Translator Disclaimer
1 March 2013 Is the Long-term Decline of Boreal Owls in Sweden Caused by Avoidance of Old Boxes?
Author Affiliations +
Abstract

Numbers of Boreal Owls (Aegolius funereus) breeding in nest boxes in northern Sweden have declined since the 1980s. The main cause of this decline is thought to be a similar decline in voles, the owls' main prey. However, an alternative reason for the decline might be the aging of nest boxes and the owls' avoidance of old nest boxes to reduce the risk of predation. In this study we tested this alternative hypothesis in an experiment during the first two years of a 3-yr vole cycle by comparing predation and the number of breeding owls in old and new nest boxes at their original location, and old and new nest boxes placed at new locations. Predation was lower at relocated nest boxes, but Boreal Owls showed no preference for any of the four nest-box treatments, and breeding parameters did not vary between treatments. We conclude that the decline in the number of Boreal Owls breeding in nest boxes is real, and not caused by aging of nest boxes.

Los números poblacionales de Aegolius funereus reproduciéndose en cajas nido en el norte de Suecia han disminuido desde la década de 1980. Se piensa que la principal causa de esta disminución ha sido un declive similar en la población de ratones de campo, la principal presa de estos búhos. Sin embargo, una razón alternativa de esta disminución puede ser el envejecimiento de las cajas nido y el rechazo por parte de los búhos de las cajas nido viejas para reducir el riesgo de depredación. En este estudio, probamos esta hipótesis alternativa en un experimento durante los dos primeros años de un ciclo de tres años de ratones de campo, al comparar la depredación y el número de búhos reproduciéndose en cajas nido nuevas y viejas en su ubicación original, y en cajas nido viejas y nuevas colocadas en ubicaciones nuevas. La depredación fue menor en las cajas nidos reubicadas, pero los individuos de A. funereus no demostraron preferencia por ninguno de los cuatro tratamientos de cajas nido, y los parámetros reproductivos no variaron entre los tratamientos. Concluimos que la disminución en el número de individuos de A. funereus reproduciéndose en cajas nido es real y no está causado por el envejecimiento de las cajas nido.

In northern Scandinavia, there has been a gradual decline in cyclic vole population densities and dampening of the cycles since the 1970s, characterized by an increased frequency of overwinter declines leading to decreased spring densities (Hörnfeldt 1994, 2004, Hanski and Henttonen 1996, Hansen et al. 1999, Solonen 2004, Hörnfeldt et al. 2005, 2006, Ims et al. 2008). Voles are the main prey of several highly specialized predators in the region, including the Boreal Owl (Tengmalm's Owl; Aegolius funereus), a small, cavity-nesting owl found throughout Europe's boreal region. The decline in vole densities in spring led to the general prediction that reproductive output and population size are likely to have declined in such a specialized predator species. Indeed, a thirty-year study of Boreal Owls breeding in nest boxes in northern Sweden has revealed a gradual decline in numbers of breeding owls (Fig. 1), and this trend has been attributed to the decline in voles (Hörnfeldt et al. 2005).

Figure 1

Percentage of nest boxes occupied by breeding Boreal Owls (≥1 egg laid) in Västerbotten, northern Sweden, 1980–2010. Some data previously published in Hörnfeldt et al. 1990 and Hörnfeldt et al. 2005.

i0892-1016-47-1-15-f01.tif

However, an alternative hypothesis, that can be derived from Sonerud (1985a, 1989, 1993) is that the decline in Boreal Owls is an artifact of aging nest boxes, because the owls avoid old nest boxes to reduce the risk of predation. The European pine marten (Martes martes) is a relatively common generalist predator in Scandinavia's boreal forest (MacDonald and Barrett 1993). It is the main predator of Boreal Owl nests (Sonerud 1985a, Korpimäki 1987), and is thought to memorize and revisit places where it has previously found food (Sonerud 1985a, 1989, 1993, Nilsson et al. 1991, Sorace et al. 2004). There is indeed an increased risk of predation in older boxes and in boxes where Boreal Owl nests have been depredated previously (Sonerud 1985a, 1989, 1993; but see Korpimäki 1987), and Boreal Owls have been shown to use new nest boxes more frequently than old ones (Sonerud 1985a). To ascertain whether the decline in Boreal Owls breeding in nest boxes is a genuine issue of conservation concern, it is important to investigate whether or not it is an inherent methodological artifact of the thirty-year nest-box study. As far as we know, however, methodological tests are rarely undertaken to validate whether observed long-term population trends are real and trustworthy.

In this study, we examine whether the decline in Boreal Owls breeding in nest boxes is due to avoidance of old boxes by comparing recent use of old and new nest boxes at their original location, and old and new nest boxes placed at new locations. If the decline of owls was caused solely by aging nest boxes leading to increased predation risk and box avoidance by the owls, we predicted that the breeding frequency of owls would be higher in relocated nest boxes than in boxes at their original sites, and as high as in the early 1980s. Alternatively, if the decline in owls was caused solely by aging nest boxes through increased wear, we predicted that the breeding frequency of owls would be higher in new nest boxes than in older ones irrespective of location, and as high as in the early 1980s.

Methods

We studied Boreal Owls near Umeå in Västerbotten, northern Sweden (approx. 64°N, 20°E). This area is within the middle boreal vegetation zone (Ahti et al. 1968) and is dominated by managed coniferous woodland with patches of wetland and low-intensity agriculture. Boreal Owls have been monitored in this area since 1980 using a nest-box program (see Löfgren et al. 1986, Hörnfeldt et al. 1990, 2005 for details), which in later years has comprised approx. 275 nest boxes. The nest boxes (made of untreated wood, 20 × 20 cm base, 8.5-cm diameter entrance hole) were placed in trees at heights of 3–4 m, at approx. 1-km intervals along 50-km roadside routes. If required, nest boxes have been repaired, replaced, and occasionally slightly relocated due to logging, etc. All boxes were kept fresh by replacing old wood shavings and removing old nesting material and other debris.

Our experiment was based on the nest-box monitoring program, after some of the original nest boxes were replaced or moved. One-fourth of the nest boxes were each (1) left as controls, i.e., old nest boxes at their original location, (2) replaced by new ones but left at their original location, (3) relocated 100–200 m away, or (4) replaced by new ones and relocated 100–200 m. Treatment was allocated sequentially, i.e., the first box along a route was designated treatment 1, the second box treatment 2 and so on. In this study, nest boxes were defined as new or relocated up to 2 yr after they were erected. As far as possible, relocated nest boxes were erected within the same forest stand to minimize effects of habitat quality. Relocated nest boxes also were erected in a tree of the same species and of approximately the same size and age class as before. The relocation distance of 100–200 m (a similar distance to that used in Sonerud's [1985a, 1989 and 1993] box relocation experiment) assured that the nest boxes were not visible from their original locations, while making it likely that they remained within the same Boreal Owl territory.

The designated nest boxes were moved in November–December 2005, in preparation for the owl breeding seasons of 2006 and 2007. In our experience, these dates were early enough to allow the owls to find the nest boxes in time for the ensuing breeding season, which starts at the end of February at the earliest. In our study area, small mammals are trapped in spring and fall as part of the Swedish Environmental Protection Agency's National Environmental Monitoring Program (Hörnfeldt 1978, 1994, 2004, 2012). Here 2006 represented the first year (increase phase) of a 3-yr vole cycle, with low vole abundance in spring and previous fall, and 2007 the second year (peak phase) with a much higher vole abundance in spring and previous fall (Hörnfeldt 2012).

All nest boxes were checked at least three times in 2006 and four times in 2007 at intervals of approximately 4 wk. Nest boxes with breeding Boreal Owls were visited more often to obtain data on clutch size and laying date. We banded brooding females and nestlings, and estimated the laying date of most detected clutches by backdating. If the clutch was discovered during laying, a laying interval of 2 d per egg was used, and if the clutch was discovered later, the wing-length of the oldest nestling was measured to determine its age, and an incubation period of 29 d was assumed (Carlsson and Hörnfeldt 1994). We assumed that nest predation had occurred when broken eggs were found. However, because pine martens sometimes plunder nests and remove all eggs without trace, some occurrences of predation may have been unrecorded, and we may have underestimated the frequency of nest predation (cf. Sonerud 1985b). We recorded nestling mortality, and thus the number of fledged young, by X-raying nest-box remains to detect aluminum bands from dead nestlings.

We tested preference by Boreal Owls for the four nest-box treatments using a chi-square test on the number of breeding attempts (≥1 egg laid) for both years pooled (owing to the small sample size in 2006; Table 1). However, we also tested for the two years separately using Fisher exact tests (valid for low sample sizes; Siegel and Castellan 1988). We analyzed the occurrence of predation using Fisher exact tests for 2007 only (no predation was observed in 2006; Table 2). For this analysis, nest-box treatments were pooled into two groups; either old vs. new or relocated vs. unmoved nest boxes. We analyzed effect of nest-box treatment on three important breeding parameters (laying date, clutch size, and number of fledglings) using least squares linear regression, controlling for the effect of year. We checked the validity of the regression models by visually examining the raw residuals.

Table 1

Number of nests (≥1 egg laid) of Boreal Owls in four nest-box treatments in 2006 and 2007. Number of nest boxes in parentheses.

i0892-1016-47-1-15-t01.tif

Table 2

Number of depredated Boreal Owl nests in four nest-box treatments in 2006 and 2007. Total number of nests (≥1 egg laid) is given in parentheses. Note that no nests were depredated in 2006.

i0892-1016-47-1-15-t02.tif

Results

The number of breeding Boreal Owls in 2006 was very low, with only 1.8% (n  =  5) of the nest boxes occupied. In 2007 occupancy increased considerably, with 15.4% (n  =  42) of the nest boxes occupied by Boreal Owls. The owls showed no preference for any particular nest-box treatment, either for both years pooled (χ2  =  0.57, df  =  3, P  =  0.90), or for the two years separately (Fisher exact test: 2006, P  =  0.84; 2007, P  =  0.76; Table 1).

In 2007 five Boreal Owl nests (12%) were lost to predation (Table 2). None of the nests in 2006 were depredated, giving an overall predation rate of nearly 11% for both years combined. We identified European pine marten as the likely predator in one case, but usually the predator could not be identified. Predation rate did not differ significantly between old and new nest boxes (unmoved and relocated boxes pooled; Fisher exact test, P  =  0.17), but predation rate was significantly lower in relocated nest boxes than in boxes that were left at their original location (new and old boxes pooled; Fisher exact test, P  =  0.01). Mean number of fledged young tended to be higher for relocated nest boxes (new and old boxes pooled) than for unmoved nest boxes (3.9 and 2.6 respectively for both years pooled). However, least squares regression analysis on this sample (controlling for year) showed no significant difference (n  =  47, F-ratio  =  2.17, P  =  0.15).

Least squares regression analysis revealed no effect of nest-box treatment on laying date, number of eggs laid, and number of fledged young after the effect of year had been removed (Tables 3, 4). Thus, owls were not selecting a certain nest-box treatment earlier than the others, and productivity did not appear to differ between treatments.

Table 3

Mean laying date (day of year), clutch size and number of fledgling Boreal Owls in four nest-box treatments in 2006 and 2007. Number of nests (≥1 egg laid) in parentheses.

i0892-1016-47-1-15-t03.tif

Table 4

Least squares regression results for the effect of the four nest-box treatments on Boreal Owl breeding parameters, controlling for year.

i0892-1016-47-1-15-t04.tif

Discussion

Boreal Owls did not show a preference for new or relocated nest boxes during two consecutive breeding seasons during the increase and peak phases of a 3-yr vole cycle (Table 1). Furthermore, the number of breeding owls occupying nest boxes was much lower during the peak breeding year than at peak densities in the early 1980s. Therefore, we found little evidence to support the hypothesis that the long-term decline in nesting Boreal Owls (Fig. 1) is an artifact of owls avoiding old nest boxes. We assume that the observed decline reflects a general decline in the Boreal Owl population that can be explained by the decline in vole populations (Hörnfeldt 2004, Hörnfeldt et al. 2005). Owls nesting in natural cavities (usually old Black Woodpecker [Dryocopus martius] nest holes) could have a different population trend than those in nest boxes. Although we do not have data on these owls, we do not suspect this and are not aware of any such indications in our study area or elsewhere. Furthermore, natural cavities are considered a rare resource for nesting owls (Lundberg 1979), and as old trees and stands are becoming more rare in Swedish forests (Esseen et al. 1997), natural cavities are more likely a decreasing resource.

Our results contrast those of Sonerud (1985a), who found that Boreal Owls in southern Norway used new nest boxes more frequently than older ones. This pattern of use was attributed to a reduction in pine marten predation risk in new nest boxes (Sonerud 1985a, cf. Sonerud 1989, 1993). In our study, predation rate was significantly lower in boxes that had been relocated, which agreed with the hypothesis that pine martens remember and revisit places where they have previously found food (Sonerud 1985a, 1989, 1993, Nilsson et al. 1991, Sorace et al. 2004). Nevertheless, the owls did not appear to prefer relocated nest boxes. Interestingly, the incidence of predation on Boreal Owl nests in southern Norway was more than four times higher than that we report here (48%; Sonerud 1985a, vs. 11% in our study); predation rates by pine marten reported from western Finland were even lower, at 5% (Korpimäki 1987). If predation risk is truly so much lower in northern Sweden (with a caveat that the predation rate may have been underestimated: see Methods), then owls should be under less selective pressure to avoid it. Indeed, the overall productivity (measured as number of fledged young) was not significantly higher in relocated nest boxes, despite the prevalence of predation in unmoved boxes. Consequently, other factors such as local food supply or other habitat features might be more important for the owls when selecting a nest site. Also, female Boreal Owls in southern Norway may be less nomadic than their counterparts in Sweden (Sonerud et al. 1988). If turnover rates of breeding owls were indeed higher in the current Swedish study, fewer individuals would have experienced local predation and may be less prone to select new or relocated nest boxes.

Our experiment encompassed two years (increase and peak phase) of a 3-yr vole cycle. Predation risk from pine marten and consequent nest-selection behavior of owls might differ during the last year of the vole cycle. However, predation on Boreal Owl nests during 2008 (i.e., the year after the end of the current study) was at a similar level, at 14%. However, we refrained from including this year in our study, because we felt that the nest boxes that had been replaced and/or relocated in late 2005 could no longer be regarded as either “new” or “relocated” after 3 years.

Cavity-nesting birds may prefer newly excavated holes or new nest boxes because old, previously used sites have a higher level of ectoparasite infestation (Møller 1989, 1994, Mazgajski 2007). Additionally, nests in old cavities and nest boxes might be more exposed to the weather due to decay and disrepair (Korpimäki 1984, 1987). However, our nest boxes were continually repaired and refilled with fresh wood shavings between each breeding season, so older boxes should function as well as new ones. Thus, it was not surprising that we observed no preference for new nest boxes in our study.

The long-term decline in numbers of breeding Boreal Owls in northern Sweden has been accompanied by a change in their population dynamics: from 3–4-yr high-amplitude cycles, through a period of low-amplitude cycles, toward nearly annual fluctuations in the early 2000s (Fig. 1; Hörnfeldt et al. 2005). It is the peak years of high owl breeding density that have undergone a marked decline, thus reducing the overall numbers of breeding Boreal Owls in the long term. Declining trends similar to that in the Boreal Owl have been observed in other vole-eating birds of prey in Sweden. Great Gray Owls (Strix nebulosa) in northern Sweden have shown a long-term decline in brood size during peak vole years, corresponding to the dampening amplitude of vole cycles in the region (Hipkiss et al. 2008). Although the data were not robust enough to reveal an overall decline in the Great Gray Owl population, the researchers predicted that decreasing peak brood sizes would eventually lead to a general population decline, analogous to that of the Boreal Owl population. Further evidence of downward trends in northern vole-eating birds of prey can be found in autumn counts at Falsterbo, a Scandinavian migration bottleneck in southern Sweden. Counts of Northern Harriers (Circus cyaneus) have declined since the late 1970s and those of Rough-legged Hawks (Buteo lagopus), have declined since the mid-1980s, and decreasing rodent abundance was suggested as the cause of these trends (Kjellén and Roos 2000).

The decline and changed dynamics of vole populations in northern Fennoscandia, which is the most likely cause of the decline in breeding owls (Hörnfeldt 2004, Hörnfeldt et al. 2005), is probably related to increasingly milder and wetter winters in the region (Hörnfeldt 2004, Solonen 2004, Hörnfeldt et al. 2005, Ims et al. 2008). During milder winters, when conditions fluctuate between freezing and thaw, and the snow period is shorter and a hard crust of ice forms on the ground, small mammals are unable to use the subniveal space as efficiently for foraging and as a refuge against predators and adverse weather. Consequently, their mortality increases which leads to more pronounced winter population declines (Aars and Ims 2002, Hörnfeldt 2004, Solonen 2004, Korslund and Steen 2006, Kausrud et al. 2008). Mild and wet winters are likely to persist and become more frequent in Fennoscandia due to global warming, and lead to further dampening of vole cycles and lower numbers of their specialist predators (Hörnfeldt 1998, 2004, Strann et al. 2002, Solonen 2004, Ims et al. 2008). The Boreal Owl is not currently classified as a species of conservation concern in Sweden or the rest of Fennoscandia (BirdLife International 2012). However, if the declining trend presented here persists, then the conservation status of the Boreal Owl will have to be reviewed.

Acknowledgments

This study was funded by grants from Olle och Signhild Engkvists Stiftelser, Alvins fond and the Swedish Environmental Protection Agency. Greg Hayward and Geir Andreas Sonerud provided constructive comments on the manuscript.

Literature Cited

1.

J Aarsand R.A Ims 2002. Intrinsic and climatic determinants of population demography: the winter dynamics of tundra voles. Ecology 83:3449–3456. Google Scholar

2.

T Ahti L Hämet-Ahtiand J Jalas 1968. Vegetation zones and their sections in northwestern Europe. Annales Botanici Fennici 5:169–211. Google Scholar

3.

BirdLife International. 2012. Species factsheet: Aegolius funereus..  http://www.birdlife.org/datazone/userfiles/file/Species/BirdsInEuropeII/BiE2004Sp2291.pdf (last accessed 12 June 2012). Google Scholar

4.

B.-G Carlssonand B Hörnfeldt 1994. Determination of nestling age and laying date in Tengmalm's Owl: use of wing length and body mass. Condor 96:555–559. Google Scholar

5.

P.-A Esseen B Ehnström L Ericsonand K Sjöberg 1997. Boreal forests. Ecological Bulletins 46:16–47. Google Scholar

6.

T.F Hansen N.C Stenseth H Henttonenand J Tast 1999. Interspecific and intraspecific competition as causes of direct and delayed density dependence in a fluctuating vole population. Proceedings of the National Academy of Sciences of the U.S.A. 96:986–991. Google Scholar

7.

I Hanskiand H Henttonen 1996. Predation on competing rodent species: a simple explanation of complex patterns. Journal of Animal Ecology 65:220–232. Google Scholar

8.

T Hipkiss O Stefanssonand B Hörnfeldt 2008. Effect of cyclic and declining food supply on Great Grey Owls in boreal Sweden. Canadian Journal of Zoology 86:1426–1431. Google Scholar

9.

B Hörnfeldt 1978. Synchronous population fluctuations in voles, small game, owls, and tularemia in northern Sweden. Oecologia 32:141–152. Google Scholar

10.

B Hörnfeldt 1994. Delayed density dependence as a determinant of vole cycles. Ecology 75:791–806. Google Scholar

11.

B Hörnfeldt 1998. Miljöövervakningen visar pä, minskande sorkstammar!. Fauna & Flora 93:137–144.. (In Swedish with English summary.). Google Scholar

12.

B Hörnfeldt 2004. Long-term decline in numbers of cyclic voles in boreal Sweden: analysis and presentation of hypotheses. Oikos 107:376–392. Google Scholar

13.

B Hörnfeldt 2012. Miljöövervakning av smågnagare.  http://www2.vfm.slu.se/projects/hornfeldt/bh/sidor/index3.html (last accessed 12 June 2012). Google Scholar

14.

B Hörnfeldt B.-G Carlsson O Löfgrenand U Eklund 1990. Effects of cyclic food supply on breeding performance in Tengmalm's Owl (Aegolius funereus). Canadian Journal of Zoology 68:522–530. Google Scholar

15.

B Hörnfeldt P Christensen P Sandströmand F Ecke 2006. Long-term decline and local extinction of Clethrionomys rufocanus in boreal Sweden. Landscape Ecology 21:1135–1150. Google Scholar

16.

B Hörnfeldt T Hipkissand U Eklund 2005. Fading out of vole and predator cycles? Proceedings of the Royal Society Ser. B.. Biological Sciences 272:2045–2049. Google Scholar

17.

R.A Ims J.A Hendenand S.T Killengreen 2008. Collapsing population cycles. Trends in Research in Ecology and Evolution 23:79–86. Google Scholar

18.

K.L Kausrud A Mysterud H Steen J.O Vik E Østbye B Cazelles E Framstad A.M Eikeset I Mysterud T SolhØyand N.C Stenseth 2008. Linking climate change to lemming cycles. Nature 456:93–97. Google Scholar

19.

N Kjellénand G Roos 2000. Population trends in Swedish raptors demonstrated by migration counts at Falsterbo, Sweden 1942–97. Bird Study 47:95–211. Google Scholar

20.

E Korpimäki 1984. Clutch size and breeding success of Tengmalm's Owl Aegolius funereus in natural cavities and nest-boxes. Ornis Fennica 61:80–83. Google Scholar

21.

E Korpimäki 1987. Selection for nest-hole shift and tactics of breeding dispersal in Tengmalm's Owl Aegolius funereus. Journal of Animal Ecology 56:185–196. Google Scholar

22.

L Korslundand H Steen 2006. Small rodent winter survival: snow conditions limit access to food resources. Journal of Animal Ecology 75:156–166. Google Scholar

23.

O Löfgren B Hörnfeldtand B.-G Carlsson 1986. Site tenacity and nomadism in Tengmalm's Owl (Aegolius funereus (L.)) in relation to cyclic food production. Oecologia 69:321–326. Google Scholar

24.

A Lundberg 1979. Residency, migration and a compromise: adaptations to nest-site scarcity and food specialization in three Fennoscandian owl species. Oecologia 41:273–281. Google Scholar

25.

D MacDonaldand P Barrett 1993. Mammals of Britain and Europe. HarperCollins, London, U.K. Google Scholar

26.

T.D Mazgajski 2007. Effect of old nest material on nest site selection and breeding parameters in secondary hole nesters – a review. Acta Ornithologica 42:1–14. Google Scholar

27.

A.P MØller 1989. Parasites, predators and nest boxes: facts and artefacts in nest box studies of birds? Oikos 56:421–423. Google Scholar

28.

A.P Møller 1994. Facts and artefacts in nest-box studies – implications for studies of birds of prey. Journal of Raptor Research 28:143–148. Google Scholar

29.

S.G Nilsson K Johnssonand M Tjernberg 1991. Is avoidance by Black Woodpeckers of old holes due to predators? Animal Behaviour 41:439–441. Google Scholar

30.

S Siegeland N.J Castellan 1988. Nonparametric statistics for the behavioral sciences, Second Ed. McGraw-Hill, New York, NY U.S.A.. Google Scholar

31.

T Solonen 2004. Are vole-eating owls affected by mild winters in southern Finland? Ornis Fennica 81:65–74. Google Scholar

32.

G.A Sonerud 1985a. Nest hole shift in Tengmalm's Owl Aegolius funereus as defence against nest predation involving long-term memory in the predator. Journal of Animal Ecology 54:179–192. Google Scholar

33.

G.A Sonerud 1985b. Risk of nest predation in three species of hole nesting owls: influence on choice of nesting habitat and incubation behaviour. Ornis Scandinavica 16:261–269. Google Scholar

34.

G.A Sonerud 1989. Reduced predation by pine martens on nests of Tengmalm's Owls in relocated boxes. Animal Behaviour 37:332–343. Google Scholar

35.

G.A Sonerud 1993. Reduced predation by nest box relocation: differential effect on Tengmalm's Owl nests and artificial nests. Ornis Scandinavica 24:249–253. Google Scholar

36.

G.A Sonerud R Solheimand K Prestud 1988. Dispersal of Tengmalm's Owl Aegolius funereus in relation to prey availability and nesting success. Ornis Scandinavica 19:175–181. Google Scholar

37.

A Sorace F Petrassiand C Consiglio 2004. Long-distance relocation of nestboxes reduces nest predation by pine marten Martes martes. Bird Study 51:119–124. Google Scholar

38.

K.-B Strann N.G Yoccozand R.A Ims 2002. Is the heart of Fennoscandian rodent cycle still beating? A 14-year study of small mammals and Tengmalm's Owls in northern Norway. Ecography 25:81–87. Google Scholar
The Raptor Research Foundation, Inc.
Tim Hipkiss, Jonas Gustafsson, Ulf Eklund, and Birger Hörnfeldt "Is the Long-term Decline of Boreal Owls in Sweden Caused by Avoidance of Old Boxes?," Journal of Raptor Research 47(1), 15-20, (1 March 2013). https://doi.org/10.3356/JRR-11-91.1
Received: 8 December 2011; Accepted: 1 June 2012; Published: 1 March 2013
JOURNAL ARTICLE
6 PAGES


SHARE
ARTICLE IMPACT
Back to Top