By Jenny Berven, RMBO Biologist
After getting walloped with some notably frigid mid-winter temperatures, many Americans looked to a groundhog, Punxsutawney Phil, to find out if spring is on its way. For many birders, the job of indicating seasons is held by a couple of hawk species.
Rough-legged Hawks usually leave their breeding habitat in arctic and subarctic Alaska and Canada at summer’s end and make their way to Colorado by November. Swainson’s Hawks form large groups, called “kettles,” in late summer and fly to South America for the winter after spending the spring and summer in Colorado and other central and western states. They travel back to U.S. the spring to nest and raise their young.
The Swainson’s Hawks’ winter absence is convenient for Rough-legged Hawks because in Colorado these two species occupy the same ecological niche, that is, habitat they use to survive. When they are at work in Colorado, both species are hunting for small mammals (like voles and mice) in grasslands, shrub-steppe and agricultural fields.
Interestingly, Swainson’s Hawks’ diets change depending on the time of year. Before taking a leave of absence from their Colorado job, they fatten up on grasshoppers. They continue their bug diet while in South America and resume their small mammal diet upon returning to the U.S. This change in the Swainson’s Hawks’ seasonal livelihood is called “niche-switching.”
One way we’ll know winter is over and spring is here is when we say goodbye to the Rough-legged Hawks and hello to the Swainson’s Hawks. (Photos: Rough-legged Hawk, top, and Swainson’s Hawk, bottom, by Bill Schmoker)