The unusual, large irruption (irregular range expansion or population increase) of snowy owls on the East Coast this season has birders all “atwitter” and has made national headlines. While everyone was observing the snowy owls, Leonard and I noticed an increase in the number of rough legged hawks (Buteo lagopus) near our home in Northern California (Modoc County) this winter, especially immature birds. Rough legged hawks winter here and return, usually in late February or March, to the Arctic to breed. Granted, our observations were not scientific, however, we do watch the raptors closely from year to year.
There are several speculations as to why the snowy owls are showing up outside of their normal range in large numbers. Generally irruptions are thought to coincide with a lack of food. When food is scarce raptors range far to find prey. Snowy owls and rough legged hawks feed mainly on small tundra and grassland mammals with lemmings being a major food source. Lemming numbers cycle approximately every four years. Yet lemmings were abundant in the Arctic during the summer of 2013. The raptors were not hungry.
Snowy owl and rough legged hawk reproduction is linked to the abundance of prey. When prey is scarce these raptors will often skip a breeding cycle and then when food is abundant they will lay larger than usual numbers of eggs. With plenty of food more nestlings fledge and survive to become adults, as happened in 2013. These large numbers of snowy owls spread out in search of more food and territory and thus are found in unexpected locales. Rough legged hawks, usually present in Northern California in the winter, increase in numbers and the proportion of immatures.
Rough legged hawks and snowy owls often irrupt in the same years because they eat the same prey. Leonard and I are speculating, but we feel the same abundance of Arctic lemmings in 2013 may be driving the snowy owl irruption and the increase in rough legged hawks (especially immature) in our area this winter.
This rough legged hawk was photographed along County Road 91 near Lookout CA.