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Peter SundeComplete reference list - peer-reviewed papersOverskaug, K., Kristiansen, E. & Sunde, P. 1995. Sex-specific diet analysis of the Tawny Owl Strix aluco in Norway. – Journal of Raptor Research 29: 137-40.
Overskaug, K., Sunde, P. & Kristiansen, E. 1997. Subcutaneous fat accumulation in Norwegian owls and raptors. – Ornis Fennica 74: 29-37.
Abstract The mean subcutaneous fat deposition (MFS) found in dead diurnal raptors and owls collected in Norway in 1987-92 was compared to detect possible overall and intra- and interspecific seasonal differences. In general, the relative fat deposition rate was correlated with body size (P lt 0.001), and was highest in winter (P = 0.01) and in females (P = 0.02). After correcting for the size effect, species-specific differences were still present (P = 0.000), but the effect of sex disappeared (P = 0.34). We propose that the relative rate of fat deposition in northern Palaearctic owls and raptors reflects adaptations to resist starvation balanced against the demands for high agility and low flying costs. This trade-off probably varies between breeding and non-breeding seasons, sexes, and species with different migratory and hunting habits.
Sunde, P. & Kvam, T. 1997. Diet patterns of Eurasian lynx Lynx lynx: what causes sexually determined prey size segregation? – Acta Theriologica 42: 189-201.
Abstract The influence of sex, body weight, physical condition, age and season on diet choice was investigated by hunting reports and intestinal analyses of 441 lynx Lynx lynx (Linnaeus, 1758) from Norway killed during 1960-1996. Of self-provisioning (³1 yr) lynx (n = 280), males preyed proportionately more upon cervids (primarily roe deer Capreolus capreolus and semi-domestic reindeer Rangifer tarandus) compared to small game (mountain hare Lepus timidus and tetraonids) than females did. Only 5.4% of the variation in prey preference towards small game and cervids (p = 0.0002) could be explained by sex. In a logistic regression model, no additive effect of weight or any other parameters was found after sex had been included. We did not find sufficient evidence for body weight (sensu stricto) being related to prey choice, but propose that sexually determined prey segregation in lynx is caused by different ranging behaviour resulting in different encounter rates with different kinds of prey.
Sunde, P., Stener, S.Ø. & Kvam, T. 1998 Tolerance to humans of resting lynxes in a hunted population. – Wildlife Biology 4: 177-183.
Abstract The tolerance of lynxes Lynx lynx to human presence and disturbance in a hunted population in Norway was studied using telemetry. Forest land within 200 metres from the nearest road or house was avoided by lynxes when resting (P < 0.01). The tolerance distance of resting lynxes towards intruding people was short (median 50 m), though strongly correlated with the horizontal vegetation cover (partial correlation, P < 0.02) and forest maturation stage (partial correlation, P < 0.02), but not with terrain inclination (partial correlation, P > 0.3). The lynxes did not enter steeper country (P > 0.4) or alter their daily walking distance (P > 0.7) after being disturbed. The data indicate that lynxes, even when suffering extensive, man-induced mortality, may tolerate high human activity within their range as long as sufficient stands of undisturbed, mature forest with dense horizontal cover are present
Sunde, P., Overskaug K. & Kvam, T. 1998. Culling of lynxes related to livestock predation in a heterogeneous landscape. – Wildlife Biology 4: 169-175.
Abstract Lynx Lynx lynx hunting in Norway is regulated through regional quotas according to the magnitude of predation on semi-domesticated reindeer Rangifer tarandus and domestic sheep Oves aries. Lynxes and semi-domesticated reindeer were studied using telemetry in an area in Nord-Trondelag County, Norway, with high lynx predation on reindeer and domestic sheep and a high hunting pressure on lynxes. The probability of an ungulate killed by lynxes being livestock as opposed to a roe deer (the only alternative wild ungulate) increased with increasing distance from fields (P < 0.0001) and roads (P < 0.0001). Hunting was the only mortality cause found for radiocollared lynxes. The culling of lynxes was biased towards the vicinity of roads and cultivated fields compared with the general distribution of radiocollared lynxes (P < 0.001) and radio-collared semi-domesticated reindeer killed by lynxes (P < 0.001). Because of the easy location in rural areas due to the well-developed road system, lynxes suffered the highest hunting mortality in habitats where the proportion of livestock in the diet was lowest, whereas lynxes inhabiting alpine areas more than 3 km from the nearest road escaped hunting. The hunters' preference for hunting lynxes near roads leads to a risk of selective reduction in regions and habitats where lynxes do little harm, whereas numbers in remote areas with high predation on livestock may remain unaltered.
Overskaug, K., Bolstad, J.P., Sunde, P. & Øien, I.J. 1999. Fledgling behavior and survival in northern Tawny Owls. – Condor 101: 169-174.
Abstract Telemetry studies of Tawny Owl (Strix aluco) fledglings (n = 53) were conducted from May to August in central Norway, where the species is at the northern limit of its range and its density is generally low. Mortality before dispersal was 61% due to starvation and predation by red fox (Vulpes vulpes), pine marten (Martes martes), and Corvus spp. Mortality was highest during the first 10 days after leaving the nest box, when fledglings were small and had poorly developed locomotion. Fledglings that died or disappeared during the first 10 days after leaving the nest box were significantly lighter than those that survived. Mean distance between fledglings and nest box increased during the summer, but fledglings remained relatively close to siblings and their mother until dispersal. The average area used by fledglings that survived to dispersal was 26 ha (n= 15), which is a figure larger than reported elsewhere for this species. Dense forest habitats were preferred, and roosting sites offering good vertical and horizontal concealment were used frequently. We propose that observed habitat and area use reflect antipredator strategies and generally low density of this strictly territorial species.
Sunde, P., Overskaug K. & Kvam, T. 1999. Intraguild predation of lynxes on foxes: evidence of interference competition? – Ecography 22: 521-523.
Abstract There is a growing focus among ecologists on the importance of predatory interactions between competing species, but because of its mixed character, it is difficult to pinpoint the possible, competitive component of the interaction. We tested degree of prey consumption in intraguild predation of Eurasian lynxes on red foxes against assumed 'pure' predation, i.e. roe deer and mountain hares. The proportion of uneaten fox carcasses was highly significantly larger than the proportion of both other prey species, indicating that interference competition may play a role in the intraguild predation of lynxes on red foxes.
Sunde, P., Kvam, T., Moa. P., Negård, A. & Overskaug, K. 2000. Space use by Eurasian lynxes (Lynx lynx) in central Norway – Acta Theriologica 45: 507-524
Abstract Habitat and spatial organisation of 11 radio tagged Eurasian lynxes Lynx lynx Linnaeus, 1758 were studied in a low-density (ca 0.3 ind/100 km2) population in a boreal-alpine environment with low and temporally varying densities (l80 ind/100 km2 in winter) of ungulate prey, primarily roe deer and semi-domestic reindeer. The use of habitat measured as 4 biome categories ranked from south boreal to alpine influenced mountain vegetation did not vary seasonally, but lowlands were much preferred to alpine habitats. Adult males moved almost 3 times farther per day in linear distance (`x = 5.9 km, n = 3) than did females with kittens (x = 2.0 km, n = 4) or subadult females (`x = 2.5 km, n = 6; p = 0.002). Subadults (n = 5) dispersed 42 +- 13 (`x +- SE) km during the first 9 months of independence, but often visited their natal range during the first year on their own. Adult lynxes roamed over very large annual ranges (males: 1906 +- 387 km2 (n = 4), females: 561 +- 102 km2 (n = 6)) that took more than 5 days to pass through, independently of sex. The only male monitored over more than 1 year maintained 2 separate home ranges each year. The larger home ranges and the possible tendency towards less defined territory boundaries than previously reported for the species, may be caused by the lower prey and population densities, though culling of adult individuals may also have played a role by continuously creating empty gaps in the territorial mosaic.
Sunde, P., Kvam, T., Bolstad, J.P. & Bronndal, M. 2000. Foraging by lynxes in a managed boreal-alpine environment. – Ecography 23: 291-298.
Abstract Foraging of Eurasian lynxes Lynx lynx was studied with telemetry and snow tracking in central Norway. In all habitats and at all seasons, medium-sized ungulates (roe deer Capreolus capreolus, reindeer Rangifer tarandus and domestic sheep Ovis aries) dominated the diet (81% of ingested biomass estimated from faeces). Mountain hares Lepus timidus and galliform birds comprised the remainder of the diet (15% and 3%, respectively). Lynxes with different life history status did not differ in prey choice, but adult males utilised carcasses of ungulate prey considerably less (16% of the edible parts) than did females with offspring (80%) and subadults (58%). Forest habitats in lowlands and adjacent to cultivated fields were the most favourable foraging habitats (indexed as the prey encounter rate per km lynx track) primarily owing to the presence of roe deer. Two family groups tracked in winter killed 0.2 ungulate per day. The importance of agricultural land as a foraging habitat and the dominance of livestock in the diet in remoter areas indicate that the lynx has responded to agriculture and reindeer husbandry during the past century by switching from small game to ungulates.
Overskaug, K., Sunde, P. & Stuve, G. 2000. Inter-sexual differences in the diet composition of Norwegian raptors. – Ornis Norvegica 23: 24-31.
Abstract Stomach contents of seven Fennoscandian species of owls and seven species of birds of prey, numbering totally 1524 birds, was allotted to six prey categories: birds, large mammals, small mammals, amphibians/reptiles, fish and invertebrates. Overall, invertebrates represented a limited proportion of the diet on a numerical and weight basis. There was no significant difference between the invertebrate proportions measured by this investigation of stomach contents vs. previous pellet analyses in Fennoscandian raptors, respectively. However, despite the fact that invertebrates play a minor role on a biomass basis, they may temporarily be important on the individual level, as judged from their frequent occurrence in some stomachs. Moreover, in the Goshawk Accipiter gentilis females had taken more birds and males more small mammals. This inter-sexual difference may be linked to respective different feeding niches between the male and the female, thus supporting one hypothesis of the possible outcome of reversed sexual size dimorphism (RSD) in raptors. However, no overall support to this hypothesis was present in the species studied.
Sunde, P., Overskaug, K., Bolstad, J.P. & Øien, I.J. 2001. Living at the limit: ecology and behaviour of Tawny Owls in a northern edge population in central Norway. – Ardea 89: 85-98
Abstract The Tawny Owl Strix aluco was studied at the northernmost limit of its geographical range (63degree20'N) in order to investigate how this extremely residential species has adapted to the marginal conditions of its northern outpost. The presence of nemoral forest vegetation was crucial for the occurrence of the species. The mean annual home range size of radio tagged females was much larger than reported in any other study. The winter home ranges were, on average, 54% larger than those in summer. In the summer, range size was negatively correlated with the proportion of mixed deciduous/coniferous forest within 1km of the nest. This could be explained by the abundance of bird prey, which comprised 61% of the summer diet. In the winter, no significant correlations with home range sizes were found. During this season, mammals comprised 95% of the diet. In extreme cases, females left the nesting area for prolonged periods during the non-breeding season. We conclude that even a species known to be notoriously residential can express a high degree of plasticity in its ranging behaviour when population densities are low, at least among the females. However, the environmental and social factors leading to some females leaving their nest areas in the non-breeding season are more complex than the result of mere lack of food.
Sunde, P. 2002 Starvation mortality and body condition of Goshawks Accipiter gentilis along a latitudinal gradient in Norway. – Ibis 144: 301-310.
Abstract Relative starvation risk and body condition were investigated in 599 Goshawks that had died in collision accidents or of starvation. Specimens were collected by the public along a 1300-km north-south (58degreeN-71degreeN) gradient in Norway, representing the northernmost geographical range of the species. The probability of a Goshawk's death being caused by starvation as opposed to by a collision accident increased with latitude with juvenile males at a disproportionately higher risk than others. Of birds killed in accidents, females generally were in better condition than males, and adults in better condition than juveniles. A season-by-latitude interaction indicated that males from northern latitudes were in poorer condition during winter and spring than males from southern parts of the country. This could also be modelled as a curvilinear relationship with daylength. There were no significant relationships between weather factors in the weeks prior to the deaths of the birds and the relative starvation probability or the condition of trauma victims. The results suggest that food limitation plays a relatively higher role in northern populations, affecting young males especially. This was also supported by the fact that the sex ratio of accidentally killed birds was increasingly female biased with increasing latitudes. It is suggested that the relatively higher mortality risk of males is due to their smaller average body size, and that selection for starvation resistance during winter is the reason behind the clinal increase of body size in Goshawks towards the northern and eastern parts of Europe.
Nybakk, K., Kjelvik, O., Kvam, T., Overskaug, K. & Sunde, P. 2002. Mortality of semidomesticated reindeer in central Norway. – Wildlife Biology 8: 63-68.
Abstract During the last decade, losses of semi-domestic reindeer Rangifer tarandus have increased in central Norway. Natural mortality in a semi-domestic reindeer herd was studied by use of mortality sensing transmitters. From 15 April 1995 to 15 April 1996, 135 of 612 animals equipped with radio collars were found dead. Adult females (>2 years old) suffered a mortality of 18.3%, of which 40.5% was due to predation. Yearlings suffered a yearly mortality of 20.2% of which 66.7% was due to predation. Calf mortality from 6 August 1995 to 15 April 1996 was 31.0%, of which 75.3% was due to predation. Predation by Eurasian lynx Lynx lynx was the predominant cause of death, with 39.3% of the total mortality. Accidents were the second most important cause of death, with 16.0% of the total mortality. Peaks in mortality were registered in September, November and January.
P. Sunde, Bølstad, M.S. & Møller, J.D. 2003. Reversed sexual dimorphism in tawny owls, Strix aluco, correlates with duty division in breeding effort. – Oikos 101: 265-278
Abstract Even though most bird species with a raptorial feeding habit express varying extents of reversed sexual dimorphism (RSD: females bigger than males), the evolutionary basis for its maintenance, as well as its possible secondary consequences for the ecological adaptations of the different sexes, is debated. We studied pairs of tawny owls, Strix aluco (females 20% heavier than males), throughout the year by telemetry to test whether any inter-sexual differences in movement patterns, resource partitioning and breeding effort correlated with RSD. Females were larger than males in all body size measures and were 16% heavier than would be expected from the difference in wing length alone. In accordance with predictions from flight economics, males moved longer distances per time unit than females, in particular during the post-fledging season, when they also fed chicks more often than the females. Males had larger home ranges than females during the post-fledging period, whereas the sexes had home ranges of equal size during the non-breeding season. Until 10 days after fledging, females foraged much closer to the offspring than males, apparently balancing their distance to offspring between the needs of offspring guarding and foraging. In males, the parent-offspring distance only increased with decreasing brood condition. The sexes did not differ in habitat use or feeding habits, rendering no indications of food niche partitioning. The study provides further evidence that selection for males to be light and energetically efficient foragers is the main evolutionary force behind RSD in raptorial birds, even when the prey base is confined by territoriality.
P. Sunde, Bølstad, M.S. & Desfor, K. B. 2003 Diurnal exposure as a risk sensitive behaviour in Tawny Owls, Strix aluco? – Journal of Avian Biology, 23 (in press)
Abstract Tawny owls Strix aluco generally roost in cryptic locations during the day. To test the hypothesis that this cryptic behaviour is an effort to avoid mobbers or avian predators, we measured diurnal behaviour and cause-specific mortality of radio-tagged birds. Non-breeding adults (assumed to be well fed individuals, optimising their own survival) roosted in less exposed locations than adults with young and newly independent juveniles. Parents roosted in the most exposed sites when their young were immature and vulnerable to predation, probably to guard offspring. Newly independent juveniles apparently selected roosting sites in exposed places to get access to food, as this behaviour was associated with lower perching heights and higher prey abundance beneath their roosting sites. They also perched in more exposed sites, closer to the ground, in summers with low prey abundance compared to summers with high prey abundance. After previous encounters with goshawks Accipiter gentilis, dependent juveniles roosted in less exposed places compared to other young. The increased risk of being mobbed was highly significant with increasing roosting exposure. Once an owl was mobbed, the intensity of the mobbing correlated positively with the mass of the mobbers, but mobbing birds never killed any owls. In contrast, diurnal raptors caused 73% of natural owl deaths (n = 15) and the predation rate by raptors was 3.8 times higher in population classes that generally roosted in more exposed locations than did non-breeding adults. We therefore suggest that depredation by diurnal raptors is the main factor shaping the diurnal behaviour of tawny owls.
P. Sunde & Bølstad, M.S. 200#. A telemetry study of the social organisation of a tawny owl population. – Journal of Zoology, London (in press)
Abstract The spatial dispersion and social interactions were studied in 11 neighbouring pairs of radio-tagged tawny owls in a deciduous wood in Denmark from 1998-2001. The numbers and shapes of territories were stable throughout the survey and similar to a mapping made 40 years earlier. The home ranges of mates were of equal size and overlapped 82% in summer and 56% in winter. The inter-mate distances were on average 2.7% shorter than expected by chance. The activity distribution of neighbouring pairs overlapped 9(95%CI: 2-15)% on average. Males and females did not differ in overlap with neighbours and there was a similar overlap between neighbours of the same and opposite sex. Both sexes vocalised more often in the peripheries than in the centres of their territory. The vocal activity during May-September varied extensively among years and months in accordance with variation in the density of juvenile floaters. Males and females vocalised equally often and were involved in disputes with neighbours at similar rates. Usually, neighbouring disputes involved either one individual from each pair or all four. Disputes involving all four owls more often involved chasing and fighting than those only involving one owl from each pair. The dispute rate between neighbouring pairs correlated positively with home range overlap. The total annual mortality was 21(95%CI: 6-33)%. Dead owners were usually replaced within 1-2 months. Two out of four cases of disappearances of radio-tagged owls from their territory due to natural causes was due to territory take-overs by invading owls, suggesting that the risk of losing fitness due to eviction was important. The apparent cooperative territorial behaviour of tawny owl pairs is probably due to improved resource holding potential of pair coalitions compared to single individuals.
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