The selection criterion of prey by owls and kestrels

When examining the diet of a predator, it is often assumed that the choice of prey available in a territory if present in abundance is the main selection criterion adopted by the predator.

This perspective follows the theory of optimization in food research. According to this theory, it is assumed that the abundance of potential prey in the hunting area becomes the key factor in determining the trophic spectrum of a bird of prey.

But is this really the case in reality?

A recent study published in Avian Research offers an intriguing look at this topic and a different version too.

An article that deserves great attention because it places valid stimuli on the evaluation of the predatory choices made by birds of prey and leads us to reflect on the field observations we make.

A team of Russian researchers, coordinated by Tatyana Kovinca, conducted accurate and prolonged monitoring from 2007 to 2019 in an area of 48 km² of the Crane Homeland Reserve, in Moscow region, Russia.

Short owl on the hunt (ph. Luca Avanzini )

The study focused on the influence of habitat structure on the composition of the diet of three species of birds of prey: the long-eared owl (Asio otus ), the short-eared owl (Asio flammeus) and the common kestrel (Falco tinnunculus).

The habitat structures were classified using the "Semi-Automatic Classification Plugin" module based on QGIS.

Flexibility in the composition of the diet of birds of prey is well known, with many factors that can influence trophic interactions between predator and prey. According to the theory of optimization in the search for food, predators select favorable habitats for hunting in order to minimize energy consumption.

The choice of habitats with high abundance of the main prey species is a key element that can determine the composition of the diet of birds of prey.

For example, the long-eared owl's diet varies based on the abundance of the main prey species. In years of low abundance of field vole (Microtus arvalis), its presence in the owls' diet decreases, favoring alternative prey.

Eared Owl

Moreover, also in my monographic book on Short-eared owl I had underlined the importance demonstrated by research throughout Europe on the species, particularly in France and Spain, on the relationship betweenMicrotus arvalisand Asio flammeus. However, the composition of the diet is not always linked only to the abundance of prey; other environmental factors, such as the presence of abundant snow cover and low temperatures, can often play a significant role.

On the other hand, the foraging opportunities hypothesis suggests that a highly heterogeneous habitat is associated with greater dietary diversity.

The diversity of fauna increases with the heterogeneity of the habitat and the increase in available resources. This is particularly relevant for birds of prey, where habitat heterogeneity affects the composition of small mammal communities and the availability of specific prey species.

Recent studies have highlighted a positive relationship between the diversity of owl diets and the landscape complexity of hunting areas. The presence of open areas with low vegetation is positively associated with the presence of voles in owl diets.

The long-eared owl, the short-eared owl and the long-eared kestrel are widespread species in central and northern Europe, as well as in central Russia. These species show a high degree of trophic niche overlap, basing their diet mainly on rodents of the genus Microtus.

The study analyzed a total of 7628 prey items. The composition of the long-eared owl's diet included prey elements belonging to all classes of vertebrates, insects and molluscs.

Data analyzed in the Russian study by Kovinca et al, 2023.

Micromammals made up 96.9% of the diet, with Northern Vole Microtus oeconomus*(43.7%) and field vole (42.1%) emerging as main prey.

The percentages of other vertebrates, as well as insects, did not exceed 2%.

As for the Common Kestrel, small mammals were the main component of its diet, although the percentage was lower than that of the two owl species (59.5%) . Field vole (19.3%) and Northern vole (20.4%) dominated its prey selection.

Insects occupied second place in the birds' diet (38.1%). In summary, the two Vole species play a significant role in the diet of the birds analyzed.

Kestrel ph. Claudio Crespi

The proportion of Field Vole was highest in the Short-eared Owl diet and lowest in the Common Kestrel diet. Similarly, the proportion of Northern Voles was higher in the long-eared owl's diet.

In conclusion, the study conducted in Russia is really interesting as it suggests that the ratio between different types of habitat elements, such as open areas and areas of forest and bush, are predictors crucial factors that determine the diet of birds of prey.

This ratio according to the article published in the journal Avian research can influence the availability of prey, with open habitats and low bushes favoring more effective hunting , even in the presence of a low abundance of prey.

Therefore this interesting study focused on evaluating the relative importance of habitat structure compared to prey abundance.

*We remind you that the Northern Vole today is also classified as Alexandromys oeconomus, the topic is debated but it is in the recent past it was classified as Microtus oeconomus. In the article it is still described as the genus Microtus.

Bibliographical references

Aschwanden, J., Birrer, S., Jenni, L., 2005. Are ecological compensation areas attractive hunting sites for Common Kestrels (Falco tinnunculus ) and Long-eared Owls (Asio otus). J. Ornithol. 146, 279–286.

Mastrorilli M., Bressan P., 2011. The short-eared owl. Noctua Book pp.208.

Mastrorilli M. , 2016. On the trail of owls. Noctua Book. pp.192.

Pyke G., 1984. Optimal foraging theory: a critical review. Ann. Rev. Ecol. Syst. 15:523-575

Korpimäki E., 1992. Diet composition, prey choice, and breeding success of Long-eared Owls: effects of multiannual fluctuations in food abundance. Canadian Journal of Zool. 70(12): 2373-2381.

Kovinca T., Sharikov A., Massalskyya T., Volkov S., 2023. Structure and heterogeneity of habitat determine diet of predators despite prey abundance: Similar response in Long-eared Owls, Short-eared Owls and Common Kestrels. Avian research 14: 1-8.

You can find a lot of information on the diet of nocturnal birds of prey in my book "On the trail of owls" 7th printing.

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