Olive-sided Flycatcher is a highly migratory species with two distinct populations that use different breeding and wintering areas. An eastern population breeds in the Northeast, west to northern Minnesota, as well as across the boreal forest of Canada all the way to Alaska. These birds move south to winter in South America. A second population breeds in montane forests of the West, occurring in areas similar to those used by Western Tanager. Like the tanager, these birds move south to west Mexico to winter. Both populations are late spring and early fall migrants, and watch for their arrival in May and their departure beginning in August.
Students of bird distribution in the United States may look at these animations and notice that the species appears to be absent from the eastern U.S. during migration. This is not the case actually, but Olive-sideds are very rare in the East during migration. It is always essential to note the scale on these maps. In this case, the occurrence peaks at .020 (2%), and at .005 (0.5%) the color scale essentially fades to very dark gray or black. The model does accurately predict movement through the East, but it is also accurate in predicting the likelihood of finding an Olive-sided as very low. In fact, the prediction is a 0.5% likelihood of finding Olive-sideds at a random point on the landscape in the East, even during peak migration. Although skilled eastern eBirders may be able to find the species at a higher frequency, this is surely due to targeting known areas (like beaver ponds, or favorite snags), or by targeting known migrant traps. If a birder were to survey random locations, the chances of finding a migrant Olive-sided might not be better than 0.5%, and might even be worse. In the West, the species is a comparatively more common migrant, although 2% is still low compared to many other species. Whenever you look at STEM maps, be sure to pay attention to the scales, and remember that these are not directly comparable from one species to another.