspects of predation

Prey abundance does not necessarily explain the degree of predation, as prey need to be accessible to the owls. Factors like vegetation, weather and prey behaviour can influence accessibility (Fig. 3). The Long-eared Owl prefers low and sparse vegetation for hunting (Nilsson 1981, Aschwanden et al. 2005). Therefore, prey lists from habitats with different vegetation structure and from different cultivation stages (e.g. before or after harvest of cereal grains) can vary considerably (Tinbergen 1932). Apodemus, for instance, is captured more frequently in or near woods (Wijnandts 1984).

Accessibility is influenced indirectly by the season (Fig. 3). Some authors reported no seasonality (Graber 1962, Marti 1974, Armstrong 1958 in Marti 1976) while others found differences between winter and summer (Degn 1976, Nilsson 1981, Wijnandts 1984, San Segundo 1988, Bertolino et al. 2001, Rubolini et al. 2003).

Weather has an influence on the accessibility of prey items. Many prey species profit from snow cover and this is mirrored in the diet lists (Elvers et al. 1979, Wijnandts 1984, Schmitz 1987, Ancelet 1987, Canova 1989). Hunting sites and hence prey composition differ in nights with and without wind (van Manen 2001). In America, however, wind did not influence hunting site selection in radio-tracked breeding Long-eared Owls (Craig et al. 1988).

The behaviour of a prey species can influence its availability for an owl. Young males of otherwise subterranean rodents display a dispersal pattern during which they can be found on the surface and are therefore more vulnerable to predation by owls. This is a possible reason for the preference of young Arvicola terrestre (Saucy 1988, Tome 2000) and Thomomys talpoides (Bull et al. 1989). Behaviour may also explain the vast differences in the genera Microtus and Pitymys in the prey lists of Long-eared Owls in Europe. While Microtus can be seen regularly on the surface, most Pitymys species, e.g. P. multiplex, live much more subterraneously (Hausser 1995).

Figure 3.

Possible factors influencing diet composition of the Long-eared Owl.

Not all species are equally attractive prey for Long-eared Owls. The attractiveness of a prey species depends on species-specific and individual qualities. Most important is probably size. Generally, vertebrates ≤ 300 g are potential prey items for Long-eared Owls (Fig. 1), with the dominate prey consisting of species ≤ 50 g. The larger prey items reported were probably weakened or injured at the time they were seized. It was assumed several times that owls fed on carcasses. Of the larger species, the smaller individuals are often preferred, e.g. in Arvicola terrestris (Saucy 1988, Tome 2000), Rattus norvegicus (Gawlik & Banz 1982, Hillis et al. 1988, Ekins & Steward 1994, Pirovano et al. 2000) and Thomomys talpoides (Janes & Barss 1985, Bull et al. 1989, Marks et al. 1994). In medium-sized prey species, i.e. Clethrionomys and Apodemus, immature individuals also tend to be taken more often (Lodé 1994). Tome (2000) found a significant size difference between individuals captured as prey by owls and those from trapping efforts in all prey species except for Apodemus sylvestris.

Shrews are avoided by Long-eared Owls (Gross 1943) and items caught are sometimes even dropped (Thiollay 1968) presumably because of their smell (or taste). Correspondingly, compared to the prey lists of other owls, especially the Barn Owl, collected in the same place at the same time, there were fewer shrews found as Long-eared Owl prey (Uttendörfer 1939, Lopez-Gordo et al. 1977, Veiga 1980, Erfurt & Stubbe 1987, Roulin 1996). Individuals with poor body condition could also be easier prey for Long-eared Owls. In the pellets, prey items with deformed bones are regularly found and their proportion decreases over the winter (Manegold 2000). Yet, it is not known whether such items are preyed upon selectively. In a lab experiment, a Long-eared Owl hunted more lab mice which were infected with Sarcocystis dispersa (Vorisek et al. 1998).