By Justine Le Vaillant and Alexandre Terrigeol
The Boreal Owl (Aegolius funereus), a small raptor from the Strigidae family, is a key species in the boreal ecosystem. However, due to its nocturnal habits and scattered distribution, knowledge about its populations remains limited, and its demographic status in North America is still poorly understood. In Canada, its population is estimated to range between 50,000 and 500,000 individuals, but large fluctuations make management decisions particularly challenging.
Indicator species and long-term population monitoring
Considered an indicator species, the observation of its biology and population trends can reflect the overall health of the boreal ecosystem. Studying this owl provides crucial insights into the dynamics of other forest species and the environmental conditions of the habitat. However, population monitoring in North America is hindered by its nocturnal lifestyle, the inaccessibility of its forest habitat, and its dispersal behavior, which limits its distribution to the north.
The Tadoussac Bird Observatory and the Friends of Whitefish Point (Michigan, USA) both launched one of the few North American monitoring programs for this species in the mid-1990s. At the time, large numbers of individuals were occasionally reported as far south as southern Québec and the United States, with no clear explanation.
Initial data showed that populations fluctuate on a 4- to 5-year cycle, often associated with a low proportion of juveniles. When prey becomes scarce in the boreal forest, an irruption to the southern edge of its range is then observed.
Population trends and condition
Due to large interannual variations, providing an absolute estimate of population size and life history traits for this species becomes much more complicated. However, recent demographic models indicate a gradual decline in abundance, with peaks during irruption years becoming less pronounced. The juvenile-to-adult ratio, an indicator of boreal reproduction, shows annual variations consistent with the demographic cycle, but does not clearly confirm a significant decline in reproductive success.
Furthermore, body condition indices recorded at Tadoussac over the past 30 years show a decrease in fat and muscle scores during fall migration. This decline in muscle development may indicate habitat degradation, possibly linked to the reduction of old-growth forests. This trend is concerning, as it may negatively affect the Boreal Owls’ survival, reproduction, and winter resilience.
Causes and implications of population decline
The strong demographic fluctuations observed in the Boreal Owl are closely linked to the abundance of its prey. Population cycles of Gapper’s Red-backed Vole (Myodes gapperi) often synchronize with irruption years, characterized by southward movements in search of more favorable conditions.
The decline in abundance and deterioration of the owls’ physical condition appear to reflect habitat degradation, which could impact the entire boreal biodiversity.
Several factors may explain this decline. Habitat loss due to expanding forestry activities and increasing disturbances, reduces the number of old trees necessary for nesting. Concurrently, the decrease in old-growth forests leads to lower cone production, which is essential for vole abundance. Finally, climate change, including temperature fluctuations and extreme events such as freezing rain, reduces food availability, affecting reproduction and recruitment.
Guidelines for conservation and future population monitoring
Although this species is not considered threatened, it is essential to emphasize the importance of monitoring all bird species, as there is a risk of significant declines in populations once considered common. Ensuring the survival of this species requires intensified conservation efforts, monitoring not only its abundance but also the health status of individuals. For example, monitoring nesting sites could provide key data on reproduction and population dynamics.
The integration of modern technologies and genetic analyses to estimate the effective population size could help deepen our understanding of this species, which remains poorly studied in North America, particularly regarding its movements and distribution.
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In summary
The Boreal Owl is much more than a symbol of the boreal forest: it serves as a valuable indicator of the changes affecting this ecosystem. Monitoring efforts, particularly at Tadoussac, provide essential clues about the evolution of northern biodiversity. It is urgent to strengthen conservation actions by adopting a holistic forest management approach and to continue research on the factors influencing its populations in order to prevent its decline.
To read the article: Le Vaillant, J., A. Terrigeol, L. Desbordes, P. Côté, J. Lemaître, A. Anctil, C. Casabona, J. Ibarzabal, L. Imbeau, N. Mackentley, C. Neri, B. Rolek, C.J.W. McClure & J.-F. Therrien. 2025. Demographic trends for the Boreal Owl (Aegolius funereus) using standardized migration monitoring data in eastern North America. The Wilson Journal of Ornithology 122(1). (téléchargement)