Snowy Owl Bubo Scandiacus Research Articles

Long‐term population decline of a genetically homogeneous continental‐wide top Arctic predator

Genetic analysis can provide valuable information for conservation programmes by unravelling the demographic trajectory of populations, estimating effective population size or inferring genetic differentiation between populations. Here, we investigated the genetic differentiation within Snowy Owls Bubo scandiacus in North America, a species identified as vulnerable by the IUCN, to (1) quantify connectivity among wintering areas, (2) evaluate current genetic diversity and effective population size, and (3) infer changes in the historical effective population size changes from the last millennia to the recent past. The Snowy Owl, a highly mobile top predator, breeds across the Arctic tundra, a region especially sensitive to current climate change. Using single‐nucleotide polymorphism (SNP)‐based analyses on Snowy Owls sampled across the North American non‐breeding range, we found an absence of genetic differentiation among individuals located up to 4650 km apart. Our results suggest high genetic intermixing and effective dispersal at the continental scale despite documented philopatry to non‐breeding sites in winter. Reconstructing the population demographic indicated that North American Snowy Owls have been steadily declining since the Last Glacial Maximum c. 20 000 years ago, and concurrently with global increases in temperature. Conservation programmes should now consider North American Snowy Owls a single, genetically homogeneous continental‐wide population which is probably sensitive to the long‐term global warming occurring since the Last Glacial Maximum.

Linking winter habitat use, diet and reproduction in snowy owls using satellite tracking and stable isotope analyses

ABSTRACT Coupling isotope values of feathers and satellite tracking of individuals have the potential to reveal multi-season linkages between wintering habitat, diet and carry-over effects on reproductive parameters in migrating birds. Snowy owls Bubo scandiacus have multiple wintering tactics as they can use both terrestrial and marine resources during the non-breeding season, but their nomadic behaviour complicates their study. We assessed if inter-individual variability in the diet inferred by feather isotopes could be explained by habitat use in winter as determined by satellite telemetry and examined possible carry-over effects on reproduction. Seventeen breeding female snowy owls were equipped with satellite transmitters and sampled for stable isotopes (δ 13C and δ 15N) in feathers. We found a positive relationship between the use of the coastal and marine environments in winter and the contribution of marine resources to the diet of snowy owls in the previous year based on feather analysis. The proportion of marine contribution to the winter diet was variable among individuals and showed a weak negative relationship to summer body mass but not with laying date or clutch size. Our integrated approach shows the usefulness of isotope analyses to infer habitat use and expand the temporal coverage of radio-tracking studies.

Age composition of winter irruptive Snowy Owls in North America

Patterns of winter irruptions in several owl species apparently follow the ‘lack of food’ hypothesis, which predicts that individuals leave their breeding grounds in search of food when prey populations do not allow breeding and are too small to ensure survival. Recent analyses, however, suggest an alternative mechanism dubbed the ‘breeding success’ hypothesis, which predicts that winter irruptions might instead be the result of a very successful breeding season, with a large pool of young birds subsequently migrating south from the breeding grounds. Here we assessed age‐class (juvenile vs. non‐juvenile) composition of winter irruptive Snowy Owls Bubo scandiacus over a 25‐year period (winter 1991–1992 to 2015–2016) between regular (North American Prairies and Great Plains) and irregular wintering areas (northeastern North America) using live‐trapped individuals and high‐resolution images of individual owls. Our results show that the proportion of juveniles (birds less than 1 year of age) varies considerably annually but is positively correlated with irruption intensity in both regions. In irregular wintering areas, it can constitute the majority (up to more than 90%) of winter irruptive Snowy Owls over a large geographical area. These results are consistent with the idea that large winter irruptions at temperate latitudes are not the result of adults massively leaving the Arctic in search of food after a breeding failure but are more likely to be a consequence of good reproductive conditions in the Arctic that create a large pool of winter migrants.

Effects of satellite transmitters on survival in Snowy Owls <i>Bubo scandiacus</i>

The use of tracking devices to monitor birds is extensive, but the effects of such instruments on equipped individuals are still insufficiently taken into account. Here we evaluate potential effects of backpack-mounted satellite transmitters (platform terminal transmitters; PTTs) on survival of 28 Snowy Owls Bubo scandiacus. Six confirmed deaths were all probably related to natural and human-induced causes. Although PTT operational time was significantly shorter than expected lifetime of Snowy Owls, five owls were observed alive after transmissions ceased. Additionally four PTTs stopped due to low battery levels, indicating end of transmitter life and not owl mortality. We found no evidence of mortality caused by PTTs, but sample sizes are relatively low and detrimental effects on equipped Snowy Owls cannot be excluded. We recommend caution when instrumenting large owls.

Snowy Owl nest failures caused by blackfly attacks on incubating females

Blackflies (Simuliidae) have been shown to influence birds negatively in several ways. In 2011, during a peak lemming year, we found that two of 28 nests of Snowy Owls Bubo scandiacus in Northern Norway and Finland failed due to blackfly attacks on the incubating female. The observations also suggest that adult survival may be influenced by these insects. The owls’ choice of breeding habitat can be decisive in their vulnerability to blackflies.

Wing feather moult and age determination of Snowy Owls Bubo scandiacus

Moult of primary and secondary flight feathers of Snowy Owls Bubo scandiacus was studied from 53 museum specimens retaining some juvenile feathers. There were no ringed Snowy Owls of known age in the skin material, and the moult pattern has thus been interpreted using the moult of Eagle Owls Bubo bubo as a model. The difference between juvenile and adult primaries is described. Greater coverts may facilitate age determination of single flight feathers. Snowy Owls start their first flight feather moult M1 by shedding the innermost 2-6 secondaries during their second summer (as 2C birds). A majority of the owls also shed primary P7 during this first moult, occasionally also P8. This moult progresses at a faster pace than seen in Scandinavian Eagle Owls. During their second flight feather moult M2, moult of secondaries advances outwards, including moult from the two focal points S2 and S5, as in the Eagle Owl. Both P7 and P8 are moulted and fresh after M2, and some owls even moult P6 or P9. After the third flight feather moult M3 it is very hard to distinguish juvenile secondaries from old M1 secondaries, and a specific moult pattern is most easily recognized in primaries. On sharp images of the upper side of wings on a flying Snowy Owl, or on birds in hand, it should be possible to age birds up to four calendar years (4C) in autumn and five calendar years (5C) in spring, before onset of the fourth flight feather moult M4.

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.

Characteristics of Nest Mounds used by Snowy Owls in Barrow, Alaska, with Conservation and Management Implications

Snowy Owl Bubo scandiacus nest mounds from Barrow, Alaska, were characterized measuring several variables from 114 nests and 2280 random mounds. Mound height appeared to be the most important trait, as nest mounds averaged about 1 m tall, and were significantly different from random mounds in all years. Generally circular nest bowls (n = 91) were dug by females and averaged 47.5 × 44.0 × 9.6 cm (length × width × depth). Mound vegetation did not conceal Incubating or brooding Snowy Owls. Tall nest mounds were advantageous in that they were drier (thinner snow cover which melted earlier allowing females to dig nest bowls), windier (relief from mosquitoes and cooler in summer) and allowed nesting owls greater vigilance against predators.

Avian psychology and communication.

The evolution of animal communication is a complex issue and one that attracts much research and debate. 'Receiver psychology' has been highlighted as a potential selective force, and we review how avian psychological processes and biases can influence the evolution and design of signals as well as the progress that has been made in testing these ideas in behavioural studies. Interestingly, although birds are a focal group for experimental psychologists and behavioural ecologists alike, the integration of theoretical ideas from psychology into studies of communication has been relatively slow. However, recent operant experiments are starting to address how birds perceive and respond to complex natural signals in an attempt to answer evolutionary problems in communication. This review outlines how a psychological approach to understanding communication is useful, and we hope that it stimulates further research addressing the role of psychological mechanisms in signal evolution.