After overwintering as a pupa in the soil, these moths fly from March to July throughout their range of British Columbia to California, east to Idaho (and Montana!), Colorado and Arizona. They are a geometer moth (in the family Geometridae). A couple of characteristics that are typical of this family include adults resting with wings spread straight out to the sides and caterpillars “inching” rather than crawling their way along. Caterpillars of many species, including this one, transform themselves into a twig when threatened, freezing in a stiff posture angled from their perch. Larvae feed mainly on leaves of mountain mahogany (Cercocarpus spp.) and Antelope Bitterbrush (Purshia tridentata). 
Photo by Glenn Marangelo on 5/17/20 in Missoula, MT