Heat-Shield Experiment

Sweltering summer heat in Texas can literally cook bird eggs or nestlings during the spring and summer months.  Historical records on a bluebird trail in Ola, Kaufman County, Texas show that as much as 10% of eggs never hatch and many nestlings die of the summer heat in single-wall nest boxes.  A study was conducted on this trail in an attempt to minimize egg and nestling casualties during the nesting season from late February through August 2002.  Preliminary tests show that providing an extra layer of material, a heat shield, on a bluebird nest box will maintain cooler temperatures inside the box.  The following report will summarize the results of this study and show an increase in the number of birds that fledged with heat shield boxes in 2002 versus the number of birds that perished from this trail in previous years without heat shields.  Ultimately, not one nestling perished from heat exhaustion in 2002 with the heat shield boxes.

PRELIMINARY EXPERIMENT

The study began at the end of the 2001 nesting season when two identical nest boxes were constructed from ¾ inch thick western red cedar (cedar).  The boxes measure four inches by five inches wide and 10½ inches tall.  The roof is slanted by 13½ degrees and the entrance hole is 1½ inches round (see Figure 1 below).   Box 1 remained with the normal single layer of wooden walls.  Box 2 was equipped with heat shields.  The heat shield walls were made of 1/8 inch birch plywood and were attached with ½ inch cedar spacers to create insulating space between the two walls.  An additional roof (outer layer) was also attached with spacers (see Figure 1 below).  Digital thermometers were installed inside each box on the back wall opposite the entrance hole.  The thermometers were equipped with FM frequency transmitters to a base unit inside the author’s home where temperature readings could be recorded.  Temperature observations began in the morning and were concluded just before midnight when the temperatures in both boxes were equal.

Click on the figures below for a full image.

There was a 2 degree difference by mid morning, a four degree difference at noon and a maximum difference of 6.3 degrees Fahrenheit at 2 pm just before a rain storm.  Based on the success of the preliminary experiment, the heat shields were applied to the author’s trail where 23 of 26 boxes were equipped with heat shields.

APPLYING HEAT-SHIELD BOXES TO THE TRAIL

Each of the 23 bluebird nest boxes was brought in for the application of the heat shields in January 2002.  Shield plates were constructed from 1/8 inch plywood as described above.  The fronts of each box were numbered according to the original trail numbers on the traditional box fronts.  Each box was coated with Weatherboss® non-toxic water-based wood preservative to protect it from the elements.  The boxes were positioned similarly to their previous year’s orientation.

The trail was first monitored on March 13, 2002.  This initial monitoring  showed that the females were willing to accept the boxes, with 15 of the 23 boxes containing complete nests.  A total of 35 eggs had been laid in 9 of the boxes by March 13th.  By the beginning of August, a total of 166 bluebirds had fledged from the 23 boxes.  All 23 boxes were used for the entire nesting season.  The 2002 nesting season yielded greater numbers of fledglings than any previous year of the trail’s 17 year history.  Not one baby bird was lost during the 2002 season due to heat stress.  In previous years, July and August monitoring events commonly saw many casualties from overheating.  However, it should be noted that the 2002 nesting season had few days where the daily temperature exceeded 100°F.

The number of unhatched eggs did not appear to be significantly changed over previous years.  However, more heat shield study is needed to evaluate whether the additional insulation affects the number of unhatched eggs.

CONCLUSION

Preliminary trials show that applying the heat shield to bluebird boxes greatly increases the number of fledglings on a bluebird trail in Ola, Kaufman County, Texas.  Temperatures in the heat shield box were consistently lower during a day of monitoring. The maximum difference was 6.3 Fº lower than the traditional box designs during the sweltering Texas summer.  This study demonstrates that providing heat shields to nest boxes decreases nestling casualties by decreasing temperatures inside nest boxes.  Future study is needed to observe long range use of heat shields on this Texas trail to ultimately determine whether these amendments increase the total number of fledglings over time or have an affect on the number of hatched eggs.  

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