Short-eared Owl—Photo: Dow Lambert

The Mysterious Irruptive Owls

by Bob Boekelheide

At least two species of owls in Clallam County are renowned for their “irruptions.” By irruption, I mean a periodic large increase of a species’ numbers, followed by periods with much lower numbers, maybe missing altogether.

Snowy Owl is perhaps the most famous of the irruptive owls. With anticipation, we formerly expected a Snowy Owl irruption around here every four to seven years, when they showed up at coastal sites such as Dungeness Spit and Ediz Hook.

The Sequim-Dungeness Christmas Bird Count data showed this periodicity quite well, recording Snowy Owls in 1977, 1980, 1984, 1991, 1996, 2000, 2005, 2011, and 2012 (Figure 1). The record CBC year was 1996, with a count of 13 Snowy Owls: 11 on Dungeness Spit, one by the Dungeness River mouth, and one way up on Burnt Hill. The back-to-back years of 2011 and 2012 were unusual, although there was only one owl seen in 2012, so hardly a trend setter. Apparently both these years were considered “irruptions.”

*Figure 1, showing the numbers of Snowy Owls and Short-eared Owls observed on the Sequim-Dungeness Christmas Bird Count since 1975. There was no SDCBC in 1990 due to an ice-storm.*

Regardless, we have not recorded a Snowy Owl on the SDCBC since 2012. Individual Snowy Owls have occasionally shown up in Clallam County since then, but unfortunately not during our CBC and only single birds. Snowy Owls have appeared at other locations in the Salish Sea, such as Boundary Bay just north of the Canadian border, where they showed a possible irruption in 2021-2022, but not around here.

Snowy Owl irruptions appear to be related to the lemming cycle in the Arctic, although there is much to be learned about the specifics of their irruptions. The accepted wisdom goes something like this: When lemmings are abundant in the Arctic, Snowy Owls increase their clutch sizes, sometimes over 10 eggs per clutch, and successfully raise lots of chicks. Lemming abundance eventually declines because predators gobble them up, not only owls, but also other predators like jaegers, gulls, and arctic foxes. The owls, particularly immatures, subsequently disperse in their pursuit to find food, traveling hundreds if not thousands of miles away from the Arctic to end up at places like coastal WA.

The story sounds good, but the Arctic is not one big monolithic habitat, and Snowy Owls are very mobile, whether irrupting or not. Regional irruptions in different areas of the Arctic do not mesh with each other, so it is uncertain which owl “populations” contribute to different irruptions, or even if there are separate owl populations at all. For instance, when we see a Snowy Owl irruption on the West Coast, do they come from nesting sites in Alaska, or the Canadian Arctic, or Siberia, or any and all of the above?

Complicating things even further, adult Snowy Owls are nomads, showing little attachment to nesting areas and likely following lemming abundance all over the Arctic. In one study, individual female owls marked with transmitters nested one year on the North Slope of Alaska, then moved the following summer to Siberia, then the next year ended up in the Canadian Arctic. It’s wrong, then, to talk about separate “populations” of breeding Snowy Owls. When choosing where to nest, adults have the ability to search over wide distances, even different continents, to find higher concentrations of lemmings before they settle down.

Not only that, not all Snowy Owls are “irruptive.” Some instead seem to behave like normal migratory birds, such as marked individuals that return each winter to the same areas of North America, particularly in the prairies and Midwest.

Another curious thing is that Snowy Owls are apparently not “starving” during irruptions, despite speculation that they disperse because they cannot find food. When they arrive in the lower 48 states, best evidence shows that they are actually in good body condition. Even though their preferred food for nesting is lemmings, Snowy Owls actually eat many different prey items during non-breeding periods, such as other mammals and birds. For instance, the Snowy Owl that hung out at Port Angeles Harbor several years ago captured birds like Buffleheads and Pelagic Cormorants.

Another complicating factor is the world-wide population of Snowy Owls is apparently much smaller than previously suspected. Formerly, estimates put the world’s population at somewhere between 200,000 to 300,000 birds, but recent genetic analyses and tracking of individual birds suggest there might be as few as 25,000 to 50,000 birds. In the whole world! That means there are fewer Snowy Owls in the entire world than there are humans in just eastern Clallam County.

Adding to the problem is climate change, because Arctic tundra is warming much faster than other biomes. Lemmings depend on snow cover for tunnels and protection during much of the year, but if the snow melts early, or if it turns to ice when thawing and refreezing, then lemming populations also suffer and owls are out of luck. Diseases such as avian influenza could also affect predators like Snowy Owls. Could all this explain why Snowy Owl irruptions might be becoming less common in Clallam County? No one knows.

Other owl species sometimes show similar irruptions, also depending on prey abundance. One that stands out is the Short-eared Owl, a bird of grasslands and marshes across North America and Eurasia, as well as Arctic tundra. We see Short-eared Owls around Dungeness during most winters, as well as frequently record them on the SDCBC (Figure 1).

*Short-eared Owl—Photo: Bob Boekelheide*

Short-eared Owls are rodent specialists, particularly consuming voles, which are chubbier than field mice with shorter tails. Voles seem to be the key to Short-eared Owl nesting success, but, like Snowy Owls, they eat lemmings in the Arctic as well as a variety of prey everywhere else.

Also similar to Snowy Owls, Short-eared Owls respond in high rodent years with larger clutch sizes and better nesting success. Their clutch sizes typically average about 5 to 6 eggs, but when prey is abundant, they might lay as many as 10 or 11 eggs per clutch and raise lots of chicks. In 2020, when we counted 11 Short-eared Owls on the SDCBC, perhaps we witnessed one of these successful nesting years. It’s possible that Short-eared Owls nest somewhere on the north Olympic Peninsula during big vole years, but typically Short-eared Owls only appear here outside the nesting season.

November and December are excellent times to be alert for both Snowy and Short-eared Owls in coastal areas of the north Olympic Peninsula. Unlike many owl species, Snowy and Short-eared Owls are active during the day, so possibly more visible during irruption years. Snowy Owls like spits and beaches, but might just as well be seen perched in towns on rooftops and utility poles. Look for Short-eared Owls flying over grassy fields and marshes, like around Dungeness and Three Crabs, particularly in the afternoon and evening twilight.

If you find a Snowy or Short-eared Owl, closely watch their behaviors and interactions. It often seems like owls have a different spirit compared with other birds, more beguiling and mysterious.

Many of the interesting facts about owls in this story came from Birds of the World, an on-line resource available through Cornell Lab of Ornithology. I highly recommend that all bird aficionados subscribe to Birds of the World, both for the information and to support the Lab of Ornithology.