This is a video about a Tengmalm’s owl.
When should you eat?
Faculty of Environmental Sciences
Czech University of Life Sciences Prague
2 March 2015
One could expect that the reproductive success of birds with large geographical distribution will be higher in the core of their breeding range than on its margin. Here we ask, how a nocturnal avian predator with wide Holarctic distribution – Tengmalm’s Owls Aegolius funereus – deals with contrasting ecological conditions of Northern (core of range) and Central (margin of range) Europe.
On a species level, populations occurring in different parts of the distribution range often face contrasting ecological conditions; these local conditions can modify the selective pressures affecting life-history characteristics, which in turn may lead to variation in reproductive strategies (Tieleman 2009). For example, Dark-eyed Juncos Junco hyemalis, which breed in North America across an extensive altitudinal gradient (Bears et al. 2009), provide a textbook example of within-species variation in life histories. Juncos inhabiting high elevations start to breed later and their reproductive season is constricted so that on average they are able to raise only half the number of young compared to conspecifics living at lower elevations. Lower reproductive success of the montane population is, however, balanced by higher between-year survival rates (Bears et al. 2009). Unfortunately comprehensive studies are scanty and our understanding of the variation in reproductive strategies among populations to maintain their viability is still far from satisfactory.
Birds of prey provide an especially suitable model for intraspecific comparisons as they tend to have large geographical ranges covering areas with contrasting ecological conditions, and show great variability in reproductive effort both within and between populations of the same species (Korpimäki & Hakkarainen 1991, Laaksonen et al. 2002). Moreover, they are well-explored example of huge intraspecific variation in life-history traits, as they are able to adjust their reproductive strategies across space and time according to actual food availability (Hakkarainen et al. 2003, Valkama & Korpimäki 1999). Furthermore, estimating the spatial and temporal availability of main foods of avian predators is relatively easy, making it possible to examine one of the main determinants of reproductive effort in birds (Martin 1987).
Variation in food abundance in both northern and temperate areas has a crucial impact on the reproductive strategies of avian predators, whereby seasons with high food availability lead to increases in clutch size (e.g. Lehikoinen et al. 2011) as well as current and lifetime reproductive success (e.g. Korpimäki 1992). However, at higher latitudes, voles, which comprise the dominant prey of most birds of prey, undergo regular 3-4 year cycles and great multi-annual and intra-seasonal changes of abundance (Korpimäki et al. 2005), while vole abundances tend to be relatively stable both within and between years, showing only moderate seasonal changes with low densities in the spring and higher densities in the autumn (Hanski et al. 1991). In addition, in temperate areas the diet of raptors and owls is additionally enriched with other species such as Apodemus mice (Zárybnická et al. 2013). However, studies comparing numerical and reproductive responses between geographically separated populations that also experience different prey dynamics have not been available so far.
Our primary intention was to compare the breeding density and performance of Tengmalm’s Owls in two geographically distant populations: a boreal population (in Finland) in the core area of the species’ breeding range and a temperate population (in the Czech Republic) at the southern limit of the breeding distribution range in the Western Palearctic. These study areas lie 1500 km from one another and differ substantially in prey availability: densities of main prey (voles and mice) at the temperate study site were consistently lower and more stable than prey populations at the boreal site where vole densities undergo both large inter-annual 3-year cyclic fluctuations and intra-seasonal changes in densities (Fig. 1). These differences in prey availability were closely related to variation in the breeding performance of the respective populations of Tengmalm’s Owls. In particular, owl breeding density showed greater inter-annual fluctuations in Finland where the number of breeding pairs positively correlated with main prey densities while no such relationship was observed at the Czech site. Clutch size was larger in Finland compared to the Czech site and positively correlated with main prey densities in both areas (Fig. 2).
See also here.