The recent subject of northward migrants is an interesting one. It is
important for people not to confuse this with postbreeding dispersal, as
is the case with the Elegant Tern, Heerman's Gull, and Brown Pelican in
the Northwest (many other examples of course). Also long-distance
migrations must be differentiated from the short-range altitudinal
migrations which bring so many interesting species to South Texas in
harsh winters.

Many people discussed seabirds as an alternative model,
but the bottom line in the case of austral transequatorial migrants is
that there are very few. It is believed that migrations evolve to
exploit a temporally limited food supply which is regulated by periodic,
predictable climate change, such as the seasonal changes on the Arctic
tundra. Since the energetic costs of migration are so great, the
benefits must be even greater in terms of food supply which in many cases
results in higher nesting success. It's a tradeoff: longer lifespan but
less kids if you don't migrate with your species or shorter lifespan but
higher reproductive success if you do. The trend of landbirds to have
greater distance migrations overall in the northern parts of the world is
a direct result of the greater proportion of landmass to water in this
part of the world. The large amounts of water in the Southern Hemisphere
act as a huge heat sink to moderate temperatures. A breeding bird survey
in Prudhoe Bay, Alaska might net you 40 species, but on a Christmas Bird
Count you'd be lucky to find 2 (Ravens + ?).

Additionally, since the retreat of the last ice sheets in the
Northern Hemisphere (10-15K years), more habitat has been open to species
colonization in the north which was hidden under ice for thousands of
years. The periodic covering and uncovering of habitat with ice
regulates the evolution of migration as well.

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
the northern hemisphere, however, 16 species which breed here migrate
ENTIRELY out of our hemisphere (15 if you don't count Cox's Sandpiper).
The large majority of boreal migrants still go pretty far, leaving 85
long-distance boreal migrants which don't go entirely below the equator.
Thus, the dichotomous nature of northern climates clearly results in many
more long-distance migrants which breed north, winter south. This can
also be seen by examining resident species (i.e. not much migration).
With the logic given, one would assume more resident species in the
southern hemisphere (since the need to migrate is not so great). This is
supported by evidence: Of 102 species of shorebirds that are residents,
52 species live entirely south of the equator. Compare that to 20 which
live entirely north of the equator, and 30 which live on both sides.

Finally, I believe that the procellariforms show evidence of being one of
few groups of birds who have successfully mastered the resources of the
northern hemisphere since the retreat of the last ice. The large
majority of procellariforms are still austral in nature: of 104 species,
52 species never occur north of the equator except as vagrants, 26 breed
south and spend nonbreeding on both sides, while only 7 breed south and
have the majority of their species (>50%) spend nonbreeding north of the
equator. Again, the evidence shows that long-distance migrations in
procellariforms are greater in the northern hemisphere than south (as an
aside there are 19 species of procellariforms which breed north of the
equator, they generally have shorter migrations).

So whether you're a landbird or a seabird, the greater amount of climatic
change in the north will persuade you to migrate more, while the more
moderate climates south will persuade you to save your energy and stay put.

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

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

From: Alvaro Patricio Jaramillo <jaramill at SFU.CA>
Subject: FLYING NORTH FOR THE WINTER

Here is another speculation based on no solid data. I feel that it
is quite likely that the nearest relatives of the Swainson's Hawks are
several neotropical species like the REd-backed Hawk (Buteo polyosoma),
Variable (Puna) Hawk (B.poelichlorous) and the White-tailed Hawk (B.
albicaudatus). This is stricly based on my own gut feeling about the
phylogeny of these hawks, they have similar flight styles and wing
shapes, as well as very similar juvenile plumages when comparing the
Swainson's with the Red-back. If my hypothesis is correct, then it is
possible that the Swainson's Hawk is a Neartic representative of a group
that likely evolved in the Neotropics, of course the reverse is also
possible. Could it be that the Swainson's hawk is actually a bird that
traded its wintering grounds for its breeding grounds? If someone is doing
a phylogeny based on DNA it would be possible to test this idea, making
a few assumptions about where the group evolved in the first place.

BTW- Plumbeous Kites that breed in South America appear to winter into
Central America, close but no cigar.

Al Jaramillo
jaramill at sfu.ca
Vancovuer, BC

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

From: Michael Smith <whimbrel at U.WASHINGTON.EDU>

> >Also what does it take for a population which has begun breeding on its
> >wintering grounds to become sedentary? In other words, why aren't these
> >South African birds genetically compelled to fly north with the rest of
> >their kind?

The irony in this is that these species most likely originated from their
wintering grounds, not becoming migratory until the opportunity presented
itself. I.e. they historically were 'sedentary', wintering and breeding
in the same general areas. Perhaps they're just trying to get back to
their evolutionary 'roots'?

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

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

From: Alvaro Patricio Jaramillo <jaramill at SFU.CA>

Michael Smith gave a very good analysis comparing migrations in both hemispheres
. >
> 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:
Rufous-chested Dotterel (I think Johnson, who wrote the Birds of Chile, called
this the winter plover as it could only be seen in the populated areas of
Chile during the Austral winter), Tawny-throated Dotterel (they show up in the
Argentine Pampas in the winter, often accompanied by Chocolate-vented Tyrants.
Their breeding grounds are in the Andes and Patagonia), Two-banded Plover (anoth
er Patagonian breeder that filters in to northern Chile and Argentina during
the winter, there is a small pampas breeding population though); Magellanic
Snipe (movemnts are suspected, but not confirmed yet), Least Seedsnipe (a
Patagonian breeder that appears to migrate to the north during the winter).

Other than shorebirds and seabirds members of the following groups are well
known migrants in South America: Grebes, Ducks, Hawks, Nightjars, Swifts,
Hummingbirds, Tyrant Flycatchers, Furnariids (Ovenbirds), Thrushes, Vireos,
Seedeaters (Sporophila), Sierra-Finches (Phrygilus), Parrots, Blackbirds and
other groups that I missed. When you get down to Argentina and Chile, you
find that a great number of birds are actually migratory. Some of the migrations
are quite spectacular down there. I agree that most of the migrations in the
south are shorter than those of BOreal birds, but I will add that we also know
very little about the migratory status of Austral birds.

I have had the pleasure of being in a 'fall' of migrants in Buenos Aires and
it was as exciting as a good day at Point Pelee! The clouds were low and dark
as a cold front shot up from the south on a late October day. Flocks of
Cattle Egrets and Whistling-Ducks were at the Costanera Sur reserve where none
had been the day previous. The skies were full of Swallows, Martins, the
occasional flock of shorebirds and groups of Fork-tailed Flycatchers. The
Snail Kites were dropping out of the skies, as were the Brown-hooded Gulls. We
even ran into a couple of overshoots, a Bearded Tachuri and a Southern Rough-
winged Swallow. If I remember correctly the daily list was about 103. I only
mention this day because it really made me realize how little was know about
migration in South America. On that day I was birding with one of the top
birders of Buenos Aires, Christian Schenke, and he had not known that many of
the species that we saw that day in flocks or in greatly increased numbers, as
compared to previous days, were actually migrants because a small proportion
may winter in the area. Many of the species that are now know as residents
down there are actually migrants, only banding and careful observation will
bring out the real story on migration in southern South America.

Al Jaramillo
jaramill at sfu.ca
Vancouver, BC.

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

From: Ronald Orenstein <ornstn at HOOKUP.NET>

> Here is another speculation based on no solid data. I feel that it
> is quite likely that the nearest relatives of the Swainson's Hawks are
> several neotropical species like the REd-backed Hawk (Buteo polyosoma),
> Variable (Puna) Hawk (B.poelichlorous) and the White-tailed Hawk (B.
> albicaudatus). This is stricly based on my own gut feeling about the
> phylogeny of these hawks, they have similar flight styles and wing
> shapes, as well as very similar juvenile plumages when comparing the
> Swainson's with the Red-back.
> Al Jaramillo

Al, Johnsgard in "Hawks, Eagles and Falcons of North America" states that
Swainson's has no obvious close relatives, but notes that Brown and Amadon
grouped it with the other species you name on the basis of notching on the
outer three primaries. So you may have something there, although it could
just as easily have happened from north to south. I would be more convinced
if Swainson's were not the ONLY long-distance migrant in the crowd. NB
Sibley and Ahlquist include the Galapagos Hawk in this complex as well.
--
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