**2. Qualitative and quantitave changes in a guild of forest owls Eurasian pygmy owl, ural owl, tawny owl, boreal owl at kamenný hrb – bankov site**

Population of Eurasian Pygmy Owl (thereafter only Pygmy Owl) at the site Kamenný hrb – Bankov has been continually monitored from 1989 till present time, till year 2021

### *Qualitative and Quantitative Changes in a Guild of Forest Owls: Eurasian Pygmy Owl… DOI: http://dx.doi.org/10.5772/intechopen.102932*

[5, 6, 12–16] but the most intensively in years 1989–1991. As an example of intensity of its monitoring in that period, we can mention years 1989–1994, when the site was visited 189 times (in 1989 realised 109 site visits/99 records), with a result of 161 records of the species at the area, even if over a half of site-visits was done in order to observe activity of owls at two occupied nest-sites. Local population of the Ural Owl was during the same period also under regular control, during years 1989–1991 by both authors, and from the 1980-ies till present time by local ornithologist Jozef Mihók, who placed a number of nest-boxes in the area, to support the breeding of the species. A low number [3–5] of nest-boxes was placed in the area in year 1991 also for the Pygmy Owl and for the Boreal Owl by first author of the paper, but these nest-boxes were not occupied by these species. In Slovakia breeding of the Pygmy Owl in a nest-box is very unusual, so far only one case was found [5, 9] and all other occupied nests in the country were found in holes excavated by Great-spotted Woodpecker (Dendrocopos major) or, especially in higher elevations also by the Eurasian Three-toed Woodpecker (Picoides tridactylus) and just very exceptionally breeds also in natural cavities – just 2 known cases so far [5]. On the other hand, breeding of the Boreal Owl in nest-boxes is quite common, as well as breeding of Tawny and Ural owl [1], but at study area, Kamenný hrb-Bankov nest-boxes were utilised only by Ural Owls, besides of natural raptor nests. Breeding of two pairs of Pygmy Owl was documented in Kamenný hrb area during years 1989–1994 [5, 7, 12–14] and in one territory breeding was documented also later on, in years 1997 and 2009 [5]. Also, food consumption, breeding biology and notes from ethology were evaluated from nests found in Volovské mountains [12], as well as course of autumn mating of the species was evaluated [16]. Curiosity of these breeding records is, that 5 subsequent breeding attempts were found in the same owl territory marked "B1": in years 1990, 1991, 1994, 1997 and 2009, while during years 1990–1997 the owls used for breeding the same oak tree, even if not in every year they utilised the same woodpecker cavity. It is almost sure, that during these years (1990–1997) more than 1 male used that territory – that presumption is likely because of short life-span of the species and it was proved, that in years 1990 and 1991 another female took part in the breeding because in both years breeding female birds were captured and ringed and the female breeding in 1991 in the same tree as in previous year had no ring. Distribution of the 8 identified breeding territories of Pygmy Owls in Kamenný hrb site during years 1989–1994 is shown at **Figure 1**. Examples of scetches describing site-visits with recorded observations of Pygmy Owls are shown at **Figures 2** and **3**. Distribution of Pygmy Owl territories at Kamenný hrb site in years 2017–2021 shows a very different picture (**Figure 4**). Despite a quite intense search after the species, especially during autumn 2020 and winter 2020–2021 only 4 occupied territories were located and effectiveness of locating of the species was very low: only 4 positive occurrences of the species despite 16 site-visits devoted to an intensive search for the species, including imitations of advertising calls. Lower density of Pygmy Owl at the site could be best explained by the already described phenomenon [17, 18] of increased competition with Tawny Owl – see **Figures 5** and **6**, that caused e.g. local extinctions of Pygmy Owl in parts of W Germany after distribution of Tawny Owl to higher elevations, what almost happened also here, at Kamenný hrb site. Very low vocal activity of the species in years 2020–2021 is probably as well a result of competition with Tawny Owl, the species remains secretive even during autumn mating and winter season, only in late autumn (XI, XII) and early spring (II) was detected some territorial activity. Only 23% of site visits were effective to locate calls of the species in years 2020–2021 in comparison to 90.8% of positive site visits in 1989 and 83% of positive visits in years 1989–1994. On the other hand, occurrence of the Tawny Owl increased at Kamenný hrb site from 4 occurrences annually (females only) in 1989 to 7–10 registrations/year (regular territorial

#### **Figure 1.**

*Distribution of 8 identified breeding territories of Eurasian pygmy owl (Glaucidium passerinum) in years 1989–1994 at the site Kamenný hrb – Bankov near Košice. Data were excerped from online databasis Avessymfony of SOS/BirdLife Slovakia – 161 data. Names and symbols of identified territories of G. passerinum: B1, K1, K3, K4, G1, Kamenný hrb, Čermeľ, B2. Known limits between individual territories are marked with black solid lines. In 2 of the territories even breeding nest cavities were found: in territory K1 nest site was known in years 1989 and 1990; in territory B1 nest sites was found in years 1990, 1991, and 1994.*

#### **Figure 2.**

*Example of original field note from year 1989, describing a detailed evening observation of activity of 2 calling males of Eurasian pygmy owl (B1 and K1) at border-line of the two territories (A. Kürthty).*

*Qualitative and Quantitative Changes in a Guild of Forest Owls: Eurasian Pygmy Owl… DOI: http://dx.doi.org/10.5772/intechopen.102932*


#### **Figure 3.**

*A "three dimensional" note of observation at a border of 2 Eurasian Pygmy Owl territories, possibly K1 and K3, in March 1989. (A. Kürthy).*

#### **Figure 4.**

*Distribution of 4 identified breeding territories of Eurasian pygmy owl (Glaucidium passerinum) in years 2017–2021 at the site Kamenný hrb – Bankov near Košice town. Data were excerped from online databasis Avessymfony of SOS/BirdLife Slovakia – 8 records. Names/codes of known territories and years, when corresponding occupied territory was checked: B1 (2017, 2020, 2021), K3 (2020), K4 (2020), Horný Bankov (2019, 2020). Known, or supposed borders between individual territories are marked with black solid lines.*

#### **Figure 5.**

*Known distribution of occupied territories of the tawny owl (Strix aluco) in years 1989–1994 at the site Kamenný hrb – Bankov near town Košice. Data were excerped from online databasis Aves-symfony of SOS/BirdLife Slovakia – 14 data. With symbol "M" were marked breeding territories occupied by territorial males; with symbol "F" is marked an area where only occurrence of a female bird was found; with a symbol "pair" is marked a territory occupied by a pair with detected nest site.*

#### **Figure 6.**

*Known distribution of occupied territories of the tawny owl (Strix aluco) in years 2017–2021 at the site Kamenný hrb – Bankov near town Košice. Data were excerped from online databasis Aves-symfony of SOS/BirdLife Slovakia – 17 data. Number of occupied territories after 30 years moderately increased, present number of supposed breeding pairs occupying the area is 5 according to known territories defended by territorial males. Main difference is against situation from years 1989–1994 is presence of at least 2 territories of males at the main ridge to east from altitude quote "Kamenný hrb", as well as at the side-ridge Kamenný hrb – Drobná and a following hill to the W – All 3 marked with blue circles; where in the previous period 1989–1994 about 30 years earlier territorial occurrence of the species was unknown.*

## *Qualitative and Quantitative Changes in a Guild of Forest Owls: Eurasian Pygmy Owl… DOI: http://dx.doi.org/10.5772/intechopen.102932*

males) in years 2017–2021 and that increase of territorial activity of Tawny Owls was the most intense in 5 territories of Pygmy Owls marked in 1989 as "B1", "K1", "K4", "Kamenný hrb" and "Čermeľ" (**Figure 1**), where territorial presence of the Tawny Owl was in years 1989–1994 almost unknown, irregular and very rare (see **Figure 4** with distribution of Tawny Owl for years 2017–2021). Also, occurrence of the Pygmy Owl at Horný Bankov – the easternmost territory occupied in 2017–2021 (**Figure 2**) is a good example of redistribution of the species to suboptimal, highly unusual habitat for the species: 80 years old oak-horbeam forest with almost no conifers, but with a lack of Tawny Owls (but occupied by Ural Owls). Another Pygmy Owl, the westernmost at the site, at **Figure 1** shown as territory "K4" sought a "shelter" from a neighbouring Tawny Owl and has literally stitched on between 2 Ural Owl territories (see **Figures 4** and **8**), in order to get protection against its Tawny Owl neighbour, moving its territory inside two territories of Ural Owls. Even if Ural Owl is able to predate Eurasian Pygmy Owls [17] and even in one of nest sites of Pygmy Owl observed at Kamenný hrb, in territory "K1" was almost sure, that one of the 5 fledglings was predated by Ural Owl [6], that risk is apparently lower for the Pygmy Owl, as close neighbourhood of the more dangerous Tawny Owl [6], which is known to be a foraging generalist and a frequent predator of birds [19]. Even if Boreal Owl does not mean direct threat to Pygmy Owl, their main prey items can overlap (small forest mammals – mice, voles, shrews), thus steep population growth of the Boreal Owl at Kamenný hrb after the year 2000 could also possibly contribute to worsening of local conditions for Pygmy Owl. Distribution and densities of the corresponding 4 owl species: Pygmy Owl, Ural Owl, Tawny Owl and Boreal Owl at Kamenný hrb – Bankov site during last 30 years are shown at **Table 1**.

Distribution of territories, nor overall density of Ural Owls, as the most dominant of the 4 observed owl species at Kamenný hrb site did not change substantially during previous 30 years – see **Figures 7** and **8**. Most of Ural Owl territories remained at the same sites as previously and density of the species also remained the same. The Boreal Owl at Kamenný hrb site went through an expressive transition from an irregular non-breeding visitor (**Figure 9**) in years 1989–1994 to a regularly occurring population of 7–8 all-year-round present territorial males (possibly pairs) in years 2017–2021 (**Figure 10**). That change is contradictory to planet-warming process, because these owls were forced to lower elevations of the mountain range apparently from higher elevations. But if we address that phenomenon from a broader regional scale of all Volovské mountains range (140,000 ha), where the Bankov-Kamenný hrb site belongs, the explanation is simple: most of spruce forests in elevations over 1000 m in central and western part of Volovské mountains with viable populations of


#### **Table 1.**

*Number of occupied territories and densities of Ural owls, tawny owls, Eurasian pygmy owls and boreal owls at Kamenný hrb – Bankov between years 1989 and 2021.*

#### **Figure 7.**

*Known distribution of occupied territories of the Ural owl (Strix uralensis) in years 1989–1994 at the site Kamenný hrb – Bankov near town Košice. Data were excerped from online databasis Aves-symfony of SOS/ BirdLife Slovakia – 69 data. Even if specifical nest sites were not identified, it is very likely that minimally around 8 pairs has bred at the site in that period. Known or supposed borders between individual breeding territories are marked with black lines.*

#### **Figure 8.**

*Distribution of occupied territories of the Ural owl (Strix uralensis) in years 2017–2021 at the site Kamenný hrb – Bankov near town Košice. 27 distribution data are displayed at detailed DFS map. The population consists of minimally 8 occupied territories of breeding pairs. In some territories even occupied nest-boxes are known. Distribution of individual territories is almost identical as in period 1989–1994.*

*Qualitative and Quantitative Changes in a Guild of Forest Owls: Eurasian Pygmy Owl… DOI: http://dx.doi.org/10.5772/intechopen.102932*

#### **Figure 9.**

*Sites with sporadical occurrence of the boreal owl (Aegolius funereus) at the same site Kamenný hrb – Bankov, between town Košice in the east and village Nižný Klatov in the south; during period 1989–1994. Data were excerped from online databasis Aves-symfony of SOS/BirdLife Slovakia – 6 data. None of determined birds was a territorial male.*

#### **Figure 10.**

*Sites with occurrence of the boreal owl (Aegolius funereus) at Kamenný hrb – Bankov, between town Košice in the east and village Nižný Klatov in the south; during period 2017–2021. Data were excerped from online databasis Aves-symfony of SOS/BirdLife Slovakia – 36 data. Data include 36 occurrences and approx. 8 territorial males. The species is now year-round present in the area as a territorial bird with supposed breeding, number of occurrences increased after 30 years (between 2017 and 2021 in comparison with period 1989–1994) from 6 to 36 and number of occupied territories from 0 to 8.*

Boreal Owls were in large extent destroyed and removed, between years 2000–2010 approx [14], thus occurrence of that owl species in elevations as low, as 400–600 m at Kamenný hrb site only reflects the tendency of the species looking for new habitats,

instead of the destroyed habitats in higher elevations of the same mountain range. Possibly good local populations of small forest mammals, such as rodents and shrews at Kamenný hrb, able to maintain a good population of Ural Owls for the last 30 years (see **Figures 5** and **6**), as their main food source; as well as numerous Black Woodpecker (Dryocopus martius) cavities in old beech stands at Kamenný hrb site could attract Boreal Owls even to these quite unique – for the species unusually low elevations – at least from a Central-European perspective.
