Assuming a similar ancestry between these hawks, it makes the most sense
that they all evolved as fairly short-hop migrants or nonmigrants, who
then speciated into the different species and migration patterns as
global climates changed. That is they all had a more-or-less sedentary
common ancestor, whose progeny developed into hawks with different
migratory patterns, some becoming boreal migrants, and some becoming
austral migrants.

Mike Smith
University of Washington
Seattle, WA
whimbrel at u.washington.edu

===========================

From: Michael Smith <whimbrel at U.WASHINGTON.EDU>
On Tue, 12 Jul 1994, Alvaro Patricio Jaramillo wrote:

> Michael Smith gave a very good analysis comparing migrations in both hemispher
es
> > I'll talk about shorebirds since that's what I know best: Of 106
> > species that have fairly long migrations, only 5 have an appreciable
> > austral migration pattern: Double-banded Plover, Wrybill, Madagascar
> > Pratincole, Australian Pratincole, Magellanic Plover. The majority of
> > the populations of these species winters significantly farther north than
> > their breeding range. However, NONE of them make it very far north (in
> > fact relative to northern species, their migrations seem very short). In
>
> His arguments still hold, but there are many more migratory shorebirds in
> the southern hemisphere. I only know the Neotropical ones so there may be
> more in Africa. In any case the following should be added to his list:

My Bad, I should have explained what I said better. When I said
appreciable, I meant that the entire population gets up and moves
somewhere farther north to winter, I based it on a 100% rule, that is
their breeding/nonbreeding ranges never overlap and further could be a
few hundred or more miles away. I was trying to come up with austral
migrants analagous to White-rumped, Baird's, Upland, Buff-breasted,
Sharp-tailed, etc. Basically, you can't. You can find austral migrants
analagous to our Westerns, Leasts, Yellowlegs, etc., which are outlined
in Alvaro's discussion.

Mike Smith
University of Washington
Seattle, WA
whimbrel at u.washington.edu

=============================

From: Alvaro Patricio Jaramillo <jaramill at SFU.CA>
>
> Assuming a similar ancestry between these hawks, it makes the most sense
> that they all evolved as fairly short-hop migrants or nonmigrants, who
> then speciated into the different species and migration patterns as
> global climates changed. That is they all had a more-or-less sedentary
> common ancestor, whose progeny developed into hawks with different
> migratory patterns, some becoming boreal migrants, and some becoming
> austral migrants.
>
> Mike Smith
> University of Washington
> Seattle, WA
> whimbrel at u.washington.edu
>
As I read Mike's response to my original ideas on Swainson's Hawk migration
it occured to me that these species that migrate from North American grasslands
to South American grasslands are problematic to understand. Shorebirds and
several other species that migrate to areas with a similar habitat (i.e.
mudflats) find stop over sites inbetween these that are also of that same
habitat. The same applies to forest birds, seabirds etc. Grassland birds
did not originally find any grasslands between their breeding and non-breeding
grounds, so then what came first, the evolution of their habitat choice or
the evolution of migration? How did the first migratory Swainson's hawk possibly
know that it could head down to Argentina and find more grassland? I am
assuming that Central America and the Andes have always been forested, even
during glaciation, perhaps there was a corridor of grassland, does someone
know?

Mike's reasoning that these long distance migrations evolved from sedentary
species that made greater and greater migrations as habitats changed makes
complete sense to me, but not for grassland birds. Bobolinks and Dickcissels,
also fit the pattern of the Swainson's Hawk. I have no good ideas on how to
explain the evolution of migration from grassland to grassland, without
proposing that the species evolved the migration before the choice of grassland
as a preferred habitat. Any ideas?

BTW- Available data suggests that Swainson's Hawks do not feed at all during
migration, but survive off stored fats. It seems that they try to get from
point A to point B as fast as possible, minimizing the amount of time spent
in the forested areas between the Prairies and the Pampas.

Al Jaramillo
jaramill at sfu.ca
Vancouver, BC

========================

From: Ronald Orenstein <ornstn at HOOKUP.NET>

Grassland birds
>did not originally find any grasslands between their breeding and non-breeding
>grounds, so then what came first, the evolution of their habitat choice or
>the evolution of migration? How did the first migratory Swainson's hawk
possibly
>know that it could head down to Argentina and find more grassland? I am
>assuming that Central America and the Andes have always been forested, even
>during glaciation, perhaps there was a corridor of grassland, does someone
>know?

I believe that it is generally accepted, following the work of Jurgen
Haffer and others, that during the ice ages the forests of Amazonia at least
broke up into refugia with arid areas in between. These could have provided
the necessary corridors.
--
Ronald I. Orenstein Phone: (905) 820-7886 (home)
International Wildlife Coalition Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2 Internet: ornstn at hookup.net
Office: 130 Adelaide Street W., Suite 1940 Compuserve ID: 72037,2513
Toronto, Ontario Canada M5H 3P5

==========================

From: "Anthony L. Lang" <tony at ZOO.TORONTO.EDU>

On Wed, 13 Jul 1994, Alvaro Patricio Jaramillo wrote:

> it occured to me that these species that migrate from North American
+ grasslands
> to South American grasslands are problematic to understand. Shorebirds and
> several other species that migrate to areas with a similar habitat (i.e.
> mudflats) find stop over sites inbetween these that are also of that same
> habitat. The same applies to forest birds, seabirds etc. Grassland birds
> did not originally find any grasslands between their breeding and non-breeding
> grounds, so then what came first, the evolution of their habitat choice or
> the evolution of migration? How did the first migratory Swainson's hawk
+ possibly
> know that it could head down to Argentina and find more grassland? I am
> assuming that Central America and the Andes have always been forested, even
> during glaciation, perhaps there was a corridor of grassland, does someone
> know?

I think that biogeographical models of what happened during Pleistocene
glaciation suggest that a lot of northern South America was grassland,
and that rainforest retreated into several refugia. This vicariance
model is sometimes invoked to partly explain the high diversity of South
American birds.

====================================================================
Anthony Lang, Dept. of Zoology | "Always keep your
Univ. Toronto, Ontario, Canada, M5S 1A1 | stick on the ice."
tony at zoo.toronto.edu | RED GREEN

=======================

From: Dave Rintoul <drintoul at KSU.KSU.EDU>

In response to another question, Tony Lang wrote:

> I think that biogeographical models of what happened during Pleistocene
> glaciation suggest that a lot of northern South America was grassland,
> and that rainforest retreated into several refugia. This vicariance
> model is sometimes invoked to partly explain the high diversity of South
> American birds.

I'll take this opportunity to ask my question. What is the meaning, and
or root, of the word "vicariance?" No biogeographers here, but I have seen
and heard the word enough times to be very curious about it; does it have
a simple meaning?

Thanks for any insights

Dave
--
Dave Rintoul Internet: drintoul at ksu.ksu.edu
Biology Division - KSU Latitude 39.18, Longitude -96.34
Manhattan KS 66506-4901 Compuserve: 71634,32
(913)-532-5832 or 6663 FAX: (913)-532-6653