Note:
The following thermal testing was done by Glenn Davis
from Hutto, Tx, just out side of Austin. Glenn has been a
landlord of purple martins since 1977 and presently has a colony
with approx 80 pairs. His housing consists of plastic and
natural gourds plus Trio castle and "M" series aluminum
houses.
This testing was done to not only show, but prove that
housing that is painted white is much cooler on the hot days of
summer than housing that is not painted at all or painted a darker
color. It is my hope that those reading this will realize the
need to paint their housing white so that the rays of the sun are
reflected away, thus keeping the young inside cooler. With
that said, the following is Glenn's report of how the tests were
done and the results of them... Enjoy
Martin House - Gourd
Temperature Tests by Glenn
Davis
|
What better time to do a temperature test on some of
today's more popular purple martin nest cavities than mid-August in
Texas. The conditions were perfect for testing the effects of sun
and heat. The tests ran from 10:30 AM until 5:00 PM and the
temperature range during that time span was from 88 to 101 degrees
Fahrenheit. Conditions were ideal, we saw partly cloudy skies early
in the testing that gave way to a clear sky with a blazing sun in
the mid to late afternoon. The wind was variable up to about 7 mph
and at times was dead still. It was a wide range of typical summer
conditions all wrapped up into one day.
Testing was done with six TELTEK brand "Informer 2"
electronic thermometers that were checked for accuracy at 75 and at
105 degrees before the testing began, all were within .5 degrees but
to assure absolute accuracy the thermometers were randomly switched
around several times during the test day.
All nest cavities were filled with a measured amount
of cedar shavings and straw to simulate a martin nest and the
thermometer probes were set at 1-1/4" above the nesting material on
special jigs. All houses and gourds were placed 5’ above ground and
the entrance holes were faced AWAY from the prevailing breeze. A
brief test was done at the 12’ level to insure that the findings at
lower levels weren't skewed. Tests were done with the entrance holes
facing the breeze as well, that will be discussed later.
Just a few different types of cavities were used for
these tests, but because some of the gourds were tested in the
vented and non-vented mode, and in different colors, and both
natural and plastic, the test covered 12 separate categories.
Included were some of the more popular "TRIO" brand housing in both
the insulated and non-insulated styles. It should be noted that the
manufacturer does not offer insulated models. The insulated model
tested was customized by adding 3/8" styrofoam insulation board to
the attic as well as drilling 6 additional attic vent holes, 3 in
each gable end. Two of the Trio houses were model "M-6" (single
floor) and both were converted to have 3 compartments measuring 6"x
6"x12". The probes were set in the center compartments. Later tests
showed that there was little difference between the temperature of
the outside and center compartments, even when the sun was shining
on the end wall panel. Tests were also done on the M-6 with 6
standard size compartments. We also tested the Trio 24 compartment
castle with standard compartments and with the "double size"
converted compartments.
More than 100 readings were taken during that six and
a half hour time frame. A few more were taken in the days to follow.
Rather than breaking them down into AM readings vs. PM readings or
cloudy sky vs. clear sky readings I thought it would be best to give
you THE AVERAGE TEMP ABOVE THE AMBIENT AIR TEMP for each style of
nest box for the entire day. That is to say if you see a rating of
"7.5 degrees" that means the interior of that house or gourd
averaged 7.5 degrees hotter than the outside air temperature. Here
are the results of that day's testing.
MARTIN
HOUSES:
Trio 24-compartment
Castle |
4.11 |
Trio "M" series w/6"x6"x12"
compartments |
4.39 |
Trio "M" series
w/insulation, 6"x6"x12" compartments |
4.61 |
Trio "M" series w/6"x6"x6"
compartments |
4.82 |
Trio castle converted to 12
oversized compartments |
4.96 |
Natural Wood House with no
venting |
19.70 |
GOURDS:
Natureline 11" plastic,
white w/vent open |
4.43 |
Natureline w/ vent
closed |
5.24 |
Natural Gourd 9" vented,
painted white |
5.58 |
Natural Gourd 9.5"
non-vented, painted white |
6.36 |
Carrol Industries 8" pale
almond plastic |
7.50 |
Natural Gourd 9" medium
brown, unpainted |
10.83 |
This is the part I was disappointed about. I thought
there would be a noticeable difference in a vented and a non-vented
gourd. Truth is...there's not much at all. In fact there's so little
it makes you wonder whether it's worth the trouble to add additional
holes which alter the structural integrity and also create another
place for rain to get in. There is another factor to consider. When
the earliest birds get here it's still cold in many cases. Vent
holes might allow valuable heat to escape, just a thought. Cold
weather testing for heat retention will tell the tale.
Bright white gourds, like the "Natureline," showed
very little gain when the two 5/8" vents were opened, the
temperature dropped less than 1 degree, .81 to be exact. About the
same advantage was realized on a natural gourd which had been
painted a shade of white similar to the Natureline. In the days to
follow temperature measurements were taken using other gourds
comparing them in the vented and non-vented modes, the original test
results repeated themselves. In the off-white (pale almond) Carroll
gourd, the interior temperature dropped almost 1.5 degrees when two
1/2" holes were cut near the top. This was the largest temperature
differential we noticed in all of the gourds we vented.
House - Gourd
Orientation |
Whether it was a martin house or a gourd, a 1.5 to 2
degree drop in temperature was noticed when the entrance hole was
faced INTO the wind. Even the slightest breeze helped evacuate some
of the heat. Gotta remember...the entrance hole is the largest "vent
hole" that the nest area has. Facing it into the wind will save you
a few degrees in temperature.
Please don't think that by omitting some brands of
housing or gourds that we had anything against them. The test was
simply done on the units we had on hand here at this colony, we
didn't go out and buy houses to test. The units tested give a good
representation of what's on the market. For example, I have every
reason to believe that the PMCA "SuperGourd" would fare about as
well as the Natureline gourd based on color, wall thickness and
gourd size. The same can be said about most any aluminum house
that's on the market. They probably compare well to the Trio brand
houses that we used, just so they're white in color with reflective
roofs and are well ventilated.
The 24-compartment Trio brand castle is a very popular
style of aluminum martin housing and it fared about as well as the
"M" series in our tests. Temp tests were also done on a unit that
had been converted from 24 standard compartments to 12 extra large
compartments. Basically, one big compartment is created by using one
compartment and the one next door to it. A 2" hole (or larger) is
cut into the wall between them so the martin pair can use the "back
room" as a nesting area. The entrance hole to this back room is
plugged off using a regular Trio door stop or putting a blank plate
in place of the regular door panel. The idea is to make a more
roomier compartment which is not a prone to owl attacks, the back
room offers some measure of safety when trying to elude the owl's
long talons. Larger nest areas also have been proven to promote
larger broods. Some were concerned about the airflow to the back
portion of this enlarged room but the test proved there was little
to worry about. The temperature in the rearmost area was less than 1
degree warmer than a compartment in an unmodified castle.
In our test we found that aluminum martin houses have
a slight edge over the gourds. The advantage is minuscule and hardly
worth mentioning. It should not sway anyone's choice. The better
gourds were always within a degree or so of the aluminum house and
gourds do have some other advantages over traditional housing.
Clearly we can see that the more heat reflective the
color, the cooler the nesting area will be in the hot summer sun. We
all probably knew this but perhaps we didn't know to what degree.
Frankly, most types of well designed houses and gourds stayed in the
"safe range" when it came to heat buildup. It's only the extremes we
should be worried about, cases where color choice and ventilation
rules were ignored. For instance, to see how bad it COULD be, we
tested an old wooden martin house which might represent someone's
first effort at a homebuilt house or some sort of a high school
woodshop project. It's roof was natural wood which had weathered and
darkened over the years, the compartments were small and there was
no venting other than a few cracks in the walls. When the outside
temperature was 100 degrees and the sun was bright, the interior
temperature measured 119.7 degrees. This is almost 9 degrees hotter
than the light brown natural gourd we tested. When the roof of this
house was painted bright white, and several 1/2" holes were drilled
for ventilation, the temp fell to 106.2 within 45 minutes.
Glenn Davis, August 1999
Back to Chuck's Purple Martin
Page
This page created and
maintained
by Chuck Abare
Woodside Gardens
The Registry of Nature Habitats
Copyright 1999 -
All Rights Reserved
Last Updated:
| |