Greetings All,
RE: Cassin's Sparrows, there are a number of phenomenon likely involved in various data presented. To parse some out, the LONG DISTANCE vagrancy of Cassin's Sparrows may or may not have much to do with summer irruptions n and w of their normal breeding ranges. Some of the long distance vagrants have been spring, some fall. I don't have time to fully flesh this out right now, unfortunately... but spring long-distance vagrancy in Cassin's may be due to inclement conditions on their breeding grounds, or to other reasons ascribed to spring vagrancy. Fall vagrancy tends to be more common in years of good nesting success (larger pool of young birds to go astray). However, relevant to what we are seeing this year (and I seem to remember that last summer there were some foothill records, including Boulder County), Cassin's Sparrows are somewhat well known for dispersing fairly far and wide if conditions in their breeding grounds are unsuitable (usually drought). This dispersal is mostly north and west. Black-throated Sparrows behave very similarly. This behavior is fairly widespread, I'd imagine, in arid-land birds, particularly migratory ones. Brewer's Sparrows in e. Washington (which nest in sagebrush flats)... during one drought year... simply moved into open lodgepole pine woodlands to breed (or at least a number of birds did) as well as disperse in small numbers into w. WA and w. OR. All fascinating stuff. These movements are called facultative, and are actually pretty common, and show amazing versatility and adaptability to local conditions, and sometimes, adjusting to novel habitats (as in the Brewer's Sparrows). However, I am not sure a westward push of RH Woodpeckers (and I think e. Warbling Vireos and maybe Orchard Orioles) is related to the movement of Cassin's Sparrows other than they probably are all demonstrations of facultative movements. Best Wishes Steven Mlodinow Longmont CO -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to cobirds@googlegroups.com. To unsubscribe from this group, send email to cobirds+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/cobirds?hl=en.
