There are two theories about this winter's irruption of Snowy Owls, the movement of hundreds of these birds from Arctic tundra down into the U.S., some as far south as Kansas. Voles are the owls' main source of food. Vole populations rise and crash every few years.
Owls? It's all about the voles
Numbers are important to these little guys
By jim williams
Theory One suggests that vole numbers crashed this summer/fall, driving birds south as they sought other food sources. Most of the owls being seen here this winter are juveniles, birds hatched this past spring. It is suggested that being inexperienced at hunting, they are having the most problems, thus are moving south in greater numbers.
Theory Two suggests that vole numbers were very high this past spring, allowing nesting Snowy Owls to be uncommonly successful in raising their broods. That would mean many Snowy Owls looking for food territories this fall. Competition would drive some of those owls south.
Maybe both things happened this year. Voles are extraordinary mammals when it comes to breeding. Robert Seabloom, professor emeritus at the University of North Dakota, writes about voles in his newly published book "Mammals of North Dakota."
Dr. Seabloom explains that voles breed from late March to late November. A female will have a litter of from two to eight babies after a gestation period of only 21 days. A female can breed when only one month old. What is particularly unusual is that the female can become pregnant while still nursing the previous litter. In almost all mammals, lactation causes a period of infertility. This provides a birth spacing that is optimal for survival of the young. Voles, however, seem to count on numbers when it comes to survival of the species. More is better.
This is understandable in light of the average life span for voles: three to six months, rarely longer than one year. More is not only better but also necessary.
Discussing the cyclic fluctuations these mammals experience over periods of two to five years, Dr. Seabloom says that populations can erupt from less than two to nearly 200 voles per hectare (slightly more than two acres), often within the same year.
So, there could have been a bountiful number of voles last spring, helping produce many young owls, followed by a vole-population crash in the summer or fall, creating a food shortage for all of those owls.
Whatever the case, we're enjoying a rare opportunity to see these beautiful birds, thanks to something about the voles.
about the writer
jim williams
Several home watch businesses joined together in the Minnesota Home Watch Collaborative to stay vigilant across the whole state.