[obol] Bird migration timing and global climate change

[Joel Geier]

Hello folks,

Just two days ago, on Thursday afternoon, I was sitting in a lecture
hall at Ecole des Mines in Nancy, France listening to the keynote
address by one of the world's eminent global climate scientists, Jean
Claude Duplessy. Little could I guess that this experience, and not the
lifer STORKS that I saw from the train between Nancy and Strasbourg a
few hours later, would be the most relevant thing for Oregon's birding
list, when I returned home.

Dave Irons has raised some valid points regarding the problems with
using "first arrivals" of bird species as indications of the effects of
ongoing global climate change, or "global warming" as it is often
referred to in the popular press. Lars Norgren has also made some good
points about the difficulty of comprehending global climate change,
within the span of a baby-boomer lifetime. I do have some disagreements
with how Dave and Lars have characterized the climate situation, and its
effect on birds, but let's start with some areas where I agree with both
of them.

There are some significant problems with using statewide "first arrival"
birding records as data for this purpose. Dave has accurately identified
the main issues:

- Increased observer density over time;

- Increased monitoring in the southern part of the state;

- Increased knowledge of how to detect first arrivals by ear;

In fact, a few years back I boxed up the entire (25+ year?) run
of /Oregon Birds/ to a Stanford researcher, so that she could have her
students copy the state field notes, and then try to analyze the first-
arrival data for a relationship to climate change. So far as I
understand, she and her collaborators were unable to come up with a
clear pattern that could be correlated to observed changes in climate.

The first issue that Dave raised (increase in observer density) is
difficult to correct for, since we lack good numbers for how observer
density has increased over time. As someone who deals with groundwater
contamination and how it spreads as water moves underground, I find it
helpful to think of bird migration as analogous to an "advective-
dispersive" process:

Later on this spring when you're standing by a stream looking for a
Northern Waterthrush, try tossing in a big handful of willow leaves and
watch what happens. The main mass of leaves will move along at a speed
equal to the average current. However, they will gradually spread out in
the stream. A few will end up way far ahead of the main mass of leaves,
and a few will lag farther and farther behind. 

This happens because different leaves encounter slightly different
fluctuations or eddies in the stream current. For most leaves, the
fluctuations more or less cancel each other out, so on average the
leaves move at the same average speed as the stream. However, a few
leaves with especially "good luck" will catch more bits of fast current,
and move far out in front, while other leaves with "bad luck" will lag
far behind. If the fluctuations in current that are encountered by a
given leaf are purely random, the leaves end up being distributed
according to the infamous "bell curve" (in statistical terms, the
"normal" or Gaussian probability distribution), which spreads out more
and more as the the leaves float downstream. Five miles downstream, the
leaves might be spread out over a distance of nearly a mile.

To draw the analogy to bird migration, the "stream" is the basic
instinctive urge to head northward in spring. The "eddies" or
"variations in current" are the various things that individual birds can
encounter along their migration trek. Maybe one bird decides to hunker
down when it hits a neotropical rainstorm, or gets spooked by a predator
so it veers to the left when its comrades veer right, with the result
that it catches a different wind speed and either moves ahead or falls
behind the migrant population as a whole. A bird that gets ahead of the
main flock one day might encounter more favorable conditions the next
day, and move even further ahead. 

Think of all the little events that can happen to individual birds as
they move north, and how each of these events can spread out the flock,
just like those leaves on the stream. By the time a given bunch of
neotropical birds on average travel from Panama to Eugene, a few of them
might be in Vancouver B.C. while a few still haven't made it past San
Diego.

Going back to the leaves on the stream, now imagine that there's a
"leafer" five miles downstream, who's very excited about seeing willow
leaves floating by. So he watches for the first one that he sees
floating by each day, and keeps track of the time. But he's kind of
busy, so he can't check the stream every minute. Chances are, he'll miss
the first few leaves, but he'll see his first leaf by the time 10 or 20
have floated by. 

Suppose this "leafer" gets a few more friends excited about his hobby,
and they check the stream at different times. The more "leafers" there
are, the better the chance that one of them will spot one of the earlier
leaves to float down the stream. So the apparent "first leaf arrival"
time will become earlier, the more "leafers" there are.

Coming back to birds, this is essentially what happens when, from year
to year, there are more and more birders checking places like Mt. Tabor
and Skinner Butte. There is just a higher chance of an Oregon birder
encountering one of the earlier arrivals.

Now suppose that one of our "leafers," who happens to be named Dennis,
decides to set up camp about 100 yards upstream of the original leafer.
On average, he's going to spot leaves a bit earlier than the rest.
That's basically what happens when someone named Dennis decides to set
up a bird banding station in SW Oregon.

Finally, suppose that our leafers improve their leaf-watching skills
over time. Wait a second, I don't even want to go there with this
"leafer" analogy! I guess I've reached the end of where it makes sense.
But I guess it's easy to see how increased skill in detecting birds
could have the same effect as more birders in the field, in detecting
early arrivals.

Now let's come around to the climate change and its effect on birds,
whether observable within our personal experiential windows or not.

First off, the scientifically accepted term is "anthropogenic climate
change," meaning the portion of ongoing climate change that is
attributable to human activities as opposed to other ("natural") causes
such as variations in earth's orbit and inclination relative to the sun,
solar sunspot cycles, volcanism etc. 

There is very little doubt that this amounts to "global warming." The
Intergovernmental Panel on Climate Change (IPCC) states in their 2007
report (http://www.ipcc.ch/ipccreports/ar4-syr.htm):

" There is very high confidence that the global average net effect of
human activities since 1750 has been one of warming, with a radiative
forcing of +1.6 [+0.6 to +2.4] watts per square meter."

It is a serious misconception to say that the main issue is an increase
in atmospheric carbon, although atmospheric carbon (principally in the
form of carbon dioxide or CO2) is indeed a major driving force of
anthropogenic climate change. In this respect, the IPCC has strengthened
their conclusions since their previous report in the 1990s. Atmospheric
carbon concentrations are a direct problem for a few things such as
coral reefs (since increased CO2 in the atmosphere affects the chemical
equilibrium relationship between the calcium carbonates which make up
coral reefs versus dissolved calcium in sea water). However, the
indirect effects of increased CO2 (along with other anthropogenic
greenhouse gases including methane and nitrous oxide) on global
temperatures are the main concern for our natural ecosystems in Oregon.

It is also a serious misconception, and frankly silly, to worry about
the political implications of using the term "global warming." Global
warming is well-documented and beyond controversy. What has been
slightly controversial, at least up until recently (and only by a
handful of naysayers), is the anthropogenic component of global warming.
With the latest IPCC report (see link above) , the anthropogenic
component is beyond real political controversy. The current U.S.
President and the three leading candidates for his job have all now
acknowledged this, though some of them took longer than others. 

Anyway, it is silly to censor one's self based on political perceptions.
There might still be a few holdouts in Congress but there are no
holdouts among serious scientists working in the field of climate
research (I'm sorry, but our so-called "Oregon state climatologist"
doesn't count in that category, even if some lesser-known scientists at
OSU, with real Ph.D.s, are legitimately part of that group). 

Lars stated that Oregon has had higher mean temperatures over some 7000
of the past 12,000 years of the postglacial epoch. I'm not sure where he
gets those exact numbers, but they are believable if one looks at the
oxygen isotope record from deep-sea sediments, which are considered to
be one of the more reliable proxy measurements for global temperatures.

However, quoting those numbers in such a way can be very deceptive,
since this obscures the fact that our Earth has been in the later
portion of an interglacial period during this time. Based on what is
well-known about variations in the earth's orbital parameters (referred
to as the Milankovitch cycles), since about 10,000 years ago when the
earth came out of the Younger Dryas glacial episode (see Daansgaard et
al., "Evidence for general instability of past climate from a 250-kyr
ice-core record," /Nature/ 364:218-220), our Earth has been in a
relatively stable or gradual cooling phase, up to the onset of the
Industrial Revolution. What's happened due to recent human activities is
that we've switched from gradual cooling to precipitous heating of the
planet.

You might think it's good, that we're staving off the inevitable next
glaciation by pumping carbon into the atmosphere. But we've overshot the
natural mark, and (according to the models) will very soon be causing
more rapid climate change than natural ecosystems have ever had to deal
with -- meaning that the rate of migration of climate belts will exceed
the natural rate at which long-lived species such as trees can shift
their ranges.

The real issue of concern is not simply climate change, or even
anthropogenic climate change. It's PRECIPITOUS climate change, like our
planet has never seen before, except in a few instances that are
recorded in the fossil record as catastrophic "mass extinctions."

What does this mean for Oregon birds?

I think it's a mistake -- and frankly unrealistically OPTIMISTIC -- to
expect that birds will be a leading indicator for climate change. I
guess that most of us were brought up with the idea that birds are a
leading indicator of environmental problems, which was true for problems
like pesticides. However, climate change is different, since the species
that are most quickly affected are the less mobile species, like plants
and frogs and salamanders.

What is clear is that major aspects of Oregon's climate have already
started to change, as exemplified by spring snow melt. Anecdotally, a
Wheeler County rancher that I regularly stop by to talk to tells me that
his irrigation water coming off the Ochoco Mountains is running one and
a half to two weeks earlier in spring now, than it did thirty years ago.
An article in this month's issue of /Physics Today/ (Levi, 2008, Physics
Today 61:4(16-18) or see www.physicstoday.org) confirms his impressions
with quantitative hydrologic data: The fraction of precipitation
remaining as spring snow and the minimum winter temperatures in western
mountain ranges, as well as the center of timing for the Columbia River
basin flows, have all shown significant shifts relative to annual
variation, over the past 50 years.

Oregon's birds will need to adapt to such changes, or else miss out on
their prime windows of habitat suitability. Over time, we can expect
that natural selection will favor subpopulations that show up a bit
earlier, on average, since they will encounter more suitable
conditions. 

However, it's an open question whether birds can adapt as quickly as the
climate is changing. Going back to the leaf analogy, early arrivals
likely have more to do with random variations in the "currents" that
affect individual migrant birds along the way, rather than any genetic
propensity to leave Panama at an earlier date. 

Thus natural selection may not be all that quick or effective in
producing bird populations with a genetic propensity to migrate earlier
in the season. Mother Nature hasn't often had to deal with changes like
the one that our species is presently imposing on the system. When she's
had to, she hasn't done so well.

Good birding while it lasts,
Joel

--
Joel Geier
Camp Adair area north of Corvallis