Date: Wed, 10 Dec 2003 15:56:26 -0500 Reply-To: Maryland Birds & Birding Sender: Maryland Birds & Birding From: David Mozurkewich Subject: Re: avian migration and those pesky hummers In-Reply-To: <5.1.1.6.0.20031210122300.009fb1e0@pop3.comcast.net> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Time for a chime-in from yet another person who doesn't really know what's going on. Charlie Vaughn may well be right with the "just-a-little-off" hypothesis. Birds heading to their wintering grounds do not all head in exactly the correct direction. Some try to go the right way but don't read their compass very well. And some will be blown off course. The result is a distribution of strays that decreases the further you get from the main wintering grounds. This is indeed what is seen for some species, I don't know enough about the wintering range of Rufous Hummers to say if this is the case here. But this hypothesis is definitely not the case for some other species. One example that comes to mind is the Varied Thrush. Its breeding range is in the northwest. Most head south along the coast for the winter. A Varied Thrush is a good bird as far east as Arizona. It is extremely rare in the southeast but much more common, although still rare, in the northeast. There is one record from England. The "just-a-little-off" hypothesis cannot explain the increase in occurrence of strays as the orientation error increases from about 45 degrees to 90 degrees. The "genetic-error" hypothesis works much better in this case; some birds head East instead of South in the fall. Back to Rufous Hummingbirds. Banding records show that at least some hummers that wintered in the southeast have returned to the same yard in subsequent years. Clearly, these birds orient themselves very well. That means they seem to be reading their compass very well. They just don't orient themselves in the same orientation that the other Rufous Hummingbirds do. The homing-pigeon analogy doesn't work here. Again, I don't know enough about wintering hummingbirds but the "genetic-error" explanation seems to be at least as good an explanation as the "just-a-little-off" hypothesis. Dave David Mozurkewich Seabrook, PG MD USA mozurk @ bellAtlantic.net P.S. There are a lot of hidden assumptions in here. ======================================================================= To leave the MDOsprey list, send e-mail to listserv@home.ease.lsoft.com with the following message in line 1: signoff mdosprey ======================================================================= =========================================================================