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VTBIRD  September 2002

VTBIRD September 2002

Subject:

FW: The Incredible Journey

From:

Wayne Scott <[log in to unmask]>

Reply-To:

Vermont Birds <[log in to unmask]>

Date:

Fri, 20 Sep 2002 12:15:27 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (212 lines)

I thought you all would be interested in this fascinating post by Pam Hunt
of NH Audubon. - Wayne Scott


------ Forwarded Message
From: Pamela Hunt <[log in to unmask]>
Reply-To: [log in to unmask]
Date: Wed, 18 Sep 2002 22:08:45 -0400 (EDT)
To: New Hampshire Birds <[log in to unmask]>
Subject: The Incredible Journey

OK bird fans, time to try to think like a Blackpoll Warbler.

Part A: The Starting Point

Blackpolls breed all the way from western Alaska to Newfoundland, with a
southward extension into the mountains of NY, ne Pennsylvania, and New
England (south to Mt. Graylock in Massachusetts).

Part B: The Destination

Blackpolls winter in South America between the north coast, Andes, and
Amazon River.

The rest of the story is about how they get from A to B (with a bit about B
to A at the end).

Based on all the available data, the vast majority of Blackpolls funnel
through the northeastern US and eastern Canada during the fall migration
(August through early November).  For our purposes, the northeastern US
extends as far south as the Outer Banks of North Carolina, although the
bulk of migration passes between Nova Scotia and New Jersey. There are
almost no fall records of Blackpolls in the US west of the Great Plains and
they are pretty rare anywhere south of Ohio, West Virginia, and eastern
North Carolina.  All of this is all based on banding and sighting data.

In other words, birds from as far away as Alaska have already travelled
across the entire continent to reach the Northeast, a distance of roughly
3000 miles.  When they pile up along the northeast coast, they may spend a
couple of weeks eating, gaining weight, and waiting for the appropriate
weather conditions.  And they put on quite a bit of weight, going from a
lean mass of 11-12 grams to as high as 20-23 grams (or from roughly
half-an-ounce to an ounce).

The prevailing winds in the Northeast are from the northwest, and birds may
wait for a particularly strong NW flow to start them on their journey.
However, given the distance they are about to travel, a short-lived cold
front is not as critical as it might appear.

The birds take off in the evening heading southeast, and essentially fly
straight out over the North Atlantic Ocean.  Mass take-offs to the
southeast detected by radar cannot be identified to species, but one
assumes that many of these birds are Blackpolls (and for the record, there
is some discussion of whether Connecticut Warblers follow a similar
strategy). The birds maintain a SW bearing until they approach the Tropic
of Cancer, at which point the prevailing winds shift to northeast (the
Trade Winds).  At this point the birds are deflected south and southwest
toward the South American mainland.  Depending on where they left North
America, this distance is between 1500 and 2000+ miles.  It can take up to
88 hours (yes, that's three-and-a-half days).

How do we know they take this overwater route?  For one thing, Blackpolls
are rare in Southeast in the fall, as already stated.  For another, they
are a regular fall migrant on Bermuda (triangle not withstanding), which
lies smack dab in the middle of the overwater route.  Finally, they have
been recorded in sometimes impressive numbers on boats far out to sea
(sometimes even accompanying shorebirds, although the latter are obviously
moving faster).

Blackpolls are also quite rare in the Caribbean west of Hispaniola.  They
are pretty common on Puerto Rico, the Virgin Islands, and the Lesser
Antilles, which again lie right in the proposed migration route.  When I
was doing waterthrush research in Puerto Rico in October, we'd catch a few
Blackpolls.  Many were actually below the normal lean weight, suggesting
they had started to metabolize muscle toward the end of the overwater trip.
Once they've hit landfall in the eastern Caribbean or northern South
America, they can feed and fatten up enough to complete whatever legs of
the migration remain.  Some may still travel 1000 miles or more southward
into Amazonia or beyond.

While most people buy into this scenario, there is one very vocal critic of
the overwater hypothesis, who instead proposes a land route via Florida and
the Greater Antilles.  A lot of the debate (such as it is) revolves around
the merits of relative abundance data and weight data.  Obviously, I come
down on the side of the plan I've outlined, largely because of the species'
a) rarity in Florida (except after storms), b) abundance in Bermuda, and c)
low weights in the Caribbean.

To summarize, the farthest a Blackpoll might travel IN THE FALL (we'll get
to spring in a second) is some 6000-6500 miles between western Alaska and
northern Bolivia.  The shortest would be perhaps 1500-2000 from New England
to northern Venezuela.  Recapture and color-banding data (some from right
here in NH on Mt. Cardigan by your's tryly) suggest an average lifespan of
3-5 years.  That's a lot of miles, not even counting the return trip.

In spring (April to May), Blackpolls appear to cross the western Caribbean
directly from Columbia and Venezuela to Cuba, and thence to Florida and the
Gulf Coast as far west as Louisiana.  There are a few records from Jamaica,
but essentially none from Central America, so it appears an overland route
is out this time as well.  From the southern US, they fan out to the north,
spreading as far west as the front range of the Rocky Mountains, and
obviously northeast to the Canadian Maritimes and New England.  In general,
this spring route is more direct for western populations, but perhaps a
little longer for the northeastern ones.  You can't blame them though -
who'd want to travel 2000 miles over water into a headwind!

Add it all together and the average Blackpoll Warbler (let's say it lives
to be 4 years old = four round trips) travels between 14,000 (New
England/Venezuela) and 50,000 (Alaska/Bolivia) miles in it's lifetime.  For
any population, roughly 8000 of these miles are over water.

For a bird this size, the whole thing is pretty mind-boggling.  Shorebirds
and seabirds are the only groups that make long journeys with such an
extensive overwater leg.  But shorebirds are significantly larger and fly
much faster, while seabirds can stop and eat whenever they want.  Remember,
this is a half-ounce insectivorus bird flying NON-STOP for over three days.
Maybe it's lucky, and can take a break in Bermuda, but most don't have that
option unless they intend to become fish food.

People often try to visualize this feat in human terms.  On the radio
Monday, Mark made the comment about how many pizzas we'd have to consume to
gain weight at the rate a sandpiper does.  But such comparisons will always
be flawed, since humans are physiologically so radically different from
migratory birds.  Rather than trying for the caloric equivalents, simply
visualize the monumental task of doubling your weight in two weeks, and
THEN exterting yourself (a fast walk, perhaps) for three days without
eating.  I suspect that we'd be barely able to move after the initial
episode of gluttony, much less carry out sustained activity of any sort.

So keep that in mind when you next see a Blackpoll Warbler.  By the end of
the month it will have launched itself over the North Atlantic, perhaps for
the first time in its life, and there's a 50/50 chance it'll be back to do
the exact same thing next fall.

Humbling.

Pam


And for the poets among you, another version of this story:

The Journey
    by Pamela Denise Hunt (Fall 1999 - February 2001)

It is autumn in the northlands
Nights get longer, leaves turn yellow
Their breeding season over
Restless songbirds start to move

>From Alaska, Canada, New England they come
Half-ounce feathered mites of green and yellow
Leaving vast forests of spruce and fir
On a journey most have never made

To the south and east an ocean awaits them
Long as a planet and a fifth as wide
A place without forests, and even food
Where wind and wave shape a watery world

They stop to feed, these tiny travelers
Flitting through coastal thickets and woodlots
Converting insects, even berries
Into a body's worth of fuel

A northwest wind provides the signal
Darkness falls, the flocks take wing
Guided by stars and hidden senses
They set off across the open sea

Three days they fly, maybe more
And some are lost to the waves below
No land in sight, no place to rest
Driven by the call of an ancestral home

The tradewinds shift, and south they turn
And land exhausted on a distant shore
A new continent, vast and green
Where a great river winds through a dwindling forest

Here they spend the winter months
As visitors in a foreign land
Feeding, roaming through the forest
And waiting for the season's turn

In lengthening days they prepare to return
Changing winter colors to those of spring
Gone are the greens and yellows of autumn
Black, white, and gray replace them

Far to the north the greening begins
Newly feathered birds are waiting
For that ancient unknown signal
To commence the arduous journey home

A different, smaller sea awaits them
Necklaced with a string of tropical islands
Over all of this they fly
Returning to the continent of their birth

>From this southeastern point of landfall
They spread west and north toward the spruces
Following the birth of insects
Preparing to settle for the summer

And thus the journey comes full circle
Nests are built and young are raised
But soon enough the days will shorten
And tiny birds fly south again


------ End of Forwarded Message

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