Greetings All,
Questions re: migration and biogeography are near and dear to my heart. The term "dispersal," as we use it, is not the same as is used in (at least some) of the ornithological literature. For those interested in migration, I think that Berthold's "Bird Migration: A general survey" published in 2001 is still the best source. It may be a bit dated by now, but I can think of no other tome that brings together so much information so well, with assured accuracy (to the knowledge at the time). In any case, Berthold refers to what we call dispersal is a subset of Intermittent Migration. There are so many strategies, that they can't really be categorized. There are birds that stay very close to where they were born, period. There are others in which young females (eg, Red-cockaded Woodpecker) move in a random direction (for that species, usually a fairly short distance), presumably to minimize in-breeding. My recollection is that female Spotted Owls do the same, only over a greater distance. Some species, mostly from arid lands, move to moister areas after breeding, and in many cases molt there. Some movements may be relatively local (as is probably the case with our Spizella sparrows), some are in the OPPOSITE direction of their final fall movement and lengthy (Canada Geese often go far NORTH after breeding, molt, then go south for winter), and others move to monsoon areas of NW Mexico and SW USA (eg, BH Grosbeak, Bullock's Oriole). These birds are generally moving to areas with better food supply, and apparently, the direction of these movements have been honed by evolution. Other birds, such as herons, disperse in all directions, also presumably to reduce competition for food resources, then move south in winter. Of course, I've always wondered, "How far does a BH Grosbeak go from its nest before it takes off for Sonora?" The answer is likely highly variable within a species and between species, and may depend on how good food resources are locally in a given year. Thus, it is nearly impossible to know what is truly going on when a Western Tanager shows up in Longmont in July (as a for-instance). It could be a bird already headed south for the monsoons, or it could be a failed breeder wandering around looking for food or mate. Pondering these things, and seeking patterns, is one of the things that makes every day birding, particularly in an area I visit regularly, fascinating to me. The avocets that I saw at Lower Latham today were not there last week (or at least, not as many) ... were they simply move a km, or did they come from the north... or had they dispersed northwards from points south. Impossible to know, but in the lack of that knowledge, there is the joy of wonder. Okay, its late, and I am waxing and a wandering (my thoughts dispersing?) at this hour. In the end, birds never stop moving, and my visits to Crow Valley and Prewitt Reservoir this summer amazed me with some of the peculiar things that have appeared at such "odd" dates. And of course, by the time the last northbound WR Sandpipers are leaving Colorado, Wilson's Phalarope females are already dispersing from their breeding locations (and methinks, generally southbound). 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.
