--------Begin forward--------
Subject: songbird migrants and climate change
Date: Wed, 21 Jun 2000 10:07:29 -0700
From: Hal Opperman <halop at accessone.com>
To: tweeters <tweeters at u.washington.edu>


The June 16 issue of Science (vol. 288, pp. 2040-2041) has a study by
T. Scott Sillett and others that documents a strong correlation
between El Ni?o events and low survival, fecundity, and recruitment
of Black-throated Blue Warblers. The adult survival rate is less
than half as great as in La Ni?a years. From 1986 to 1998 the
authors studied a nesting population in New Hampshire and an
overwintering population in Jamaica, in both cases in relatively
undisturbed habitat. Knowing that fecundity in this species is
limited by food availability, and that its main summer prey is
lepidoptera larvae, they measured total larval biomass and found a
positive correlation with annual fecundity, with prey biomass being
low in El Ni?o years and high in La Ni?a years. They then found that
the wintering population numbers in Jamaica dipped following an El
Ni?o summer -- in other words, recruitment to the breeding population
was lowered.

This study adds to the growing evidence that the El Ni?o Southern
Oscillation (ENSO), in the authors' words, "impact[s] demographic
rates and food resources of many animal taxa, including seabirds,
raptors, Pacific island passerines, primates and rodents, and
arthropods." Their study shows that "populations of migratory birds
are...susceptible over a range of spatial and temporal scales to
shifts in global climate patterns."

It is worth quoting their concluding paragraph in full:

"Evidence is accumulating that bird populations are being affected by
global warming associated with long-term climate change. Global
warming could also be increasing the severity of ENSO events. If
this is true, we predict that variance in demographic rates of
migratory bird populations will become amplified, leading to elevated
extinction risk, especially for small populations. Because many
migratory bird species are declining in abundance and therefore of
conservation concern, field research and demographic modeling efforts
should focus on understanding how events throughout the annual cycle
are interconnected and on how multiple limiting factors, both natural
and human-related, determine population size."

Elsewhere in this issue of Science (pp. 1975-1976), Bernt-Erik
Saether contributes a Perspectives piece stimulated by the previous
article. Sillett et al., he says, "show that large-scale regional
variations in climate have a two-fold effect on the demographics of a
migratory bird species, affecting both its survival in the tropics as
well as its reproductive performance in the north. One frightening
consequence of these findings is that they illustrate how difficult
it will be to reliably predict the effects of large-scale regional
climate change on ecological systems."

If we want to keep on having a diversity of birds and other organisms
to share the planet with us, we need to start formulating
conservation policy more and more at the systemic level, with good
science behind it. Sooner rather than later.

Hal Opperman
Medina, Washington
mailto:halop at accessone.com

------end forward--------------

--
Mike Patterson Alas, to wear the mantle of Galileo,
Astoria, OR it is not enough to be persecuted
celata at pacifier.com by an unkind establishment,
you must also be right.
---Robert Park
http://www.pacifier.com/~mpatters/bird/bird.html