First, this is a fascinating discussion, and I thank all who have
contributed their thoughts.
Mr. Garrett, one thing I didn't see mentioned in your post is the
percentage of TRKIs that make this reverse migration. You say
In fact, most of the Tropical Kingbirds (and other
"reverse migrants" from the northern Neotropics)
are hatch-year birds.
What percentage of the hatch-year bird population makes the reverse
migration? If it's just a small percentage, then the 180-degree
navigation mistake seems more likely to me. If it's a significant
portion or even a majority, then the "mistake" idea seems less likely.
Do you (or anybody else) have any data on this?
Menlo Park, Calif.
Kimball Garrett wrote:
> Tom et al.,
> I am reluctant to wade into this, but fearing that the birding
> "audience" out there is going to come away from this thread with a very
> twisted understanding of bird migration, I had to throw in my 2 cents
> worth. One of the true migration biologists on Calbirds can certainly
> add a more coherent and correct version of what I am about to say, but
> perhaps their time is better spent doing biology...
> Bird migration is a highly evolved phenomenon that essentially takes
> advantage of seasonal differences in resource availability in different
> regions. Most migration patterns are completely or almost completely
> innate - birds are programmed to go where they go when they go there.
> (The very interesting phenomenon of less hard-wired "facultative"
> migration is a different matter). An individual bird does not sit there
> and think "Hey, I'll go to California instead of southern Mexico (or
> wherever) because I have a feeling things are better there." In fact,
> most of the Tropical Kingbirds (and other "reverse migrants" from the
> northern Neotropics) are hatch-year birds. They have no clue where
> California is and what is there once they get there. They're hard-wired
> to go southward (more or less) after the breeding season, and some
> individuals simply go the wrong way. [Once one successfully winters in
> California, that's a different matter - birds are also programmed to
> repeat migration routes that are successful.]
> Our Tropical Kingbirds, etc., have merely gone the "wrong" direction.
> [By the way, I very much doubt they come from "Southeast Arizona" -
> there probably aren't more than a couple of dozen nesting pairs of TKs
> in Arizona, as opposed to hundreds of thousands farther south in Sonora,
> Sinaloa, etc.]. This "reverse migration" isn't limited to Neotropical
> birds - just ask the folks in the Canadian Maritimes, n./nw. Alaska,
> etc. about North American birds that go north in fall by mistake.
> In short, Tom's suggestion that Tropical Kingbirds (etc.) reach
> California in fall from the south because they have made a strategic
> ecological "decision" to do so flies in the face of all we know about
> bird migration. Believe me, the resources (=insects) these kingbirds
> need to get through the winter are much more available in central and
> southern Mexico, etc., than they are here in California. It's possible
> that all of our plantings and the abundance of certain insects (like
> introduced honeybees) make it more likely now than a century or more ago
> that TKs will survive the winter in California, but it seems highly
> unlikely that a northward migration strategy has evolved in that time
> frame. [I acknowledge that very interesting work in Europe and
> elsewhere has shown that novel migration strategies can evolve rapidly,
> but no such work has been done on the phenomenon of northern Neotropical
> birds going north in fall.]. Even if our wintering TKs return to Mexico
> to breed and their offspring show a genetically-based tendency to go
> north like their parent, this is still a far cry from the "Dude, I think
> I'll try California" theory Tom espoused.
> Thanks to Steve Hampton for pointing out the work of Dave DeSante that
> demonstrates (at least in some cases) that misorientation is more likely
> to be 180 degree off than other vectors. So perhaps TKs do go north
> more often than they inappropriately go west (not a good strategy if
> you've hatched on the west coast of Mexico) or east. But it's still
> misorientation, not a pre-planned vacation.
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