Snowy Owl (Bubo scandiacus)

 Marina Hayward

 

 

General Description

Snowy owls are well known for their enchanting wintery appearance: snow white feathers and large, luminous yellow eyes. It is very appropriate for blending into their primary habitat, the Arctic tundra. This is a place where they will experience freezing temperatures, as well as 24 hour daylight in the arctic summer and 24 hour night in the arctic winter. Therefore, they must have particular adaptations that allow them to endure such harsh living conditions.

White Colouring

In the winter, the unique white colouring and dark markings on snowy owls allows them to completely camouflage into their snowy environment. The amount of dark markings on their white plumage indicates the gender. All young snowy owls get whiter as they get older, but this is especially true for males. Some old males literally become as white as the snow, although most tend to have small marks of dusky plumage and retain up to three tail bands. Females, on the other hand, are darker than males with dusky spotting, never become totally white, and may have up to six tail bands (Montana Field Guide, 2017).

Body Measurements

Snowy owls are one of the largest owl species and certainly the heaviest owls in North America, due to their thickly insulated bodies (Cornell, 2015). A dense layer of down over the entire body, including the legs and toes, provides insulation. This allows snowy owls to retain a very warm body temperature between 37°C and 40°C in a frigid environment that can reach -40°C. (Denali Education Centre, 2017). They have a body size between 20 to 28 inches, with a wingspan between 4.2 to 4.8 feet, and weigh between 3.5 to 6.5 pounds. The average lifespan of a snowy owl in the wild is 10 years. However, the oldest snowy owl on record is a 23 year old female that was recaptured during banding operations in Montana, in 2015. She was first banded in Massachusetts, in 1992 (Cornell, 2015).

Vocal Sounds

In the nonbreeding season, the snowy owl tends to be silent because they have no reason to communicate when they are without a mate. Snowy owls are generally solitary birds, but can be seen in a group on rare occasions, as seen in the image below. In the breeding season, males can be quite vocal in acts of territorialism or mating calls. Both males and females will make sounds similar to dog barks, shrieks, hissing, and bill-snapping, while nestlings make cheeping noises (Montana Field Guide, 2017).

Hunting and Diet

Snowy owls have exceptional eyesight and hearing; this allows them to detect hidden prey easily. They are also diurnal, they hunt throughout the day and night. The arrangement of an owl’s feathers deafens any sound that could be produced during flight, making them silent killers. A snowy owl’s favourite prey is the lemming, a plump, small rodent that is related to the vole. An adult may eat more than 1,600 lemmings a year, that’s about five lemmings consumed every day. However, snowy owls will also hunt rabbits, various rodents, fish, and birds like ptarmigan, ducks, and geese (National Geographic, 2017).

Migration

Snowy owls will occasionally spend the entire year in their Arctic tundra breeding grounds, although they often migrate to Canada, the northern United States, Russia, Europe, and Asia. Snowy owls will often occupy open fields, marshes, and beaches. The availability of prey may determine how far south they will migrate (National Geographic, 2017).

Reproduction

The breeding season starts around May or June. In the dry summer months, snowy owls stick out conspicuously on the brown tundra. However, adult snowy owls don’t need to worry too much about getting the attention of predators. This is because for such an attractive and demure raptor in appearance, the snowy owl has evolved a ferocious attitude. They are one of the most aggressive owls, known to attack animals much larger than themselves, like wolves and even polar bears. So even though nestlings may be a tempting meal to some predators, many will seriously regret trying to take the owlets once the parents begin attacking (National Geographic, 2017).

The nest is built on the ground, usually with some vantage point of the area. Females generally lay a clutch of 3 to 11 eggs, where clutch size depends upon the availability of food. Incubation lasts for 32 days and the young fledge at 50 to 60 days. Male snowy owls hunt for food, while the female incubates eggs and defends the nest (Denali Education Center, 2017).

Conservation

According to the International Union for Conservation of Nature (IUCN) Species Programme, snowy owls are listed as least concern. This is because their remote breeding grounds are free from direct human disturbance, they have an estimated global population size of 200,000 and they are protected from poaching because they are raptors (Cornell, 2017).

Interesting New Research about Snowy Owls

1. The first record of a Razorbill (Alca torda) being eaten by a Snowy Owl has been made! Microscopic feather characters and osteological comparisons were used to analyze the owl’s stomach contents to confirm findings (Dove and Coddington, 2015).
2. A protozoan parasite, Crytosporidium Baileyi, was discovered in a seven week old snowy owlet kept in a zoo, it caused severe proventriculitis and the owlet died. The proventriculus is the narrow glandular region before a bird’s stomach, located between the crop and the gizzard. This is the first reported case of C. baileyi being present in a raptor species in Asia, and the first time this parasite has been associated with proventriculitis (Nakagun et al, 2017).
3. In an experimental study, the irruptive migration of snowy owls correlates with the breeding success hypothesis, where high abundance of food during the summer permits for high production of young, and thus increases the number of snowy owls migrating south the next winter (Robillard et al, 2016).
4. Platform terminal transmitters (PTTs) are satellite transmitter backpacks for large owls. A study on the survivability of snowy owls with PTTs was conducted to see if these transmitters were having negative effects, including mortality. However, no unnatural mortality of equipped owls was observed, but the researchers still recommend caution when using PTTs on large owls, since the sample size was small (28 snowy owls) and there could still be negative effects present. Further research is required (Heggøy et al, 2017).
5. Marine food sources contribute to variability in stable isotopes (carbon 13 and nitrogen 15) of snowy owl feathers. Some snowy owls are consuming more marine resources during the nonbreeding season, this was determined by satellite tracking, behavioural studies and feather analyses. There was 4-19% variability among different feather types on the body, and 3-71% variability among a single feather type, for contribution of marine sources by diet reconstruction models. This study showed that the contribution of marine resources to the winter diet of snowy owls could not be relied upon a single feather type; and, diet varied when different feathers were being made. Recommendations for future studies include: sampling many feathers from various body regions, except wing feathers, ‘to investigate winter diet or habitat use’ (Robillard et al, 2017)

References

Dove, C.J. and Coddington, C.P. 2015. Forensic techniques identify the first record of snowy owl (Bubo scandiacus) feeding on a razorbill (Alca torda). Wilson Journal of Ornithology, 127(3), pp.503-506. Web. 9 October 2017. http://www.bioone.org/doi/abs/10.1676/14-176.1?journalCode=wils

Heggøy, O., Aarvak, T., Øien, I.J., Jacobsen, K.O., Solheim, R., Zazelenchuk, D., Stoffel, M. and Kleven, O. 2017. Effects of satellite transmitters on survival in Snowy Owls Bubo scandiacus. Ornis Norvegica. Web. 9 October 2017. https://brage.bibsys.no/xmlui/handle/11250/2458927

Nakagun, S., Horiuchi, N., Sugimoto, M., Tomikawa, S., Watanabe, K. and KOBAYASHI, Y. 2017. Proventriculitis associated with Cryptosporidium baileyi in a snowy owl (Bubo scandiacus) and its epidemiological investigation. Journal of Parasitology. Web. 9 October 2017. http://www.journalofparasitology.org/doi/abs/10.1645/17-54

Robillard, A., Therrien, J.F., Gauthier, G., Clark, K.M. and Bêty, J., 2016. Pulsed resources at tundra breeding sites affect winter irruptions at temperate latitudes of a top predator, the snowy owl. Oecologia, 181(2), pp.423-433. Web. 9 October 2017. https://link.springer.com/article/10.1007/s00442-016-3588-3

Robillard, A., Gauthier, G., Therrien, J.F., Fitzgerald, G., Provencher, J.F. and Bêty, J. 2017. Variability in stable isotopes of snowy owl feathers and contribution of marine resources to their winter diet. Journal of Avian Biology. Web. 9 October 2017. http://onlinelibrary.wiley.com/doi/10.1111/jav.01257/full

Snowy Owl — Bubo scandiacus.  Montana Field Guide.  Montana Natural Heritage Program and Montana Fish, Wildlife and Parks. Web. 9 October 2017. http://FieldGuide.mt.gov/speciesDetail.aspx?elcode=abnsb06010

Snowy Owls. Denali Education Center: Alaska. Sundog Media LLC. Web. 9 October 2017. https://www.denali.org/denalis-natural-history/snowy-owls/

Snowy Owl. 2015. All About Birds. Cornell Lab of Ornithology. Web. 9 October 2017. https://www.allaboutbirds.org/guide/Snowy_Owl/lifehistory

Snowy Owl. 2017. Animals. National Geographic. Web. 9 October 2017. http://www.nationalgeographic.com/animals/birds/s/snowy-owl/