SNOWday Science: Are Snowy Owls Diurnal or Nocturnal?

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Use the SNOWstorm Tracker to explore the travels of Snowy Owl “Assateague” for 24 hours along Reed’s Beach, Cape May, New Jersey and draw your own conclusion about whether “Assateague” is diurnal or nocturnal.
Red arrows indicate daytime or diurnal movements.
Blue arrows indicate nighttime or nocturnal movements.

Backgrounder

First some definitions. Nocturnal animals are active at night. Diurnal animals are active during the day.

The existing literature is pretty definitive that Snowy Owls are not strictly nocturnal or diurnal. A few examples follow:

PBS’s Nature: Snowy Owl Infographic (2012)
“Unlike most owls, which are nocturnal, snowy owls are diurnal—they hunt and are active both day and night.”
National Geographic: Snowy Owl (2014)
“Unlike most owls, which are nocturnal, snowy owls are diurnal—they hunt and are active both day and night.”
Cornell Lab of Ornithology’s All About Birds: Snowy Owl, Life History
“Unlike most owls, Snowy Owls are diurnal, extremely so. They’ll hunt at all hours during the continuous daylight of an Arctic summer.”
The Birds of North America from Cornell Lab of Ornithology and American Ornithological Union: Snowy Owl (1992)
Differing from most owls in being largely diurnal, the Snowy Owl hunts in all weather during winter and the continuous light of arctic summer

Now, scientists know that Snowy Owls are both diurnal and nocturnal, depending on the circumstances. But among most birders, the prevailing wisdom about Snowy Owls is that they are diurnal. Fortunately, the kind of GPS technology we’re using at Project SNOWstorm is shedding interesting new light on this question.

Snowy Owls breed above the Arctic Circle, the land of the midnight sun, so they have to be able to hunt when in daylight during the summer. So obviously they are diurnal during the summer.

Observations of Snowy Owls in the northern U.S. during irruptive winters like the current one show that the owls are pretty inactive during the day. So much so that many birders and (and some ornithologists) have previously thought that many of these irruptive owls must be starving since they are seen eating so little.

So one question to ask is what happens if Snowy Owls have a choice of hunting when it is light or dark? That would really be the way to determine if they were diurnal or nocturnal. These preferences may be based on the individual and its circumstances. We’ll need a large sample size of tracked Snowy Owls in a variety of situations before we can make any wide ranging assessments of whether they are diurnal or nocturnal. The test will be if we predict individual owl behavior based on the time of year, the habitat, owl age, owl sex, and other relevant factors.

Enter Project SNOWstorm and its solar-powered GPS-GSM transmitters. These transmitters record the owl’s latitude, longitude, altitude, and speed every 30 minutes — that’s 48 locations times seven days a week.

With this level of detail, it is finally possible to start pulling the curtain away from the hidden nocturnal lives of Snowy Owls. How cool is that?

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  • I would love to see this analysis done for all the other Snowy Owls carrying transmitters–is the pattern just as clear; does it hold from one 24-hr. period to another? So many questions! Fortunately (and finally!), a way to get some answers now! My thanks and compliments (on a job well done) to all the Project SNOWstorm cooperators!!

  • Ryan Brady

    One hypothesis may be that on the wintering grounds individuals in poor body condition are more likely to be diurnally-active, while those that are not food-stressed are active at the preferred crepuscular or nocturnal periods. During most irruptions here in WI my experience has been that far fewer Snowies are active diurnally. Of course, that doesn’t mean they can’t be seen in “non-active” mode during the day.

    • Scott Weidensaul

      I think that’s been the general assumption, but all the owls we’ve tagged this winter (and virtually all that banders in multiple states have been catching) have been very healthy and very fat. So that wouldn’t account for differences in our tagged owls. One thing that’s not immediately evident from the tracking data is weather — snowies are definitely more active (and active earlier in the afternoon) on gloomy, cloudy days than on sunny ones.

      • Following the 5 or so SNOWs east of Janesville the last month and a half, there’s been a noticeable pattern with the immature females being more active diurnally than the (assumed) mature birds. There are two obviously immature females, a mature male, and two ‘unknowns’ that seemingly could fit as an immature male or mature female. Other than one occasion when it was very cloudy and gloomy, the mature male hasn’t been seen perched on a power pole or other high object except during twilight hours. As noted, the immature females have been seen hunting during all times of the day – and they are very territorial. On at least 3 occasions, I have seen the young females push other SNOWs off of their hunting perches. Of of the unfortunate victims got blindsided and pushed off a grain elevator and tumbled several feet to a platform before composing him/herself and flying off. I’m unsure if the ‘victim’ was a mature female or immature male…here’s a link if you want to make a guess http://scottweberpal.zenfolio.com/img/s5/v126/p341496544.jpg I want to say mature female based on the tail banding, but it seems a bit smaller than the immature bird that seems to have dominance over it. In one of the other cases, a different immature female pushed the mature male off his perch in a tree near dusk.

        The bird in the link above has also been seen chasing Eurasian Collared-Doves several times but has yet to be successful in catching one. Based on my completely non-scientific observations, the older birds tend to be more active during the crepuscular hours while the younger birds tend to be more opportunistic and going after food when they can.

  • Cathy Cogswell

    I watched a Snowie Owl for 3 days in a row in Massachusetts. Each day I arrived at ~6:30 a.m. stayed for a few hours and then returned later in the afternoon/evening. In the early morning the owl’s eyes were wide open, its’s head and neck in the upright position and it was perched on something higher like a sign, a small tree etc. Interestingly, almost like a switch flip, as the rising sun reached a certain position on the horizon it flew down to a hump on the sand and the eyes went to a half- way position or were closed. If it heard a “humanesque” noise, the eyes would open, but only half way. As the sun started setting, I observed the same behavior – eyes fully open, head and neck upright and a higher perch. I called this “hunting” mode. The hunting mode seemed to begin at some point during the “sun setting period” and end at some point during the “sun rising period”. Circadian related i.e.- temperature and position of the sun in artic winter vs. summer? All very fascinating to me.

  • Aurora

    are snow owl’s nocternal???