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Sapsucker secrets (fwd)



Hi Carolinabirders,

Found something interesting on the on-line archive of the Georgia 
listserv on Sapsucker habits. The study was conducted in their breeding 
range in Michigan; I wonder how closely their winter behavior matches 
this?

I also noticed while I was there that Texas is having even greater GBBC 
problems than we are, with rampant Purple Finch and American Tree Sparrow 
 and Juniper Titmouse sightings from all over the state! Sounds like 
Allison and her crew are already on the job editing things though...

Cheers,

Josh

------------------------------------------------------------

Subject: Sapsucker secrets
Eran Tomer <etomer@EMORY.EDU> 
Date: Mon, 21 Feb 2000 06:51:25 -0500

Hello all

I have recently come across a new study on Yellow-bellied Sapsuckers which
was not only fascinating but also unusually well done.  Since I found it
so informative, I thought I'd summarize its main points on GABO for those
who might be interested in the peculiar behavior of this common bird.

***

Laurie Eberhardt from the University of Florida performed this study in
a forest in Michigan, where the Yellow-bellied Sapsucker breeds.  The
project addressed several interesting questions concerning sapsuckers and
sap:

- Do sapsuckers have a preference for certain tree species for drilling ?

- Why do sapsuckers select individual trees for drilling and not others ?

- Why do sapsuckers construct their sap holes in such patterns ?

- How do sapsuckers actually obtain the sap ?

To answer these questions, many aspects of sap trees, non-sap
trees, sapsuckers' "sap wells" and  sapsucker behavior were examined and
measured. The researcher also imitated sapsucker drilling artificially, in
several fashions, and experimentally excluded sapsuckers from certain sap
trees to investigate the effects of the birds' drilling. It should be
noted that the work and analysis were done in an unusually thorough and
scientifically-rigorous manner, which is sadly lacking in many bird
studies.

The results show that, for one, Yellow-bellied Sapsuckers really do have a
distinct preference for certain trees, at least in Michigan.  Out of 36
"sapsucker trees" which were studied, 17 (or 47%) were Paper Birch and
the rest were Red Maple, Juneberry and Bigtooth Aspen (8, 6 and 5 trees,
respectively).

The birds also preferred particular individual trees of these species to
others. For example, when leaving the nest, they flew beyond the nearest
sap-species tree to another, seemingly similar individual farther away
(64.9 meters farther, on average).  In other words, the sapsuckers
regularly flew extra distances to favored trees instead of drilling in sap
trees closer to home.  This was interesting since preferred trees were
similar to neighboring unsued trees in terms of growth measures (e.g. soil
moisture) and physical measures (e.g. thickness of the bark). Even more
interesting is the fact that factors such as soil moisture and sunlight
affect a tree's quantity and quality of sap but the birds were not using
this as a basis for tree selection. Hence, it seems that the birds choose
trees for their very individual qualities.

Supporting this idea was the way in which sapsucker trees were arranged.
The trees which sapsuckers chose for drilling were not randomly scattered
individuals in the forest but clumped together, and the birds almost
always returned to drill in tree which have already been drilled in
previous years. These observations are related since trees around a
favored sap tree would also be drilled some, being in the center of the
bird's activity area. Several factors could make particular trees
attractive for sapsuckers and this study did not attempt to discern which
ones, but favored trees were shown to have distinctly richer sap (or more
sap per unit area).

Drillings were mostly located 3.6 to 10.6 meters above ground and almost
invariably on the tree trunk instead of limbs (1 exception). Drillings
were, however, quite close to the base of limbs. Sapsuckers drilled new
holes right above old ones, between 1 and 8-9 per day (but many more in
Juneberry), and fed from them for 1.5 to 5 days.  They expanded existing
holes before drilling new ones and at any one time, only the top holes in
a hole cluster would produce sap. Such drilling patterns appear to be very
deliberate and made in direct relation to the arrangement of sap
"channels" inside the tree. Specifically, by creating a ring of holes
around the trunk, and then others to form rows of holes, the sapsucker
creates a wood tissue block and thus a build-up of sap right above the
row, which is why new holes are added right above existing ones.
In addition, by drilling close to tree limbs, the bird is
presumably extracting sap which is rich in nutrients due to the closeness
of the sugar-producing leaves. (In which case, why doesn't the bird
regularly drill on branches and instead uses mostly the trunk ?  The
author tentatively cites various predation risks as possible factors but
this question is out of the scope of the study).

Sapsucker drilling did in fact have an effect on the trees which they were
using: those trees had increased moisture levels near the drillings and
were strikingly less healthy than other trees.  This was not the result
when the researcher artificially imitated the birds' drilling.  The bad
state of sapsucker trees could be either the effect of the birds' drilling
or its cause, since the sap of "sick" trees was shown to contain more
proteins than that of healthy ones.  This study could not responsibly
determine which was the case, cause or effect.

The researcher had a very difficult time trying to obtain sap by imitating
sapsucker drilling at various places on trees and at different depths into
the wood, which was repeated both on sapsucker-used trees and unused ones.
As a matter of fact, the only success came from drilling immediately above
sapsucker holes and then allowing the woodpecker access to the artificial
excavations. The author concludes that either artificial drilling was not
a good imitation of the bird's specialized technique, or that the bird is
using an additional step in obtaining sap (e.g. perhaps it has saliva
which prevents sap from hardening).

***

Fascinating stuff.  The Red Maples by my workplace, all used by
sapsuckers, have had an explosion of shelf fungi growing on them for the
second year in a row now, and right over sapsucker holes. Until now I
wasn't sure what to make of it - were sapsuckers killing the trees on
which they feed?

Best regards

- Eran Tomer
  Atlanta, GA
  etomer@emory.edu

------------------------------------------------------------

Joshua S. Rose
Duke University
Department of Zoology

jsr6@acpub.duke.edu
http://www.duke.edu/~jsr6/