The Disjunct Range of the Burrowing Owl (Athena cunicularia)

Burrowing Owl photo by Suranjan Mukherjee for the 2010  Nature Conservancy Calender.

Range maps of the burrowing owl exhibit a curious distribution.  They live in central Florida, 1500 kilometers to the west in states concentrated along a corridor of the shortgrass prairie region, and certain areas of the southwest from Texas through most of California.  These diminutive owls are also found on the pampas of South America.  They prefer prairie and dry scrub habitat.  Because central Florida has an extensive area of dry scrub prairie type habitat, burrowing owls have been able to persist there, but at some times during the Pleistocene, this habitat must have been more widespread throughout the southeast, so that this kind of environment existed in patches connected to the bird’s current range in the west.  Evidence from a new study provides tantalyzing clues to when eastern burrowing owls became isolated from western populations.

Shaded areas represent current range. Cross hatching represents summer migrating populations.   Curved lines represent possible Pleistocene range.

Ornithologists differentiate North American burrowing owls into two subspecies: Athena cunicularia floridana (the eastern subspecies) and Athena cunicularia hypugaea (the western subspecies).  Recently, scientists analyzed burrowing owl genes and determined these two subspecies split about 350,000 years ago and have been isolated from each other ever since.  (Other genetic studies suggest the split between Florida scrub jays and the western scrub jays, two species that occupy the same habitat as the burrowing owls, also dates to 350,000 years ago.)  Although the time frame is not an exact match, the Stagell interglacial dates to about 420,000 BP to 395,000 BP, and I believe this is when the two subspecies of owls (and jays) were isolated from each other.

The Stagell interglacial is unusual because of its length.  Most interglacial periods last from 10-15,000 years, but this one lasted 25,000 years because of an extended period when earth’s orbit around the sun was circular, rather than elliptical.  An elliptical orbit is one of the triggers of glacial periods.  During an interglacial period, the climate becomes warmer and wetter, fostering the spread of the kinds of forested environments that burrowing owls avoid.  Subsequent Ice Ages, or glacial periods, expanded habitat favorable to burrowing owls, but the corridor of their preferred habitat that connected the southeast with the west was never re-established.   Incidentally, this may have influenced cheetah distribution as well.  Rare cheetah fossils in the east date from before the Stagell interglacial, but not after.

I believe burrowing owls were more widespread throughout the southeast during the Wisconsinian Ice Age (~118,000-11,000), even though forests blocked the recolonization of some areas, but fossil evidence supporting my hypothesis may not exist.  Burrowing owls choose dry upland habitats away from river bottoms where floods might have buried their bones.  And they didn’t inhabit the well forested mountains, near where caves (another source of fossils) existed.  Opportunities for fossilization of burrowing owls in most of the southeast outside Florida were minimal.  However, other western prairie-loving species are known to have inhabited the southeast during the last Ice Age, including 13-lined ground squirrels, badgers, coyotes, prairie chickens, and upland sandpipers.  If habitat existed for these species, it did for burrowing owls as well.  Moreover, in Georgia there were lots of extensive sand dunes that would have supported the scrub-like habitat burrowing owls now inhabit in Florida.

There is an interesting behavioral difference between eastern and western burrowing owls.  The former dig their own burrows, but the western subspecies has completely lost this ability and is wholly dependent upon the burrows of prairie dogs, ground squirrels, armadilloes, and other creatures.

These interesting creatures are under threat from urbanization.  Let’s hope man can prevent their extirpation.  The following is a compilation of facts about burrowing owls I thought fascinating but didn’t pertain to the above essay.

–Their burrows can be as little as 6 inches below ground or as deep as 3 feet.

–They hide the scent of their burrow with manure of other animals.

–Their young make rattling noises similar to that of the rattlesnake.  This wards off predators.

–They’re small owls–only reaching 11 inches tall, and they’re related to a few other species found around the world–all small.

–They mostly eat large insects, such as grasshoppers, but they also consume rodents, birds, lizards, snakes, crayfish, and carrion.

–Burrowing Owl habitat in Florida expanded following real estate development and the felling of forests.  They moved into vacant lots, golf courses, and airport fields.

—One study found that burrowing owls increase in suburban neighborhoods that are 45%-60% developed but decline when neighborhoods reach 70% development.

–An extinct Pleistocene species of burrowing owl was the dominant upland predator on Cuba and other Carribean islands.  It grew to an astonishing 3 feet tall.  It became extinct the same time man colonized the islands.

References:

Forbush, Edward Howe

A Natural History of American Birds of Eastern and Central North America

Houghton Mifflin 1939

Korfata, Nicole M.; David McDonald, and Travis Glenn

“Burrowing Owl (Athene cunicularia) Population Genetics: A Comparison of North American Forms and Migratory Habits”

The Auk April 2005

Millsap, Brian

“Survival of Florida Burrowing Owls Along an Urban Development Gradient”

Journal of Raptor Research 36 (1): 3-10 2002

www.allaboutbirds.org/guide/Burrowing_Owl/lifehistory

www.nbbd.com/godo/efrFloridaBurrowingOwl.pdf

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