>All,
> Conceivably a bird species that migrates far enough across the
>equator could be leaving in our late summer or early fall and arriving in
>the southern hemisphere in their early spring just in time to raise a
>clockwise brood down south. Then, just when the empty nest funk begins in
>their late summer or early fall, off they go again to the north for a
>counter-clockwise affair. Do any migratory species actually raise two
>broods a year, one there and one here?
>
> Bruce Harrison
> McMinnville, Oregon

Bruce,

There are many factors that affect whether a bird breeds in an area,
timing (early spring) is just one of them. There are energetic
considerations, moult timing, historical constraints, and competetive
interactions that a bird has to deal with as well. Many birds have a
drastically different ecological niche in the winter when compared to the
summer. For example, the strictly insectivorous and very pugnacious Eastern
Kingbirds becomes a social, flocking berry eater when in South America! The
striclty insectivorous Swainson's Hawk becomes a rodent hunter when breeding
here in North America. The Franklin's Gull, a bird specialized to breed in
small pools and marshes of the Prairies and Great Plains becomes a bird of
sandy Ocean beaches when in Chile and Peru. I could go on with similar
examples, in fact its probably truthful to say that few birds fit into the
same or similar niche here as they do in their wintering grounds. The
reasons for this are various. Part of this may be due to competition with
birds that are year round residents in the Neotropical areas where these
migrants winter. Basically, the available niches that would allow a bird to
successfully breed are taken by the residents, making it impossible for a
migrant to be able to breed. Perhaps migrants could breed, but it wouldn't
be worth it. Consider a situation where a Wilson's Warbler goes down to
western Mexico and nests and brings up a brood. The overall production down
there is likely going to be lower than in a temperate area in the summer
(this is a general rule) so it may raise half the number of offspring than
in the summer in Tweeterland. Now, the energetic expense of holding a
territory, making a nest, producing the eggs, feeding the young etc. etc.
may take so much out of the bird that it may decrease its chance of being
able to sucessfully migrate back north, this is a realistic proposition. The
overall loss of production of young in a lifetime by decreasing the
probabity of making it back up north (where more young can be raised), or
increasing the likelihood or mortality, is likely high enough for it not to
make sense to do a second nesting down south. Of course individual birds
don't make these decisions, but evolution does. The new population of Barn
Swallows in Argentina would be a great place to study this type of event,
but nearly nothing is known about these birds. It is not impossible that
these are actually not Barn Swallows, but an African species of swallow that
has colonized Argentina. As far as I know there are no photos, or specimens
of birds from this population, so their identification is up in the air as
far as I am concerned. In any case, productivity, mortality, and all these
ecological parameters could be calculated from a study on this Argentine
population of swallows and used to address the question of why Barn Swallows
don't usually breed in Argentina, or why it has made sense for this
population to try it out. Apparently Common Sandpiper (a European breeder)
has recently been found breeding in southern Africa, is that right Dennis?

Good birding,

Alvaro Jaramillo "You are better off not knowing
Half Moon Bay, CA how sausages and laws are made"
alvaro at quake.net Otto von Bismarck,
but I saw it in a fortune cookie!

http://www.quake.net/~alvaro/index.html