Date: Wed, 10 Dec 2003 09:10:40 -0500 Reply-To: Maryland Birds & Birding Sender: Maryland Birds & Birding From: Maurice Barnhill Subject: Re: Human intervention MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit With the understanding that I am an interested amateur and not a professional, here is my understanding of the genetics of dispersal. Birds have the ability to determine direction from various clues, and these abilities are of course under genetic control. Migrating birds can find their way from breeding grounds to wintering grounds and back, and this ability is also apparently under genetic control. The evidence for the latter is that in many species young birds are independent before their first migration and therefore cannot be taught where the wintering grounds are. A stray bird either has some defect in one of these systems or has been displaced by weather. With hurricanes as the major exception, it is unlikely that weather can cause a bird to be displaced by many hundreds of miles. Clearly birds routinely get displaced overnight by tens of miles, but nonetheless they usually wind up in their normal wintering locations. Something must go wrong internally for them to wind up thousands of miles away from their normal targets. Weather clearly affects the timing of such arrivals just as it affects the timing of arrival at normal locations, but most birds simply don't migrate if they can't go in the direction they "want" to go. So most of the time something is wrong with the orienting system of a bird that winters far from its normal wintering ground. This something must be genetic, although it doesn't have to be a simple as a mutation in a single gene. Nothing in biology is simple, and there is no guarantee that the offspring or even a fraction of the offspring of a bird which strays in winter will inevitably also stray. However, it is very likely that some of them will at least have a tendancy to stray, especially if the defect is a misperception of the proper migratory direction. If this straying of offspring leads them to go to a new wintering area which is better than the conventional one, a higher percentage of the straying birds will return to breed than of the nonstraying ones, and the tendancy to stray will spread. Range extensions presumably happen in just this way. If the strays do not survive, obviously there is no change in the species normal winter range. This process is Natural Selection applied to the determination of an acceptable winter range. Now if we could find all the strays, protect them over the winter, and release them or, worse, carry them back to the breeding grounds, we could artificially establish a more favorable wintering ground. Done efficiently and persistantly, this intervention could gradually change the wintering behavior of a species. Fortunately, we cannot and will not find enough of the strays to affect the entire species. My original comment was that if we feel -- mistakenly in my view -- that we ought to intervene to prevent an individual stray from dieing as a result of its unfortunate choice of wintering ground, we are very likely to permit it to have offspring that also stray into impossible winter climates and die as a result. We will then in the long run have harmed more individuals than we have helped. We will not have affected the species as a whole, because we simply don't find enough individuals to affect the species, as Bob Mumford pointed out in his response. We can and probably are affecting the winter ranges of some species by the presence of feeders, because there are a lot of feeders over a wide area and a significant number of individual birds can find feeders. These birds may survive the worst of the winter weather due to their better nutrition as a result of access to feeders, thus allowing the winter range of the species to expand. To get back to the original point, I think it is likely that stray western hummers in the east are genetically different from most individuals of the species. The difference may be more complicated than a single changed gene. If the birds do not survive the winter, obviously the change will not spread through the population. If lots of such birds were to survive to breed, spread of the change would be likely, and establishment of a new, alternative wintering ground would be the result. Gerald & Laura Tarbell wrote: >Could Maurice and/or Bob or anyone with a knowledgeable opinion ring in >on the probability that our two SI hummers could really carry a gene >for wandering or aberrant migratory behavior? To some of the rest of us >this sounds like a real stretch, but I'm not going to say it's not >possible. Even if they do, what is the possibility that the gene could >spread through the population with any speed or accuracy? > Jerry the curious > > > -- Maurice Barnhill mvb@udel.edu [Use ReplyTo, not From] [bellatlantic.net is reserved for spam only] Department of Physics and Astronomy University of Delaware Newark, DE 19716 ======================================================================= To leave the MDOsprey list, send e-mail to listserv@home.ease.lsoft.com with the following message in line 1: signoff mdosprey ======================================================================= =========================================================================