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Magic of Transformation!

TheSnowyOwlBook

The Snowy Owl (2013)


The jury may be out on whether a leopard can change its spots or not, but a Snowy Owl certainly can and will! It used to be believed that the male Snowy is white with little barring and gets whiter with age, and the female has more barring and gets darker with age. But, in reality, Snowies cannot be aged or sexed with certainty. The darkest males and the lightest females are selfsame to the human eye. Also, no two owls are identical, and even throughout an individual’s lifetime, Snowies may choose to increase or decrease their coloration based on the environment and their physiological condition.

The female’s barring serves as a camouflage to help her hide herself and her chicks in the nest, but not in the way that we automatically reason. Animals don’t see colors like humans do. For example, several animals see our black power lines as glowing and flashing bands. We are also not always who they display to or hide from! So their visual appearance is tailored specifically to their needs, keeping in mind their primary ‘ultra-awesome’ predators and preys.

Several of the owl’s prey have UV vision. While the white plumage of the Snowy may seem to humans as allowing them to camouflage themselves against the snow, it is in fact highly UV absorbing, making the owl conspicuous to its predators! To them, neither the Snowy Owl’s white plumage nor its dark parts reflect in UV or near UV. In fact their dark parts peak in the near infrared part of the spectrum. So the Snowy Owl appears starkly grey against the white background of the snow. Moreover, Snowies breed in the open tundra, where they can be seen from great distances both in white winter and in colorful summer. They are the largest avian predators on the tundra and can protect themselves from intruders. Having said that, it ends up spending as much time fighting competitors and bullies as it does minding its business, because they are all equally plucky; a prerequisite to everyone’s survival in inhospitable terrains.

In fact, the males that do all the hunting during the breeding season, reflect significantly more light than the females, so their white color is not an adaptation to camouflage the bird, but is an impairment during hunting! But, this is the sacrifice that the birds make, because pigmentation production costs energy that is better used to minimize energy expenditure during the moult, when all Snowies loose one or two primaries on both wings (the moult intensity is higher for non-breeding Snowies). At no point during the moult do the birds lose their ability to fly.

Because the white of the Snowies is an impediment, it is only the best hunters that can pass on their genes to further generations. In fact, a female might be mighty pleased to know that the male can hunt even without pigmentation!

While their preys have UV vision, Snowy Owls themselves don’t. To Snowies, white is white. On sunny days, they orient themselves toward the sun so that the snow’s albedo enhances their visual display. This is how they broadcast their territorial claims, prey information, and show-off to conspecifics. The lighter Snowies display longer than the darker ones.

Even though Snowies are not UV-sensitive, their vision is one of the most highly developed of any owl, and can track distant objects in all variations of ambient light! Unlike other owls, the Snowy Owls are diurnal (active during the day). Some believe this is the case because they have adapted themselves to the long nights and days in the Arctic! In fact, the Snowies' multifocal tubular eyes are one of the most ecologically adapted of any bird in the world. The species eyes are roughly 1.5 times more sensitive than those of humans, but with lower limited field view, and increased ability to see in low light levels. Like all owls, the Snowy can turn its head 135 degrees in either direction giving it a total of 270 degrees “field of view”. It can even turn its head almost upside down.

Interestingly enough, the primary reason for the eyes being set at the front of the head is not to give them binocular vision as much as to accommodate large ears. The Snowy Owl is distinct from the other avian predators in taking prey from under the snow. It even prefers to hunt through the snow than over thawed surfaces. Such hunting is clearly done using sound, as much as vision.

And even though their pupils maintain an unchanging diameter of 12mm giving them a small depth of field, they have well-developed rictal bristles around the eyes and beak that help them feel for their newborns, and assess their well-being, or detect the shape and softness of the captured prey.
Snowy Owls have a highly developed and complex emotional nature; perhaps incomparable with humans because of the Spock-like gentle-practicality to their nature, which is distinct and multi-layered.

Take sex for instance. Snowies are the only birds that are known to have recreational sex. They copulate often, even after the chicks are born, and even in seasons when conception is impossible. And the copulation is solicited by both males and females with beautiful displays and mewing. And in fact, even when the female is nesting her chicks, she might take off for just a bit for a little sex break. But, all sex begins with a food ritual, where the male, who usually delivers food with his talons, indulges in some beak-to-beak lemming action. Snowies derive a great deal of pleasure from consuming a lemming, and particularly the lemming over any other species, and visibly so.

During the sex ritual the couple then touch each other with the bristles close to their beaks, close their eyes and make a soft whistle-chipping noise, which is then followed by the coacal kiss and some cackling calls. Observers say Snowies show a clear “shine of pleasure” after sex.

Even though copulation during a couple’s life together is solicited either by males or females, their breeding activity begins with the male displaying. And if the female is impressed with his undulating flight, they may choose to live monogamously ever after and make lots of babies!

The Snowy Owl is a classic example of a bird with a circumpolar breeding distribution. They breed throughout the Arctic and Arctic-fringe area, but in very few locations: Canada, the USA (Alaska), Russia and Greenland (these four having the majority of the breeding populations), sometimes in Norway, Sweden and Finland, and rarely if ever in Iceland and the UK. Breeding attempts south of the Arctic Circle are sporadic, and if successful, do not result in large numbers of young.

But, if a random point is chosen within the traditionally defined range, the chances of seeing Snowy Owls breeding there would vary from no better than 75% to as little as .01%. This is because, Snowy Owls may alternate eruptive breeding with long periods of absence from any particular place.

It is not known how Snowy pairs are formed. They arrive on the breeding grounds already paired in April. Individuals pairs in a loose boid settle in the same neighborhood, and maintain comfortable distance from each other (this is where the reflective white of their plumage comes in handy). This distance is determined by the lemming density, so that it could be anywhere between 1-8 sq.kms. Needless to say, shorter distances have a higher probability of conflict than larger, especially among owls that are of the same age and sex.
That being said, vagrant Snowy Owls that choose not to travel in boids, have reduced chances of survival or successful breeding. One might therefore assume that if there is a breeding Snowy Owl pair in a region, there are sure to be some more in the neighborhood.

Snowy Owl is also a rare documented example of an avian species with a fixed (but still variable) laying date, in all of its habitats; perhaps due to non-photic cues. However, earlier arrivals breed earlier and lay larger clutches, later arrivals breed later and lay smaller clutches.

Snowy owls are the most prolific large carnivorous bird laying up to 14 eggs in one season. Their asynchronous egg laying process makes the Snowy Owl unique among all birds. No other species produces a large clutch over such a long period of time; not to mention, at low ambient temperatures of -18 ° C. Since the average clutch constitutes 25– 43% of the female’s body mass, synthesis of such a clutch is a significant physiological burden.

The clutch size automatically adjusts to the hunting performance of the male, as well as the resource qualities of the breeding habitat. The females produce clutches based on spring lemming abundance which may not always be a good indicator of the summer population, as the lemming abundance may not stay constant throughout the summer. So they are forced to take a gamble, where the chance of success is as high as the chance of failure.

The female’s ability to maximise clutch size therefore implies a corresponding ability to reduce it when needed, and asynchronous hatching makes this possible, where the eggs are not laid all at once, but one after the other with a fairly long laying interval between the them!

When the female is ready to reproduce (typically in mid-May), her body switches on an egg-production conveyor belt, and the first egg is laid a fortnight after that. As soon as the first egg appears, the female begins to multitask: she incubates the laid egg, produces further eggs, and ensures that the male owl maintains a constant supply of food.

Observers share that the interval between the eggs laid increasing with the number of eggs being laid. Clutch completion therefore varies from 2-22 days.

In addition to gambling on clutch size, it is believed that the Snowy Owls might physiologically manipulate the sex ratio of their offspring based on the spring lemming population. Unlike humans, we use the ZW sex determination system to determine the sex of birds. The females are heterogametic sex (ZW), and the males are homogametic sex (ZZ). The book doesn't explain the motivation behind the sex-ratio for Snowies very well, but I did find an interesting article on sex-ratios of Tree Swallows, and can only speculate in what way it may or may not be similar to Snowies.

During the egg laying period, the female’s body has to generate about 55g of high quality protein and 5g of calcium every other day; this amounts to 2.5% of the female’s body weight. This is the equivalent of if a 75kg human had to produce 1.8kg of meat every other day, which is unachievable for us, and yet, the female Snowy Owl does it annually, even in stormy weather.

When all is good, the female incubates the clutch non-stop (even when buried in snow during snowstorms). Observers report that the eggs are quite resilient to cold. There have been instances where eggs hatched even after a female failed to cover it sufficiently. However, if an egg is left open to the air for too long, it could quickly freeze and crack, or the chick could die after hatching!

In the first week of the chicks’ birth, they are ill-equipped to weather the cold temperatures without their mother protecting them. But, as they grow older, they are equipped with thick grey down all the way to their toes. Some think Snowy chicks' downs are among the best insulators in the world!

Even though chicks hatch asynchronously, and the older chicks are bigger than the younger ones, the maximum size differences are not achieved at the nest. As the older chicks start to get big, their emotions are a complex mix of tenderness and rivalry. By the time they are 8-10 days old, the benefits of freedom outweigh the benefits of cuddling and nest-safety, and they start to demonstrate escape behavior. At this point, they spend half the time developing outside (nidifugous), roaming 1-10 meters from the nest, and half the time within the nest (nidicolous), returning only to be brooded by their mom. By the age of 16-18 days the chicks permanently move into the open tundra. But even outside the nest, they rely entirely on their parents to deliver food to them. So they make whistling noises to call out to their parents and in fact compete to be seen first when a parent returns to the site with food. By the end of the month, they will have dawdled up to 3 sq.kms from the nest. They are still remarkably spotted by their parents through their whistling calls!

Snowy Owls need to keep their body temperature almost constant at all times. This is a task which is a challenge in the Arctic, where the weather is temperamental. So Snowies change the thickness of their plumage all the time. Their body temperatures too are about 38.54 ° C, which is almost a degree lower than other owl species. There is a complete lack of circadian patterns in their body temperature. Their temperature rises when they are in the dark or are active, and drop when they are deprived of food; whereas, exposure to cold does not have such any effect. Since food is their only source of energy consumption, they balance their need to conserve energy, with their need to expend it to find food.

The global Snowy Owl population is currently estimated at 290,000 (I think!). But, because they vary both spatially and temporally, the author suggests that estimating their population size is akin to counting insects in a locust swarm, without knowing where the front or the rear of the swam is. But, they slow down during the breeding season. The breeding decision is dependent on local food sources, and their ability to reach it, which again is dependent on where they spent the winter, and their ability to detect lemming trends. Their numbers therefore follow the fluctuations of the main prey.

Lemming numbers demonstrate cyclical variations both in time and space. The unpredictability of this primary food has made the owl nomadic. The owl breeds only in years when the average body weight of the prey is greater than 40g. And the heavier the body weight of the prey, the larger the brood size. If the lemmings are too small, the owl either does not breed at all, or attempts to switch to an alternative food source. If lemmings breed a lot, the number of large individuals will decrease, whereas the number of young will increase to the detriment of the Snowy owls. Moreover the average body weight of lemmings declines over the course of summer so that at the end of the summer the Snowy Owls face problems with the food delivery rate. If the average body weight of rodents declines below 35g, the parents try to switch prey and deliver heavy prey to the nest and take smaller ones for themselves to eat. Because of this, adults and chicks have significantly different diets, which too is unique to Snowy Owls.

In order to appreciate the decision making process that owl has to go through in selecting the right lemming, it is important to understand the energy budget of Owls.

Snowies spend most of it’s time still- and ground-hunting. It can fly about three hours in a day. In this time, it has to catch 6– 8 rodents to feed itself, 6-8 to feed his female, plus up to 64 more rodents to feed the brood. 80 lemmings in about 200 minutes means the male must catch a rodent every two and half minutes to sustain a brood of 14 chicks. The smaller the rodents, the more flights an owl needs to make for the chicks to eat enough to survive.

On the other hand, a grouse requires significantly greater hunting time. The male would need to fly farther, spend more time in flight to an uncomfortable level and be more agile. The owl would be able to bring in as little as one grouse per brood per day. However, grouse weigh about 500– 550g, which is enough to a feed the female and two chicks.

Moreover, many arctic predators eat the same limited varieties of prey species. So the way the predators are able to survive is that they have each carved their own ecological niche, which is weight based. For instance, the Snowies take the biggest lemmings, the Rough-legged Buzzards medium-sized, and the Short-eared owls the small ones.

Unlike many other birds of prey, Snowies are caring mothers. In the Magic of the Snowy Owl, we see the youngest chick struggling to compete with its older siblings for food. As it becomes weaker, the mother broods it tenderly and attempts to save it. Even the owlets eventually rally around it and show affection. But when the chick dies and all movement stops, the mother, she lets her chicks stuff their face with the late sibling! Snowy Owls eat each other all the time, but as a last resort, when food is negligent and they are struggling to survive. They don’t actively cannibalize their kind, and will only eat already dead owls. For the most part, they are happy to let their kind decompose naturally.

The book however shares a story of how one starved female deserted her nest and was seen a week later carrying a carcass of a male Snowy around. It is not known if the male was killed by the female, although observers think he may have died of starvation. But, even in urban airports, there have been reports of Snowies eating other Snowies.

Snowy Owls, are one of the few birds gifted with the ability to consume their own body tissues. It is this extreme adaptation that allows them to survive severe snowstorms, even though the weight loss may not be reversible. But, it is also that same desperation that drives them to cannibalize their own kind.

Cannibals or not, Snowy Owls are food pirates! In 1982, in Wrangel Island, observers recorded 36 instances when owls attempted to pirate lemmings from unsuspecting foxes. They were successful most of the time, except in a few cases when the foxes either managed to swallow the lemmings or hide them in the snow. During the same period, there was no record of a single hunt for lemmings by the owls.

On the plus side, their aggressive behavior also makes them desirable among species seeking extra protection from other predators. Brent Geese in fact are known to produce larger clutches and lay larger eggs when their nests are close to a Snowy Owl’s nest. So, ask a Brent Geese what it thinks about the Snowies kleptoparasitism! It’s all good.

Want more? Read my other Snowy Owl post!
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