Surnia Research Articles

The northern hawk owl Surnia ulula invasions in Europe

Abstract The northern hawk owl is a real irruptive species that respond to irregular changes in the food supply. When prey levels are adequate, it breeds and winters within northern forests. Decreased prey availability can start winter invasions, the timing and magnitude of which are the subject of this study. Mainly the citizen data were extracted from the national bird websites to obtain data on the number of northern hawk owls observed in 2010–22. This paper demonstrates that through citizen data large amounts of information can be collected over wide areas, entire Europe in this case. From Finland to Poland and Czechia the invasions years were very similar, 2013–14, 2017–18 and 2021–22 but in Sweden and Norway three clear irruption years were a year or two before. In Denmark, the clear invasions years were 2013–14, 2016–17 and 2019–20 but Poland, peak years were not at all as clear as in the other countries. The invasions started earliest in Finland in September and peaked in November. In Estonia and Latvia peaks occurred from November to January. In Poland, irruption peaked a month later in December but continued until April like in Finland, Estonia and Latvia. The origin of the irrupting owls in the region has been debated. In Norway, authors are convinced that owls originate from Fennoscandia but this paper indicates that mass invasions have to originate from northern Russia. However, only in Denmark there was one Russian northern hawk owl ring recovery. Further ring recoveries and preferably GPS tagging and satellite tracking of the northern hawk owls are awaited to have a better picture of invasion movements and future conservation needs.

Hawk owl irruptions: spatial and temporal variation in rodent abundance drive push and pull dynamics

Bird irruptions are thought to be triggered by low food availability in breeding areas, thereby causing emigration (push factor). However, few studies have tested whether emigrating individuals are drawn towards areas of high food availability (pull factor). The Northern hawk owl (Surnia ulula), a rodent specialist, occurs irruptively to southern parts of Fennoscandia. We analysed whether irruption size during 1980–2020 in southeastern Norway was related to rodent abundance at four sites 450–990 km to the north-northeast (potential source areas) and at two sites in southeastern Norway to test push and pull dynamics of irruptions. Irruptions occurred when rodent abundance in potential source areas were low, supporting the push hypothesis. High rodent abundance in potential source areas 1–2 years before irruptions suggested that irruptions were preceded by high reproduction. Upon arrival to southeastern Norway, hawk owls did not encounter high rodent abundance in their main habitat (boreal forest). However, hawk owls stayed in boreal forest in hills in years with higher microtine rodent abundance, but occurred in farmland areas in the lowlands when microtine rodents were less abundant. Use of lowlands coincided with higher than median numbers of wood mouse (Apodemus sylvaticus) for 87% of the hawk owls settling in the lowlands, thus suggesting support for the pull hypothesis. In conclusion, hawk owl irruptions to southern Fennoscandia were triggered by low food availability in northern areas (push factor), and appeared to be drawn by high food availability in southeastern Norway to some degree (pull factor).

Fire severity and pre-fire forest composition affect Northern Hawk Owl presence in burned forests in western North America

Wildfire is an important natural disturbance shaping the boreal forests of North America. Not surprisingly, many species have adapted to benefit from the structural or functional changes in response to fire. Of these post-fire species, the Northern Hawk Owl (Surnia ulula caparoch) is one of the least studied species on the continent. We surveyed 18 recent fires in the boreal forest region of Alberta, Canada, ranging in age from 1 to 14 years post-fire, to improve our understanding of post-fire habitat relationships for this species. Hawk owls were present in fires ranging from 2 to 11 years post-fire, and occupied burns ranging in size from 1,000–272,000 ha. Hawk owls were strongly associated with areas of older deciduous-dominated forest, based on pre-fire forest inventory. Hawk owl responses to pre-fire forest age structure and composition were consistently stronger at the 1000 m radius (home range) extent than the 400 m (nest site) extent. We also found hawk owls responded differently to time since fire, with an immediate (e.g. 2–6 yr) response to low severity fire and a delayed (10–11 yr) response to high severity fire. We recommend future surveys for this species be conducted earlier in the day, ideally within a few hours of sunrise, based on the timing of detection in this study. Finally, the retention of large patches (e.g. > 300 ha) of older, deciduous-dominated habitat within post-fire forests may be an important management consideration for the conservation of hawk owls and other post-fire species.

Risk Factors and Prognosis for Humeral Fractures in Birds of Prey: A Retrospective Study of 461 Cases from 2000 to 2015.

The objectives of this retrospective study were 1) to characterize humeral fractures in raptors at a rehabilitation program, 2) to determine risk factors for closed and compound humeral fractures in the raptors, and 3) to investigate prognostic factors for humeral fractures in raptors. Of the 6017 birds of prey admitted over 15 years to the Université de Montréal raptor rehabilitation program, 461 birds presented with a humeral fracture. The highest occurrences of humeral fractures were documented in short-eared owls (33%; Asio flammeus), Northern hawk-owls (24%; Surnia ulula), long-eared owls (20%; Asio otus), and Northern harriers (19%; Circus hudsonius). Humeral fractures were more common in males, and species from the intermediate weight group (300 g to 1 kg) were more likely to have compound humeral fractures than birds from the lower and higher weight groups. Humeral fractures of the middle and distal third of the bone were diagnosed as compound more often than fractures of the proximal third. A correlation between the occurrences of humeral fractures and the relative average width of the humerus for each species was observed and suggests that species with slender humeri have an increased likelihood of compound humeral fractures. Following the triage criteria used at this rehabilitation program, surgical (119 birds) or nonsurgical (77 birds) treatments were attempted. Release rate for birds without concomitant conditions in which a treatment was attempted was 39.3% (59/150) and did not significantly differ between closed and compound fractures, which suggests that the triage criteria used for compound fractures in this program were adequate. Our findings emphasize that raptors should not be systematically euthanatized when presented with compound humeral fractures to a rehabilitation facility.

Relative magnitude of two Northern Hawk-Owl (Surnia ulula) irruptions to southern Norway: comparison of citizen data and survey data

Cover photo: Northern Hawk-Owl Surnia ulula. Photo: Terje Lislevand. 
 The Northern Hawk-Owl Surnia ulula occurs as an irruptive species to southern parts of Fennoscandia. Large numbers of individuals were recorded in the autumns of 2016 and 2020 but assessing the relative magnitude of irruptions is challenging. Systematic surveys allow direct comparisons but are time-consuming and will therefore be limited in time and space. Citizen data may provide large amounts of information for wide areas but may in particular suffer from spatial biases in the observation effort of birdwatchers. I compared the 2016 and 2020 irruptions of Northern Hawk-Owls to southern Norway, and found that citizen data indicated that numbers in 2016 were ca. 2–3 times larger than in 2020. However, the relative magnitude differed geographically, and the 2016 irruption was larger in western and southern counties, whereas the difference was smaller in eastern counties (in particular in Innlandet county). Systematic surveys in eastern regions (Oslo and Akershus) indicated that Northern Hawk-Owl densities were similar in the two irruption years. Overall, Northern Hawk-Owls were recorded at the same rate (approximately one owl per 16 km), and density was estimated to be 0.09–0.18 individuals/km2 in 2016 and 0.08–0.13 individuals/km2 in 2020. Thus, citizen data and survey data from the same geographical region concurred. However, due to the geographical variation in relative irruption size and spatial variation in observer density and biases in observation effort, the overall difference between the two years is difficult to assess. The 2016 irruption was likely larger than the 2020 irruption, but the difference was probably smaller than suggested by citizen data.

Acoustic discrimination of predators by black-capped chickadees (Poecile atricapillus).

Smaller owls and hawks are high-threat predators to small songbirds, like chickadees, in comparison to larger avian predators due to smaller raptors' agility (Templeton et al. in Proc Natl Acad Sci 104:5479-5482, 2005). The current literature focuses only on high- and low-threat predators. We propose that there may be a continuum in threat perception. In the current study, we conducted an operant go/no-go experiment investigating black-capped chickadees' acoustic discrimination of predator threat. After obtaining eight hawk and eight owl species' calls, we assigned each species as: (1) large, low-threat, (2) mid-sized, unknown-threat and (3) small-, high-threat predators, according to wingspan and body size. Black-capped chickadees were either trained to respond ('go') to high-threat predator calls or respond to low-threat predator calls. When either low-threat predator calls were not reinforced or high-threat predator calls were not reinforced the birds were to withhold responding ('no-go') to those stimuli. We then tested transfer of training with additional small and large predator calls, as well as with the calls of several mid-sized predators. We confirmed that chickadees can discriminate between high- and low-threat predator calls. We further investigated how chickadees categorize mid-sized species' calls by assessing transfer of training to previously non-differentially reinforced (i.e., pretraining) calls. Specifically, transfer test results suggest that mid-sized broad-winged hawks were perceived to be of high threat whereas mid-sized short-eared owls were perceived to be of low threat. However, mid-sized Cooper's hawks and northern hawk owls were not significantly differentially responded to, suggesting that they are of medium threat which supports the notion that perception of threat is along a continuum rather than distinct categories of high or low threat.

Инвазии ястребиной совы в лесостепь Западной Сибири

The article presents data on the invasions of the hawk owl (Surnia ulula) into the forest-steppe of Western Siberia on the territory of the Novosibirsk and Kemerovo regions, Altai Krai and Krasnoyarsk Krai territories.

Microstructure of the Primary Remex of Owls (Strigiformes)

An electron microscopic study of the microstructure of the primary remiges in owls (Strigiformes) was conducted using 13 species: Nyctea scandiaca, Bubo bubo, Asio otus, Otus scops, O. sunia, Aegolius funereus, Athene noctua, Glaucidium passerinum, Surnia ulula, Strux aluco, S. uralensis, S. nebulosa, and Tyto alba. Owls have a number of species-specific microstructural primary remex characteristics that are taxonomically important. First of all, these are the structural features of the pennaceous barb, which differ markedly not only at the ordinal, but also at the species level: cross-section configuration, pith architectonics in cross- and longitudinal sections, and cuticular barb structures. This work emphasizes that the identified elements are of taxonomic significance only when comparing the specific, strictly analogous sections of the barbs in different bird species. Based on the present study, we conclude that along with the typical elements of feather architectonics characteristic of representatives of other bird orders, the owls show a number of peculiar microstructural features. Thus, as the result of an analysis of the microstructure of the owls primary remex pennaceous barb vanule, the unique features of the distal barbules and the structure of the apical section of the barb with tightly adjacent and elongate proximal and distal barbules could be distinguished. These characteristics are revealed to cause a dense fleecy structure of the vane dorsal surface and the presence of a complex of peculiar “bunches” that form the cleft edge, i.e., the purely specific traits of the owls feather that are not found in representatives of any other bird orders we have investigated.

The Northern Hawk Owl in Montana: A Summary of Breeding Biology, Diet, Habitat Association, and Records (1994–2015)

We studied Northern Hawk Owls in Montana from 1994 through 2015. We report 36 breeding records, 18 of which included a known nest location. Owls reused one nest in subsequent years, but this nest was included only once in our analysis of nest-site characteristics. All nests were in moderate to severely burned forests from 1–11 yr post-burn. Nests were found in black cottonwood (Populus trichocarpa; n = 6), quaking aspen (P. tremuloides; n = 6), Engelmann's spruce (Picea engelmannii; n = 3), and western larch (Larix occidentalis; n = 2). Average nest tree height was 19.6 ± 3.4 m (95% CI, n = 17), average nest height was 14.1 ± 1.6 m (95% CI, n = 17), and average diameter at breast height was 43 ± 4 cm (95% CI, n = 17). Average measurements of nest tree height and nest height were larger than those reported from elsewhere in North America. Diet, based on 274 prey identified from pellets, was 88% voles, most frequently Microtus pennsylvanicus (51%, n = 140). Only 1 of 72 banded owls was recaptured or recovered over the course of this 22-yr study, suggesting a non-sedentary or nomadic population with low site fidelity. The lone recapture represents the first report of a philopatric Northern Hawk Owl in North America. Although this owl is still considered a rare breeder in the conterminous United States, we documented breeding records in 11 of the last 12 yr of this study. All but four known breeding records in Montana occurred within Glacier National Park.

In‐flight RCS measurements of drones and birds at K‐band and W‐band

This study presents the in-flight radar cross-section (RCS) data of drones and birds at K-band and W-band obtained from extensive experimental trials. The focus of this study is to demonstrate the RCS characteristics of these targets in practical scenarios, hence experimental results are used exclusively. Owing to variations in orientation, aspect angle and target motion, the measured RCS values of these targets fluctuate significantly during their flight. Three very well-calibrated frequency modulated continuous wave radar systems, one operating at the K-band (24 GHz) and two at W-band (94 GHz), have been used to collect data for RCS analysis. Three drones of different sizes (DJI Phantom 3 Standard, DJI Inspire 1 and DJI S900 Hexacopter) and four birds of prey of different sizes (Northern Hawk Owl, Harris Hawk, Indian Eagle Owl and Tawny Eagle) have been used for data collection. The results demonstrate that the RCS scales broadly with the size of the target, consistent with the optical scattering regime and that the RCS values for each target are comparable at the K-band and the W-band. The statistical distribution of RCS for each target falls within a certain range which is useful for predicting the performance of a drone detection radar.

Northern Hawk Owl (Surnia ulula) Scavenges and Defends a Lynx (Lynx canadensis) Carcass in Winter

ABSTRACT.— Scavenging has been increasingly documented for difficult to observe species through advances in motion-triggered photography. Here, we document several instances of scavenging over multiple days by a Northern Hawk Owl (Surnia ulula) on a Canada lynx (Lynx canadensis) carcass. These observations are of particular interest because the owl visited the carcass 25 times, with the longest visit lasting >3 hrs. In addition, the owl routinely defended the carcass against Black-billed Magpies (Pica hudsonia). These observations suggest scavenging may represent a larger role in the diet of Northern Hawk Owls; further research is required in order to understand its importance for this and other raptor species.

Record nesting year for Great Gray and Nothern Hawk Owls in Riding Mountain National Park region during the spring of 2009

The spring of 2009 proved to be a record year for breeding Great Gray owls (GGoW) (Strix nebulosi) and Northern Hawk owls (NHoW) (Surnia ulula) in and around Riding mountain National Park

First Nest Records of Northern Hawk Owls (Surnia Ulula) In Montana, With Notes On Behavior and Food Habits

Abstract In 1994, Steve Gniadek (SJG) located a Northern Hawk Owl (Surnia ulula) nest in Glacier National Park (GNP), Montana. This was the 1st nest found in Montana and the northwestern United States. Prior to this discovery, only 3 states in the contiguous United States (Minnesota, Michigan, and Wisconsin) had breeding records for this species. Historical evidence suggests that Northern Hawk Owls previously bred in Montana. A photo taken in 1990 of a misidentified fledgling in GNP is the 1st verified breeding record. Subsequent breeding has been documented in the vicinity of the 1994 nest, and elsewhere in Montana and the northwestern US. The 1994 nest cavity was 13.7 m high in a 42-cm diameter-at-breast-height (DBH) dead Trembling Aspen (Populus tremuloides), in an area that burned at moderate to high intensity in 1988. At least 7 nestlings fledged. Based on pellet analysis, the adults ate predominantly voles (Cricetidae, 96.3%), and they were observed to pluck and cache their prey.

Density, numbers and origin of Northern Hawk Owls <i>Surnia ulula</i> in southern Norway during the 2016 irruption

One of the largest known irruptions of Northern Hawk Owls Surnia ulula in southern Norway started in mid August 2016. During September-October I conducted 246 km surveys in boreal forests in Oslo and Akershus counties, southeastern Norway. A total of 16 owls were detected. Depending on assumptions regarding detectability, a density of 0.106-0.204 owls/km 2 was estimated, corresponding to 371-714 individuals in boreal forests of the two counties in total. Southeastern Norway has about 55,600 km 2 boreal forest which implies a total of 5,894-11,342 owls in this habitat. On the bird reporting websites artsobservasjoner.no and nofoa.no there were 1,366 reports of Northern Hawk Owls from all counties in southern Norway during September-October, representing 661 different sites. In Oslo and Akershus counties, estimates suggested that < 10% of all hawk owls were detected. A 5-10% detection rate suggests that there were 6,610-13,220 owls in southern Norway. However, in many counties hawk owl reporting rates measured as proportion of all bird reports that concerned hawk owl, were much higher than in Oslo and Akershus, which could imply higher density. By adjusting density (using the values 0.106-0.204 owls/km 2 from boreal forest in Oslo and Akershus) for county-specific owl reporting rates, the number of hawk owls present only in boreal forest in southeastern Norway was estimated at 14,466-27,872 individuals. This number excludes additional owls in agricultural areas and owls in other parts of southern Norway. Overall, a total number of about 10,000-20,000 individuals is considered most likely. I review available evidence of the origin of these owls using number of reports and spatial distribution of hawk owls in northern Norway, Sweden, Finland, Estonia and Russia during 2015 and 2016. I conclude that the hawk owls came from northern Fennoscandia where large numbers of Northern Hawk Owls bred in 2015, and not from more eastern areas such as Russia.

Bionics - Natural but Innovative Methods Improve the Aeroacoustic Engineering

This paper presents the aeroacoustics result of the trailing edge noise prepared wings two species of owl - Barn owl (Tyto alba), called “good hearing” and Northern hawk-owl (Surnia ulula), called “good seeing”. The acoustic performance comparisons between the Barn owl and Northern hawk-owl show that there are differences between the sound intensity the studied wings. These results prove that the special sound suppression characteristics of wing feather play an important role for their silent flight. Therefore the flat plates with the cutting trailing edge have been studied. At lower speeds, a better noise reduction effect was obtained for the plates with edges as elliptical arcs. On the basis of the above tests it can be concluded that other types of notches on the trailing edge (not just a sawtooth) also reduce the aerodynamic noise of flat plate.

C-2029 : Winter nesting of the birds near north pole in the Northern Hemisphere–Yakutia/Russia

In winter on Yakutian taiga 10 species of birds nest (3.7% breeding fauna). Evening temperatures reach −20 ÷ 30 C°. In March Bubo bubo nest (4 nests) and in April Surnia ulula (4 nests). In March-May the egg-laying of Loxia leucoptera begins (50.0–66.7% clutches; n = 10), at the end of March to early May, Corvus corax (93.2–100.0; 47) and Perisoreus infaustus (75.0–87.6, 93). In April and May egg-laying starts in Strix nebulosa (52.9; 17), Nucifraga caryocatactes (33.3–100.0; 13), Accipiter gentiles (0–33.3; 4), and Haliaeetus albicilla (33.3; 3), and stretches up to July for Acanthis flammea (1.0–27.3; 637). In the middle taiga in winter nesting is earlier in the calendar and phenological periods than in other parts of their range, 87.6% pairs of the Siberian jay (n = 89). Also here their clutches are larger (2–5, average 3.99 ± 0.06 eggs; n = 90). In the woods of valleys species begins egg-laying earlier than in the watersheds – the end of March (22.7%; n = 44) versus the beginning of April (38.5, 26), has a larger clutches – 4.13 ± 0.09 (n = 40) versus 3.75 ± 0.1 (28), and nesting mostly on the trees (84.5%; n = 73), and watersheds – on the willows (67.2, 70). There are other mechanisms that allow to species to breed at winter temperatures and in various landscapes. Pairs are constant and strictly territorial, nest on the trees and shrubs (n = 192) at a height of 0.5–8.0, on average, 1.6 (willow) −2.9 (pine) m, mainly on south side (50.0 %). Feathers, down and rag compose up to 22.8 % weight of the nest (n = 5), a supporting part (dry twigs) – 63%. The numbers of the population in the site of the watershed taiga (10.6 km2) in spring and autumn are, respectively, 59–84 and 55–87 individuals. In November 2012 residents amounted to 71.9%, immigrants – 18.7 (n = 64). In the autumn of 2009, the ratio female and male (n = 25) was equal, and for juv (29) the number of males was larger (1:1.5), but for the year it aligned for them also. On eggs females spend an average of 96.7% of the day. Chicks hatch for 1.5 days on average, they are fed in the end of April, from 5:25 to 19:45 h, after 1.5 h.

Attempted predation of Northern Hawk-owl Surnia ulula by Common Kestrel Falco tinnunculus?

Attempted predation of Northern Hawk-owl Surnia ullula by Common Kestrel Falco tinnunculus?

Observaciones De Aves Raras En España, 2011

Report on rare birds in Spain 2011. Current report gives details about 452 new records on the presence of 100 rare bird taxa in the whole of Spain. Acceptance index was of 88,94% from a grand total of 579 examined records including pending, duplicated and regional rarities. Details on 12 new taxa for Spain are included for the first time in any rarities report: Asian white-winged scoter Melanitta deglandi, Fea's petrel Pterodroma feae, first and second records, black-bellied storm-petrel Fregetta tropica, eastern imperial eagle Aquila heliaca, sandhill crane Grus canadensis, south polar skua Stercorarius maccormicki, Thayer's Gull Larus thayeri, american hawk-owl Surnia ulula caparoch, buff-bellied pipit Anthus rubescens, eastern black redstart Phoenicurus ochruros phoenicuroides, semicollared flycatcher Ficedula semitorquata and Cretzchsmar's bunting Emberiza caesia. Deserta's petrel, Thayer's gull and hawk owl are new to the Spanish list which was updated in 2012 having considered the rest of the above accepted firsts. The report deals with record numbers of goosander Mergus merganser, buff-breasted sandpiper Tryngites subruficollis and pectoral sandpiper Calidris melanotos besides overwintering episodes of Baird's sandpiper Calidris bairdii and broad-billed sandpiper Limicola falcinellus. Other interesting records are third records for Spain of African white-backed vulture Gyps africanus, upland sandpiper Bartramia longicauda or Baltic gull Larus fuscus fuscus.

Moult and Breeding of Captive Northern Hawk OwlsSurnia ulula

During 2003–07, the moult of four captive Northern Hawk Owls Surnia ulula was studied in Poznań Zoological Garden (Poland). The aviary was under daily observation and all breeding events were recorded. It was recognized that captive conditions differ from that in the wild (food availability, expended energy — especially by males, and stress) but the study of captive owls allowed a detailed study of moult in relation to the breeding cycle of Northern Hawk Owls. Identification of dated moulted feathers enabled us to reconstruct the moult process of primaries, secondaries and rectrices. During this study the four birds had different breeding status: single, paired without breeding, paired with unsuccessful or successful breeding. In nine cases, the moult corresponded to patterns described in literature. In every case all primaries and rectrices were renewed each season. Males started moulting 3–11 days earlier than females and shed all secondaries every year while females replaced from 50–100% of secondaries annually. Rate of moult was not related to ambient temperature. During a successful breeding season, the moult of both sexes was two-phased: a short first phase with low moult intensity lasting until young owlets left the nest box followed by a longer, more intense second phase. Breeding females (egg laying and incubation) started to moult a few weeks earlier than non-breeding females but completed moult at approximately the same date. In contrast, breeding and non-breeding males had similar patterns of moulting intensity.

Bad News and Good News: Population Changes of Finnish Owls During 1982–2007

In Finland, monitoring of ‘common’ birds of prey is based on two projects run by the Finnish Ringing Centre: the Raptor Grid (since 1982) and Raptor Questionnaire (since 1986). The Raptor Grid has produced sufficient data for analysing population trends in six of ten owl species. The overall trend during 1982–2007 was significantly negative in the Eagle Owl Bubo bubo (-2.2% per year), Long-eared Owl Asio otus (-4.7%) and Tengmalm's Owl Aegolius funereus (-3.1%), and significantly positive in the Ural Owl Strix uralensis ( 1%); no significant trend was detected in the Pygmy Owl Glaucidium passerinum or Tawny Owl Strix aluco. The Eagle Owl population increased to the middle of 1990s, but has since decreased by 5% per year. The decrease coincides with the closing of 90% of local open rubbish dumps, which offered a stable and rich food supply to the Eagle Owls. The decrease in the Tengmalm's Owls can partly be attributed to the decrease in the amount of old forest. The Pygmy Owl population increased steeply (>5% per year) during 1994–2003, then crashed following a mass invasion and has since started to recover. The geographical distribution of the Raptor Grid study plots is not suitable for monitoring the northern nomadic species like the Snowy Owl Bubo scandiacus, Northern Hawk-Owl Surnia ulula, Great Grey Owl Strix nebulosa and Short-eared Owl Asio flammeus. Annual totals of active nests and occupied territories reported with the Raptor Questionnaire do not indicate any long-term population changes of these species. However, intensive cooperation over larger areas across national boundaries in northern Europe is urgently needed for reliable monitoring of these nomadic owls.