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A Bird's Home


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.


Trio 24-compartment Castle


Trio "M" series w/6"x6"x12" compartments


Trio "M" series w/insulation, 6"x6"x12" compartments


Trio "M" series w/6"x6"x6" compartments


Trio castle converted to 12 oversized compartments


Natural Wood House with no venting



Natureline 11" plastic, white w/vent open


Natureline w/ vent closed


Natural Gourd 9" vented, painted white


Natural Gourd 9.5" non-vented, painted white


Carrol Industries 8" pale almond plastic


Natural Gourd 9" medium brown, unpainted



Gourd Venting

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.

Brands Tested

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 Trio Castle

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

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