>Date: Sat, 18 Nov 2000 09:45:47 -0500 >Reply-To: Jean Iron >Sender: "National Birding Hotline Cooperative (Chat Line)" > >From: Jean Iron >Subject: [BIRDCHAT] Northern Owls Moving South >Comments: To: ontbirds@hwcn.org >To: BIRDCHAT@LISTSERV.ARIZONA.EDU > >Dear Ontbirders and Birdchatters, > >Bruce Murphy of Cobalt on Lake Timiskaming (6 hours north of Toronto, >Ontario) asked me why so many Northern Hawk Owls and other owls are moving >south this fall. > >In general, owl movements in fall and winter are caused by lack of food >such as Meadow Voles. Recently, some owls have found less to eat as Least >and Eastern Chipmunks and Jumping Mice have gone into hibernation. Owl >populations increase when vole and other small mammal numbers are high. >When prey numbers crash (high and low cycles), the owls move or starve to >death or both happen. The above comments apply to the northern forest owls >such as Great Gray, Boreal and Northern Hawk Owls. Not all these species >eat exactly the same prey, for example, Boreal Owls eat voles, but may be >able to survive better in poor vole years because they also prey on >Northern Flying Squirrels. Interestingly, Boreal Owls are sometimes seen >around bird feeders in central Ontario. These Boreals catch Short-tailed >Shrews that are eating the spilled seeds buried in the snow under the >feeder. Southward movements of northern Great Horned Owl are caused by >crashes in Snowshoe Hare populations. Snowy Owls move south when lemming >populations crash on the tundra. The Northern Saw-whet Owl is a regular >migrant, but in years of high Deer Mouse and Red-backed Vole numbers, many >Saw-whet Owls apparently stay on territory all winter in Algonquin Park >where the snow cover is deep. > >Cold and Deep Snow: It is a myth that northern owls are forced south >because of cold weather and deep snow cover, but you'll hear this repeated >all the time. Some owls such as the Eastern Screech Owl and Long-eared Owl >are greatly affected by extremely cold weather and deep snow cover, but >these conditions generally do not affect the true northern forest owls. The >life cycles of northern forest owls are adapted to cold weather and deep >snow. Most years we see the boreal forest deep in snow and if prey >populations are high, the owls stay on territory. > >Snow Crusts: Sometimes crusts form on top of deep snow following a thaw or >rain in mid-winter. Do crusts prevent northern forest owls from catching >prey? Crusts may actually help owls find prey! A metre or yard below the >snow surface at ground level, the temperature is above freezing; bacterial >decomposition of dead leaves and plants goes on all winter. This decay >releases carbon dioxide gas which is trapped under the crust. The buildup >of carbon dioxide gas agitates the voles and causes other behavioural >changes, forcing voles to the surface and out through openings into the >talons of waiting owls. > >Northern forest owls, cold temperatures, deep snow, crusts and voles are >part of the boreal forest ecosystem in winter. Great Gray, Boreal and >Northern Hawk Owls are adapted to cold and snow. Extreme cold, deep snow >and crusts do not force northern owls south, but lack of food does. > >Ron Pittaway >Box 619 >Minden, Ontario K0M 2K0 > >Jean Iron >President, Ontario Field Ornithologists >9 Lichen Place >Toronto, Ontario M3A 1X3 >Canada >Phone: 416-445-9297 >e-mail: jeaniron@sympatico.ca >Web Page: http://www.interlog.com/~ofo ================================== Phil Davis Davidsonville, Maryland USA mailto:PDavis@ix.netcom.com ================================== ======================================================================= To leave the MDOsprey list, send e-mail to listserv@home.ease.lsoft.com with the following message in line 1: signoff mdosprey ======================================================================= =========================================================================