Research: Migration is Coming!
The migration banding season for Northern Saw-whet Owls (Aegolius acadicus) is almost upon us! Starting tonight, the Arkansas Saw-whet Owl Project will be back in full-force for the 7th season in a row. When the first owl landed in our nets in fall of 2014, little did we know this is where we would end up. Studying this secretive creature on the fringes of its range has its drawbacks, including the low number of migrants we’ve experienced during the last 2 seasons. Last year, the drought broke with a handful of owls. This year, we have evidence from colleagues to our north that 2020 could be one of the biggest migration flights for eastern saw-whet owls since 2012 (which was a big one)!
For those who don’t know, the movement ecology (migration behavior) of saw-whet owls is pretty unusual. The species is regularly migratory, but the magnitude of their migration depends on how they did during that year’s breeding season. In some years, the owls find few mammals on the breeding grounds, resulting in few young raised successfully. Naturally, we expect a year like this to result in a migration where we don’t see many owls. That's what happened in 2018 and 2019. On the flip side, saw-whet owls do really well in some years. In a year where the owls do well, we expect a large number of birds moving during migration; particularly a large number of hatch-year birds that were born that summer (Weidensaul 2015). This happens in cycles, with peaks roughly every 4 years (2016 was our last peak).
This year, banding sites up north are already having a grand time capturing large numbers of saw-whet owls. Mostly hatch-year birds, too, which is very exciting! Below is a visual of the sites that are “on the map” so far and have captured plenty of owls.
Over the course of the project so far, almost 100 saw-whet owls have been captured and banded at our field site in Madison County, AR. This, plus a telemetry survey of some of our owls in 2016-2017, confirm saw-whet owls are in Arkansas during fall and winter, and may be quite common (stay tuned for publications). Prior to beginning this project, they were considered a vagrant species this far south, known only from a handful of winter records (Arkansas Audubon Society, James & Neal 1986).
This year, we will continue migration monitoring driven by a few exciting new questions to be on the lookout for.
1. Where else do saw-whet owls occur in northern Arkansas?
Since discovering that saw-whet owls migrate through and winter in Arkansas, this has been one of the most pressing questions. They certainly do not just occur at our single field site. Using autonomous recording units (ARUs), we hope to explore what other sites saw-whet owls may occupy in the region, with an emphasis on forests with coniferous trees (pine, cedar). ARUs are a popular tool for detecting difficult-to-find species. And, if you know much about saw-whet owls, then you know they definitely fit into this category! Saw-whet owls are thought to be silent during the nonbreeding season, but really they just call infrequently and almost never use their obvious breeding call. Instead, sounds they make may be obscure and difficult for the untrained ear to recognize. ARUs will be deployed in forested areas with pine and cedar throughout the region and set to record sound on a certain schedule. This technology will extend our reach in the region and allow us to monitor potential saw-whet owl habitat without actually being present.
You can listen, too! Below I’ve linked some of the sounds you are likely to hear from a saw-whet owl this time of year. These vocalization are not uncommon and are believed to either be alarm calls or contact calls (sounds used to communicate with other saw-whet owls). Both have been heard at our field site during migration.
"Skiew" call (an alarm vocalization):
"Chirp" calls (alarm vocalization, contact call?):
2. Why do we mostly capture females?
This is a question resonated from the mouths of saw-whet owl researchers across the continent. There are several possible explanations for why we see a higher proportion of females captured. First and foremost, most researchers use an audio lure to attract owls to their net arrays. This lure is typically programmed with the male saw-whet owl’s breeding vocalization. As you might expect, we think it draws in mostly females and deters males. A recent study in Michigan confirmed this by using a separate audio lure programmed with a known female vocalization, thus capturing a more even proportion of males and females (Neri et al. 2018). As with most of science, explanations are rarely simple. For saw-whet owls, there are also reasons related to their movement ecology that may drive how many of what sex we are capturing. For example, male saw-whet owls are thought to winter further north, which could be part of why we capture mostly females (Beckett & Proudfoot 2011).
This may seem strange, but is actually not uncommon in birds! Males of many northern breeders winter “up north”. Some commonly cited reasons include: 1) males may remain closer to prime breeding territories, which could be a rare resource, especially if you are a cavity-nesting owl; 2) males may be better parents when they remain on a year-round range and are familiar with the local prey base (Korpimäki 1987). BUT even northern banding sites capture fewer males, so that isn’t the only thing going on here. Which is why we will continue the use of a separate net array with a female-only lure.
And just to make darn sure we aren’t missing something, we will be collecting blood samples to sex our owls using molecular techniques. The current field sexing method may be missing something, too, further exacerbating our skewed sex ratio.
3. Are saw-whet owls suffering health consequences from parasitic infection?
While we’re “in there” collecting blood samples for genetic sexing, we will also be collecting blood samples to take a look at what sort of parasite infection our little saw-whet owls may have, and how that infection could impact their health. This is a really interesting collaboration with a friend and disease ecologist at the University of Arkansas. Birds are amazing creatures and respond differently to parasitic infection. The ectoparasites (parasites outside the body) that saw-whet owls—and most birds—carry are commonly called flat flies (family Hippoboscidae). These creepy creatures look like a house fly that survived after being smashed under a book. Despite being LOADED with flat flies, little is known about how saw-whet owls may be affected internally. We hope to find out!
We can’t wait to see how all these moving parts come together this year, so let’s hope the owls cooperate! Stay tuned for more updates on the fly.
Arkansas Audubon Society. [online]. 2004. Bird Records Database. Arkansas Audubon Society. <http://www.arbirds.org/results.asp> (Accessed 15 September 2018).
Beckett, S. R., and G. A. Proudfoot. 2011. Large-scale movement and migration of Northern Saw-whet Owls in eastern North America. Wilson Journal of Ornithology 123:521-535.
James, D. A., and J. C. Neal. 1986. Arkansas birds: their distribution and abundance. University of Arkansas Press, Fayetteville, AR.
Korpimäki, E. 1987. Sexual size dimorphism and life-history traits of Tengmalm’s Owl: a review. In Biology and Conservation of Northern Forest Owls, Symposium Proceedings, Winnipeg, Manitoba. USDA Forest Service General Technical Report RM-142:157-161.
Neri, C. M, N. Mackentley, Z. A. Dykema, E. M. Bertucci, and A. R. Lindsay. 2018. Different audio-lures lead to different sex-biases in capture of Northern Saw-whet Owls (Aegolius acadicus). Journal of Raptor Research 52:245-249.
Weidensaul, C. S. 2015. Peterson Reference Guide to owls of North America and the Caribbean. Houghton Mifflin Harcourt, NY.