All: At David's request, I am providing my working definition of the term "fallout" as it relates to birds and their occurrence at particular localities (not the definition used at Three Mile Island or Chernobyl).
"An occurrence on the ground of birds that were in active migration but that were precipitated due to weather and/or geologic conditions adverse to their continuing migration." Further clarification: "on the ground" -- not migrating; in or around vegetative cover or literally on the ground "active migration" -- birds actively migrating; in the process of migrating from point A to point B, with A and B having no particular definition other than being points on or between, usually, breeding location and molting and/or winter location Weather is most often the cause of fallouts, with birds traveling on a broad front running into adverse weather (strong opposing wind and/or precipitation) selecting to interrupt migration due to survivability concerns. However, large bodies of water can precipitate birds in a fallout should birds find themselves unexpectantly over water after dawn. This happens fairly frequently along the Great Lakes and oceanic coasts. This is a regular aspect of the trans-Gulf migration system, as the birds crossing the Gulf cannot physically make the crossing in one night's flight. However, when approaching land after making this huge water crossing (going in either direction, S or N), only the birds in poor condition will put down at the first land when traveling under benign weather conditions, with those in good condition continuing well inland in most cases. The cause of spectacular fallouts on the Gulf Coast (Mexico or the U.S.) is due to a combination of good lift-off conditions at the sources (southern U.S. in fall, Yucatan in spring) and adverse conditions on the other side. In spring, these adverse conditions are usually provided by an approaching cold front, with its colder temps and, particularly, northerly winds. Under these conditions, even those birds in good conditions (read, "suitable fat reserves") will precipitate onto the first available land. However, fallouts can be very local and be caused by individual thunderstorms. This type of fallout predominates in my experience of the phenomenon in CO, particularly the huge fallout at Chico Basin Ranch on 20 May 2000. Another aspect of fallouts is that the number of birds falling out can be quite small, and these sorts are often/usually difficult to detect. They are most detectable when waterbirds migrating diurnally get precipitated by a storm onto the nearest water body and which often depart as soon as the weather clears, with the best CO example that I recall being a particular fallout of a flock of scoters at Chatfield Res. that was present for only 30 minutes or so. However, many birds may stay for some time, at least until the next night, and the huge fallout of Greater Scaup (and one Tufted Duck) at Barr Lake in spring a few years back is the best example I know of that. Weather and bird migration is a fascinating subject!! Tony Leukering Villas, NJ -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/cobirds?hl=en.
