Threats

Threats to the Boreal Owl (Aegolius funereus)

The main known factors that may negatively affect breeding populations of the Boreal Owl are described below, together with a brief assessment of their significance.

Shortage of Nesting Opportunities
In commercially managed forests, particularly spruce stands, there can be a scarcity of suitable natural nest cavities—primarily those excavated by the Black Woodpecker (Dryocopus martius). The availability of nesting sites can thus represent a significant factor limiting the density of breeding Boreal Owl pairs. The lack of natural cavities can be partially mitigated by installing suitable nest boxes.

Predation
One of the factors influencing the nesting success of the Boreal Owl is predation, particularly by the Pine Marten (Martes martes) and potentially the Beech Marten (Martes foina). Martens are capable of entering certain nest cavities and boxes, where they may prey upon eggs, nestlings, and incubating females.
A major advantage of the nest boxes we install for the Boreal Owl is their structural protection against predation. The boxes designed by us feature sheet-metal cladding on outer sections, which also extends around the entrance hole. This metal sheeting prolongs the lifespan of the nest boxes while significantly restricting martens from entering the nesting cavity. The effectiveness of this design feature is confirmed by our long-term field experience. By contrast, in some nest boxes lacking protective elements, we repeatedly record the presence of martens.

Food Supply and Availability

The supply and availability of food—primarily small mammals—are among the most critical factors influencing the breeding density and reproductive success of the Boreal Owl. In years of small-mammal population peaks, the number of breeding pairs can increase substantially. Furthermore, spatial clustering of pairs can occur in food-rich localities, with several pairs nesting within a relatively small area.
In years of food scarcity, the Boreal Owl may skip breeding entirely, or its reproductive success may be severely reduced. If a significant decline in food availability occurs during the breeding process, it can lead to clutch abandonment, nestling mortality due to starvation, or premature termination of nesting. Based on our field experience, cases of failed nesting attempts with fully feathered fledglings are also known.

Other Negative Factors
Additional causes of Boreal Owl mortality include predation by larger owl species, particularly the Eurasian Eagle-Owl (Bubo bubo), Tawny Owl (Strix aluco), and Ural Owl (Strix uralensis). Mortality can also occur through collisions with road vehicles or railway traffic.
Secondary poisoning resulting from the ingestion of small mammals affected by rodenticides can also be considered a potential risk. However, the significance of this factor for Boreal Owl populations remains insufficiently documented.

Author: Karel Poprach