Seminar at Department of Population Ecology Thursday 28th of September 2000
Living at the limit: ecology and behaviour of Tawny Owls in a northern edge
population in central Norway
Peter Sunde
The Tawny Owl (Strix aluco) was studied at the northernmost limit of its geographical range in central Norway (6320’N) in order to
investigate how this extremely residential and territorial species adapted to the presumed marginal conditions of its northern outpost.
Playback recordings (male hoots) revealed that presence of southern broadleaved forest vegetation was crucial for the occurrence of
the species The response frequency to playback at identified owl territories (<28%) were considerably lower than in high density
populations in UK (94%) and Italy (>50%), indicating less ability to detect intruders and/or less willingness to invest in territory display.
The mean annual home range size of radio tagged females was much larger than reported in any other study, indicating that the
environment was of considerably poorer quality for the species than in any other study. The winter (1 October-15 March) home ranges,
measured with the fixed kernel method, were on average 54% larger than those of the summer (1 June-30 September), indicating that
larger areas was needed to fulfil their metabolic requirements. The sizes of the summer and winter ranges were not correlated,
indicating that different ecological factors determine the owls’ spatial needs during the two seasons. In summer, home range size was
negatively correlated with the proportion of mixed deciduous/coniferous forest within 1km of the nest box. This may be explained by the
abundance of bird prey since 61% of the summer diet consisted of birds, and mixed deciduous/coniferous forests sustained the highest
densities of birds during this season. In winter, no significant correlations with home range size were found. During this season small
mammals comprised 95% of the diet. It is noteworthy that the owls had the narrowest diet niche during winter when food was most
scarce. This indicates that the classical generalist feeding strategy was difficult to practise.
In extreme cases, females left the nesting area for prolonged periods during the non-breeding season, sometimes as early as
September when prey were still readily available. It is concluded that even a species known to be notoriously residential is able to
express a high degree of plasticity in its ranging behaviour when population densities are low and/or environmental conditions marginal.
However, the environmental and social factors determining whether some females leave their nest areas in the non-breeding season
are more complex than the result of mere lack of food.
Note. These data were collected 1994-98. For novices considering the possibility of starting a radio-tracking study, this seminar should
also give a practical demonstration of how a telemetry survey not should be done.