Date: 11/18/17 8:24 am From: Joe Roller <jroller9...> Subject: [cobirds] Amateur's response to your questions about stray birds.
William, I am an amateur but experienced birder, and I can mention a few factors that seem to correlate with spatial patterns of stray birds. Finding out of range birds is one of the many joys of birding.
Most of these are observations, not careful scientific studies. 1) Young migratory birds (hatched this year) seem to get lost more than adults, presumably due to lack of experience with their best migration paths. For some species, young birds migrate later than adults, so the more capable adults might have passed through, with hatch year birds seen later by a month or so. That can help explain temporal, but not spatial patterns.
2) Sex does not seem to be a factor - both go astray. Of course for many species, the sexes look the same, so one cannot tell. 3) The most unusual season to find strays is during the breeding season. By then most birds are where they are supposed to be. So look for strays (vagrants) during migration.
4) Species: some species are more likely to have vagrants than others. Sedentary species rarely stray.
5) Habitat: most strays are found where the habitat is "right," and they stay longer if there is something to eat, shelter, etc. However, there's a tendency for vagrants to show up in strange habitats, e.g., a migrant warbler on the Pawmee Grasslands, away from trees. There are well-known migrant "traps," usually a bit of water and vegetation in the middle of a vast prairie or agricultural zone, Last Chance, Van's Grove, Two Buttes.
6) Some birds seem to have predictable patterns of vagrancy - herons, flycatchers often have post-breeding dispersal northward in later summer, fall. Birders know that if they look in the right season, at the right places, they will finally find strays. In the spring, some vagrants seem to be "overshoots," species migrate northward toward their breeding grounds in AZ, TX and overshoot to CO, e.g., 7) Vagrancy can correlate with lack of food in breeding range - e.g., when Snowy Owl's raise a lot of young, the youngsters can go way south, as there is not enough food for these inexperienced raptors in the Arctic. Abundant food and habitat can attract strays - "Incursions" of eruptive species, e.g., Common Redpoll, Red Crossbills this year, Snow Buntings in the thousands at rare intervals.
8) Definite connection with human-caused factors; mostly negative, but humans built reservoirs, lakes, that migrant waterfowl, loons, etc, never had available on the Great Plains in the past.
Bird feeders and ornamental plantings attract many species in winter. Also how can grassland species stray when humans have diminished their habitat and numbers by 90%? Passenger Pigeons dont stray no more! Nor do Eskimow Curlews.
Just a few thoughts. Hope others can expand on this simplistic overview.