Sort the oddity from the flock. Uncommon or out-of range birds will often join a flock of common, but closely related species. Large, mixed-species flocks of waterfowl, gulls or shorebirds are good places to look . Here in Vermont, winter aggregations of birds by the edge of ice on Lake Champlain often contain a rarity or two. Several years ago, I was sorting through a raft of ducks that included quite a few Common Goldeneye (expected on the lake in the winter), but, upon closer inspection, one of the “Common Goldeneye” proved to be a severely out-of-range Barrow’s Goldeneye. Looking at eBird data for Vermont, that’s how Barrow’s is reported – one or a few at a time, in winter, probably mixed in with a flock of other ducks. In a similar situation, an aggregation of a few hundred (mostly ring-billed) gulls on the lake proved to contain single individuals of both Glaucous and Iceland Gulls – both rare in Vermont.
Another trick is to watch eBird, birding lists and other sources for reports of a species you’d like to see – even if it’s nowhere near you. Some species appear out of their accustomed range in irruptions, significant numbers of birds that leave their accustomed range in the same year, probably for reasons of weather or prey density. Both Snowy and Great Gray Owls are known for this behavior. Snowy Owls are rare most places south of the US-Canadian border, except in the far upper Midwest. However, in an irruptive year, numerous birds can be seen as far south as Pennsylvania, and isolated specimens are found even farther south. If you hear of Snowy Owls south of their usual haunts in some places, it’s a fair chance that an irruption is going on, and one may show up closer to you.
