EAGLE OWL

The territory-index of the Eagle Owl has varied between 0.45 and 1.09 (Fig. 1A) and the nest-index between 0.11 and 1.38 during 1982–2007. The overall log-linear decrease of the population indices from 1982 to 2007 was 2.2% or 4.1% per year depending on whether calculation was based on the numbers of occupied territories or active nests, respectively (Table 1).

The time series based on the numbers of territories included a highly significant non-linear component (Fig. 1A), which meant that the requirements of the log-linear regression were not fully filled. When the time series was split into two halves, the problem of non-linearity disappeared. During the first half of the period the population change was in fact positive (+1.2%; NS), but during the second half of the period the population decreased steeply: the territory-index by 5.2% (P < 0.001) and the nest-index by 8.7% (P < 0.05) per year. If this trend would continue, the population would halve in 14 years.

The overall picture (Fig. 2A) of the annual population changes and trend during 1986–2007 was the same on the basis of the Raptor Questionnaire data: the annual totals of active nests (range 71–537) and occupied territories (range 427–1106).

According to Raptor Questionnaire data, all productivity parameters, i.e. clutch size, productivity (number of young produced per active nest) and brood size (young per successful nest) have slowly decreased during 1986–2007, but statistically significant has been only the decrease of the brood size by 0.46% per year (P < 0.05). There are many ring recoveries of Eagle Owls ringed as nestlings and found dead, but not enough live encounters for a survival analysis based on combined data sets (cf. the Tawny Owl account below; Francis & Saurola 2004).

Of all Eagle Owls ringed in Finland 1913–2007 (n = 14516) and later reported dead, with information of the cause of death (n = 1731), 41% collided with or were electrocuted by power lines, 21% were hit by car and 6% by train, and 7% were deliberately killed. During the last two decades very few Eagle Owls have been killed on purpose. In contrast, power lines are still a very significant cause of Eagle Owl deaths. Thus, power line companies should invest more for developing safer structures for birds.

The Finnish Eagle Owl population was increasing (= partly recovering) during the 1970s and 1980s due to (1) breeding season protection since 1966 and full protection since 1983, (2) increase of suitable nest sites and hunting areas on clear-cuts created by forestry, and (3) especially, excellent year-round food supply of Norwegian Rats Rattus norvegicus at the numerous poorly managed local rubbish dumps (Saurola 1985b). Since the middle of the 1990s about 90% of the local rubbish dumps have been closed. This dramatic change of the food supply has certainly been very important factor behind the steep negative trend during the last 15 years (Valkama & Saurola 2005).

Table 1.

Overall population changes per year during 1982–2007 in Finland of six owl species. Annual population indices (cf. Fig. 1) were based on the numbers of occupied territories and on active nests (Raptor Grid study plots). Power: the number of years needed to detect an annual change of 5% with the observed between-year variation and at a power of 80%. Changes that were affected by increasing monitoring effort, are shown in parentheses. Note: a continuous annual change of 1% means that the population will halve or double in 70 years; changes of 2%, 5% and 10% suggest halving/doubling in 35, 14 and 7 years, respectively.

Table 2.

Average annual productivity (large nestlings produced per active nest per year) of nine owl species in Finland during 1986–2007. Given are the medians of annual means of productivity, and the change in productivity per year. Power: the number of years needed to detect an annual change of 5% with the observed between-year variation and at a power of 80%. Sample size n is the total number of nests included.

In Estonia, the recent distribution of the Eagle Owl is concentrated to the western coast and islands; no major population changes have been detected since the early 1990s (Nellis 2006). In many other European countries the Eagle Owl population has been increasing, e.g. in Germany during 1988–2002 by 5.3% (P < 0.01) per year (Mammen & Stubbe 2003, 2005).

Figure 1.

Annual population indices of six species of owls calculated by using the program TRIM and the numbers of occupied territories found from the Raptor Grid study plots during 1982–2007. Base year is 1994 with index value 1.0. Vertical bars indicate the standard errors. Thin line connects yearly indices to show year-to-year trajectory.

Statistically significant (see Table 1) log-linear regression line is shown by a thick solid grey line, except in panel A by a dashed line. In panel A the solid line indicates the statistically significant 7-point LOESS smoother (see text). In panel B the indices of the first part of the study period are shown differently, because they are biased by an increasing effort (see text).

PYGMY OWL

During 1986–2004, the number of nest boxes constructed especially for the Pygmy Owl Glaucidium passerinum increased steeply, from 385 to 6180 and has after that fluctuated around 6000. Because this increase took place in the Raptor Grid study plots as well, the steep ‘increases’ of both territory- and nestindices during 1982–2007 (Table 1, Fig. 1B) and of the annual totals of active nests (range 20–798) and occupied territories (range 110–1214) reported by the Raptor Questionnaire (Fig. 2B) are heavily biased. At least during the first years of the study period the ‘increase’ must have been caused by the increased detection probability of pairs breeding in nest boxes instead of woodpecker cavities.

In autumn 2003, an exceptionally large invasion of Pygmy Owls took place (Valkama 2003). In spring 2004 the territory-index had crashed down to the level of 1993 (Fig. 1B) and remained much lower during the good vole years of 2005 and 2006 than before the crash. This suggests that the changes detected by the Raptor Grid were relevant and not biased during the second half of the study period. Thus, the population really increased steeply by 5.6–8.7% (P < 0.01) per year during 1994–2003, then crashed and after that started to recover again (Fig. 1B).

These data show for the first time that Pygmy Owls, which participated in the mass invasion, really disappeared from the Finnish population. Did they emigrate from Finland and started to breed somewhere else or did they simply die off during the invasion? There is no data to answer this question. But, in any case, these data indicate that invasions are an important part of population regulation of the Pygmy Owl. In conclusion, the Raptor Grid data does not suggest any long-term trend of the Finnish Pygmy Owl population during 1982–2007 (Table 1).

TAWNY OWL

During 1982–2007, the territory-index of the Tawny Owl fluctuated between 0.50 and 1.08 (Fig. 1C) and the nest-index between 0.32 and 1.03. The time series of the territory-index is very similar (r = 0.80), but not quite as ‘nicely’ regular as the one of the Ural Owl (Fig. 1D). The reason is that the Tawny Owl is, of these two generalist feeders, less dependent on vole cycles than the Ural Owl. No long-term trend of the Finnish Tawny Owl population was detected (Table 2, Fig. 1C).

The annual totals of active nests (range 143–560) and occupied territories (range = 288–789) from the Raptor Questionnaire give precisely the same pattern of changes recorded in the Tawny Owl population during 1986–2007 from the Raptor Grid (Fig. 2C).

Because the Tawny Owl is a resident species (Saurola 2002, Saurola & Francis 2004), and because the Ringing Centre has encouraged ringers to collect data for capture-recapture analysis (Saurola 1987), it has been possible to calculate reliable and accurate time- and age-dependent survival probabilities for Finnish Tawny Owls (Francis & Saurola 2004).

Survival rates averaged 33% (range 26–43%) in the first-year of life, 64% (57–71%) in the second, and 73% (67–79%) in subsequent years. Approximately 50% of the annual variation in survival could be explained by the stage of the vole cycle and severity of winter weather.

Further, a matrix model based on Raptor Questionnaire data on productivity (cf. Table 2) and survival probabilities suggested that the Finnish Tawny Owl population has been decreasing during two years out of three and then recovering to the previous level during the third year of the vole cycle. The model predicted the numbers of breeding pairs to be low in one of three years, with no long-term trend (Francis & Saurola 2004), which is consistent with the Raptor Grid and Raptor Questionaire data (Fig. 1C, Table 1).

Figure 2.

The annual numbers of all occupied territories (columns) and active nests (lower parts of the columns in blue) of owls found in Finland during 1986–2007 and reported by the Raptor Questionnaire. The numbers are shown both as national totals and by the areas of local ornithological societies. Note: The scale in the panels for all local areas is the same, but different in the panel for the entire country.

Continued

Continued

Tawny Owl populations were reported as stable in Estonia during 1991–2002 (Elts et al. 2003), ‘stable?’ during 1970–2000 in Britain and Ireland (Hardey et al. 2006), and decreasing by 3.6% (P < 0.01) per year in Germany (Mammen & Stubbe 2003, 2005).

URAL OWL

During 1982–2007, population indices for the Ural Owl have fluctuated between 0.66 and 1.44 (territories; Fig. 1D) and between 0.29 and 1.5 (nests). The pattern of fluctuations is very clear: first there were five cycles of three years (two high years and one low), followed by two cycles of four years (three and one) and then again one cycle of three years (two and one). The same pattern can be seen in the annual totals of active nests (range 213–1236) and occupied territories (543–1732) during 1986–2007 (Fig. 2D).

Ordinary log-linear regression analysis shows that the territory-index has increased by 1.0% (P < 0.05) and the nest-index by 1.2% (NS) per year during the last 26 years (Table 1). According to program TRIM, both trends are statistically significant (P < 0.01). My own study population in southern Finland (61.0°N, 24.5°E) has remained at the same general level during the last 35 years; the annual number of active nests has varied between 15 and 130 (Saurola 2003, 2007)).

The Ural Owl is, together with the Tengmalm's Owl, the best studied owl species in Finland (see Kontiainen et al. 2008, Saurola 2007 for references). The Ural Owl is a generalist feeder, but its reproduction is highly dependent on 3–4-year cyclic fluctuations of microtines (e.g. Brommer et al. 2002, Saurola 2003; Table 2). Neither the Raptor Questionnaire data nor 43-year data from my study area indicate any long-term trends in productivity.

Preliminary results of a combined analysis of all Finnish recovery and recapture data (cf. Francis & Saurola 2004) from 1968 to 2005 indicate that the median annual survival during the first year of life is 37% (range 17–68%), is 74% (range 61–81%) during the second year of life, and is 85% (range 75–90%; Saurola & Francis unpubl. data) after that. Because vole populations crash after the peak, first year survival of fledglings hatched during the peak vole year has averaged only 27%. This means that the majority of the high number of fledglings produced during the peak year are ‘wasted’ and never become recruits (cf. Saurola 1989). No long-term trend in survival has been detected.

The Estonian Ural Owl population has been relatively stable since the beginning of the 1990s, but increased strongly during 1971–90 (Elts et al. 2003, Lõhmus pers. comm.).

LONG-EARED OWL

During 1982–2007, both territory-index (range 0.08–1.60) and nest-index (range 0.05–1.4) have varied widely (Fig. 1E). The general pattern of highs and lows is very similar to that of the Ural Owl. The main differences are the low values in good vole years 1982 and 1983, and the low period without proper highs from 1995 to 2001. In contrast to the Ural Owl, the long-term population change of the Long-eared Owl has been negative: -4.7% (P < 0.05) per year based on territories and -3.1% (NS) per year based on nests (Table 1). According to the program TRIM both trends are statistically significant (P < 0.01).

The lowest annual total of breeding attempts (i.e. nests plus fledged broods) reported during 1986–2007 was 28 in 1993, while the highest total was 616 in 2003 (Fig. 2E). The range of annual totals of occupied territories was 61–781 during the same period. Data from the Raptor Questionnaire shows the same wide annual fluctuations as data from the Raptor Grid, but does not suggest any decreasing population trend during 1986–2007.

In Finland, the Long-eared Owl is a regular migrant (Saurola 1983); only a small number of individuals may winter successfully, provided the circumstances are favourable. There is no capture-recapture data available to study the natal and breeding dispersal of the species. Ring recoveries reported by the general public indicate that, on average, Long-eared Owls reported dead during the breeding season were several hundred kilometres away from their natal area (Saurola 2002). Thus, Finnish Long-eared Owls seem to be seminomadic, which probably explains why the population highs and lows do not exactly match with those of the sedentary Ural Owl.

In Estonia large annual fluctuations, but no long-term trend has been observed (Lõhmus 2004, Rein Nellis unpubl. report). In contrast, in Germany the Long-eared Owl population has decreased in 1988–2002 by 1.7% per year (P < 0.05; Mammen & Stubbe 2003, 2005), and in Britain and Ireland the population was reported as in ‘decline?’ for the time period of 1970–2000 (Hardeyetd. 2006).

TENGMALM'S OWL

During 1982–2007, the annual population indices of the Tengmalm's Owl have fluctuated widely: the territory-index between 0.19 and 2.21 (Fig. 1F) and the nest-index between 0.28 and 3.02. The ordinary loglinear regression analysis indicated a significant negative trend of the territory-index by 3.1% (P < 0.05) per year during the last 26 years (Table 1), but due to the large between-year variation, the negative trend (-2.3%) of the nest-index was not statistically significant. According to program TRIM, both trends were statistically significant (P < 0.01).

Further, the Raptor Questionnaire data from 1986–2007 (Fig. 2F) show that the annual totals of active nests (range 201–2265) and occupied territories (range 387–3643) of the Tengmalm's Owl have decreased, especially all over southern Finland. In many other parts of the country the period elapsed between the population peaks have been much longer than 3–4 years, as used to be in the 1970s and 1980s.

What factors could have caused the decrease? Both the annual Raptor Grid indices (Fig. 1F) and the annual totals from Raptor Questionnaire (Fig. 2F) show that the population first increased steeply and rapidly during the 1980s before the decrease started in the early 1990s. Thus, before trying to analyse the causes of the apparent overall negative trend, one should find out why the numbers of the species ‘exploded’ in the late 1980s. Did the Finnish population receive, due to particularly favourable vole years, an exceptional influx of immigrants from the east? Respectively, could then an exceptional emigration have been involved in the rapid decrease of the population? Unfortunately these questions remain open. Because few breeding owls have been ringed in Russia, very few Russian-ringed Tengmalm's Owls have been encountered in Finland.

Several studies have shown that productivity, survival and dispersal of the Tengmalm's Owl are highly dependent on 3–4-year cyclic fluctuations of microtines (e.g. Laaksonen et al. 2002, Hakkarainen et al. 2002, Saurola 2002). Because the Tengmalm's Owl is a forest-dwelling species, continuous degradation of the forest habitat in Finland may play an important role behind the population decrease of the species. According to the results of 9th (1996–2000) and 10th (2004–2006) National Forest Inventory the total areas of 60–120 year and > 120 year forest age classes have decreased by 8% and 6%, respectively, during only ten years (Korhonen et al. 2007).

Laaksonen et al. (2004) showed that the lifetime reproductive success of the Tengmalm's Owls increased when the proportion of old forest increased in the territory (through higher number of breeding attempts), and decreased with an increasing proportion of agricultural land because of decreased fledging success in years when vole populations crashed during the breeding season. Further, the predation pressure by avian (Hakkarainen & Korpimäki 1996, Hakkarainen et al. 2004) and mammalian (Sonerud 1985, Korpimäki 1987) predators probably increases when the amount of optimal forest habitat decreases.

In northern Sweden, the breeding population of the Tengmalm's Owl has declined by 75% with respect to the peak densities from mid-1980s to early 2000s; this decline has happened in parallel with the decline in spring and autumn densities of vole populations (Hörnfeldt et al. 2005). In Estonia the Tengmalm's Owl has decreased during 1991–2002 (Elts et al. 2003) and, as well, in Germany by 2.1% (P < 0.01) per year during 1988–2002 (Mammen & Stubbe 2003, 2005).

SNOWY OWL

During 1986–2007, breeding Snowy Owls were reported by the Raptor Questionnaire only in three years (Fig. 2G). In 1987, five breeding attempts from 21 occupied territories were verified, in 1988 the corresponding numbers were 15 and 16, and in 2007 five and five. The average annual productivity was quite poor in all three years: 0.38, 1.79 and 0.4 large nestlings per active nest and 1.50, 2.27 and 2.0 per successful nest, respectively.

The nomadic Snowy Owl breeds numerously in Fennoscandia only occasionally, when an invasion from the east coincides with microtine peak (e.g. Mikkola 1983, Saurola 1997, Svensson et al. 1999, Jacobsen 2005). During the last decades, the maximum annual total for all Fennoscandia has not been more than about 100 breeding pairs. The mass occurrences have always been concentrated in relatively small areas (Wiklund & Stigh 1986), which vary in location from one invasion to the next. According to historical and partly anecdotal records, Snowy Owl invasions 100 years ago were much larger than nowadays: e.g. in 1907 about 800 eggs were collected from 100 nests in north-western Finnish Lapland (Mikkola 1983)! Satellite telemetry projects such as Fuller et al. (2003), will continue to offer new and exciting data on the long-distance movements of Snowy Owls, information important for understanding population trends in this species.

NORTHERN HAWK-OWL

During 1986–2007, the annual totals of occupied territories of the Northern Hawk-Owl reported by the Raptor Questionnaire have varied from 4 in 1997 to 175 in 1988 and the numbers of active nests from 1 to 119, respectively (Fig. 2H). In 13 years (1990–2002) breeding Northern Hawk-Owls were almost absent from Finland; the annual numbers of active nests were less than 20 except in 1996 (27). Finally in 2003, Northern Hawk-Owls came back again; nests were found both in Lapland and in some areas in central Finland.

The majority of the breeding records of the Northern Hawk-Owl have been made in the northern half of the country, where the density of ringers, other birdwatchers and people in general is quite low. This means, that the numbers of nests and territories reported are only a (small) fraction of the real annual totals. On the other hand, the nest site and territory of the Northern Hawk-Owl are easier to find than those of the other owls, because the Northern Hawk-Owl is a day-active and easily perceptible species, revealing the nest to the intruder by aggressive and noisy behaviour. Thus, the time series data from the Raptor Questionnaire probably reflects the real annual fluctuations of the breeding population.

In principle, Northern Hawk-Owls may breed throughout Finland. According to indirect historical records (egg collections and numbers of birds killed) Northern Hawk-Owls decreased in Finland during the first half of the 20th century, especially in the southern part of the country (e.g. Mikkola 1983). Sonerud (1997) concluded in his overview on population fluctuations: “Hawk Owl seems to have been more common in Fennoscandia during the first and last quarters of the past 100 year than in the intervening time”.

Ring recoveries give an idea of the scale of the nomadic movements of Northern Hawk-Owls. Of nestlings ringed in Finland, two were encountered east of the Ural Mountains at 2795 and 2659 kilometres to the east from their natal sites and three others were recovered in southern Norway at 1200–1400 kilometres southwest of their natal sites (Saurola 2002).

GREAT GREY OWL

The annual total of active nests of the Great Grey Owl reported by the Raptor Questionnaire has varied between 4 and 100 during 1986–2007 (Fig. 2I). The minimum and maximum totals of the occupied territories detected during the same period were 7 and 131. Contributions to the national total from local areas are very different from year to year and no trend during the study period can be suggested (Fig. 2I). The wide annual variation of numbers can be attributed to the vole cycles and semi-nomadic lifestyle of the species (Stefansson 1997, Saurola 2002).

The numbers of clutches of the Great Grey Owl in egg-collections collected during 1880–1910 suggest that the species must have been quite numerous in Finland one hundred years ago; in 1910–30 only few nests were found, in the 1930s a couple of good years were registered and during 1940–60 the species was almost absent from Finland (e.g. Sulkava & Huhtala 1997). Since the 1960s the number of breeding records has slowly increased to the present level. Correspondingly, the numbers of the Great Grey Owl have increased in northern Sweden since the middle of the 20th century (Stefansson 1997).

Sulkava & Huhtala (1997) reported two conditions that coincided with the increase of the Finnish Great Grey Owl population during the last five decades. The area of forest clear-cuts has increased since the 1950s providing open and vole-rich hunting areas for the owls. In addition, with the change in the attitude towards all birds of prey, illegal persecution of owls decreased and has essentially stopped during the last decades. Nonetheless, part of the perceived population increase may have been due to the more efficient monitoring and ringing of all birds of prey (Saurola 1985a, 1987).

SHORT-EARED OWL

The time series of the territory-indices (range 0.19–7.27) based on insufficient data from the Raptor Grid suggested a very strong decrease (-8% per year; P < 0.01) of the Short-eared Owl in Finland during 1982–2007. However, corresponding decreasing trend is not indicated from the larger Raptor Questionnaire data set (Fig. 2J).

During 1986–2004 the annual totals of nests and territories of the Short-eared Owl reported by bird ringers fluctuated as widely as those of the Northern Hawk-Owl and Great Grey Owl: from 1 to 132 active nests and from 26 to 280 occupied territories (Fig. 2J). Then, in 2005, the annual totals jumped surprisingly up to 304 nests and 586 occupied territories. In 2006 the numbers were again ‘normal’: 55 nests and 158 occupied territories.

The Short-eared Owl may breed all over Finland, but the present distribution is concentrated to the northern half of the country (Väisänen et al. 1998). However, the majority of breeding data reported by ringers comes from Bothnia, an area in western Finland with large peat bogs and agricultural fields (Fig. 2J). This means that neither the Raptor Questionnaire nor the Raptor Grid data on the Short-eared Owl is fully representative for the entire country.

Short-eared Owls migrate away from Finland in the autumn; some of them extend their migration at least to northern Africa (Saurola 1983). Ring recoveries have proved that a part of the Short-eared Owls are highly nomadic, changing their natal and breeding areas more than 1000 km from one breeding season to the next (Saurola 1983, 2002). In the future, more fieldwork is needed for surveying Short-eared Owls, as well as the three other northern species in Lapland.

In Estonia the Short-eared Owl population has decreased strongly during the last two decades (Elts et al. 2003). In Britain and Ireland, similar to the Long-eared Owl, the Short-eared Owl has been in ‘decline?’ (Hardey et al. 2006).