By Jim Ochterski, RBA Conservation Committee
From Doran Road to the grasslands of Nations Road, Short-eared Owls capture our focus as birders in the early winter. Bitterly cold and inhospitable sunsets are tolerated for long moments as we scan for the silhouettes soaring low. We strain to listen for the faint “barking” sounds of this owl over gusty winds. Sometimes we see them, sometimes we don’t.
Do think you are looking at male or female Short-eared Owls when you see them this winter? It is hard to say for sure, but recent research suggests that you are more likely to be observing females earlier in the winter and probably males later in the winter in this part of North America.
There is so much variation in how males and females look and behave in the world of birds. Sexual dimorphism (distinct appearance of female or male genders of a species) is obvious for Northern cardinal, Mallard, and Red-winged blackbird. Yet for Blue jays and Black-capped chickadees, you might as well forget it – males and females are quite indistinguishable in the field.
Differential migration by sex is recognized in more than 60 North American and European species – males and females wind up migrating in different groups. Short-eared Owls are among this group and we can use that tendency to more accurately speculate which gender we are observing. This has implications for conservation, since the first migrants select the most suitable habitat and territory. Our conservation efforts need to appeal to the territory-selecting gender.
Two different theories account for why some females and males migrate apart from each other. One possibility is that females are forced to migrate farther as a way of avoiding competition – a dominance factor. Another possibility is the sex who benefits most from having favorable habitat will arrive earlier at breeding grounds and so does not migrate as far – an arrival-time factor.
Researchers at the University of New Mexico examined the collection records of almost 1,200 Short-eared Owls from 35 museums and scientific collections, and ran statistical analysis on where they were recovered during a 150 year period. Male Short-eared Owls tended to be found much farther north (closer to their breeding grounds) than females. Female Short-eared owls migrated much farther away from their breeding grounds – the average female owl migrated 213 miles farther south compared to males in the winter. Interestingly other owl species (Northern Hawk, Boreal, and Long-eared Owls) exhibit the same pattern of females migrating farther than males. The timing of migration was sex-specific as well. Even though both genders left their breeding grounds at the same time in the fall, females showed a faster southward shift than males.
When the Short-eared Owls arrive in the Genesee Valley in early winter, we are probably seeing a greater proportion of females. Later in mid-to-late winter, the males would be more likely to show up in our midst. Not that you will be able to tell by looking at them. Female Short-eared Owls are about 19% larger than males, but we are not as likely to see males and females together for comparison. Either way, I hope you stay warm when you see these majestic, low-soaring predators. It’s worth the trip every time.
Source article: Differential migration by sex in North American Short-eared Owls by C. Witt and R. Dickerman, Western Birds 43:236–247, 2012