Monitoring Summary. The stainless steel Museum Birdhouse performed about the same as a Stokes wooden birdhouse when temperatures were in the 80's. At higher temperatures, the stainless house was only a degree or two warmer than a wooden house but still well below a level that is dangerous to newly hatched birds. The extreme conditions during monitoring more than likely resulted in temperature buildups higher than would be experienced under actual use for all three birdhouses tested. For any birdhouse, an effort should be made to face the entrance hole away from direct sunlight and the house should be located where it receives at least some shade during the very hottest part of the day.
Temperature monitoring was conducted in order to make sure the temperature buildup inside the Museum Birdhouse was not harmful to any bird that would nest in the house. There is surprisingly little detailed information available about temperature levels inside either commercially available or homemade birdhouses, whether they are wood or metal. So in order to answer the question, it was necessary to monitor both a commercially available wooden birdhouse and the stainless steel Museum Birdhouse. As a "control house", a well-ventilated white box was also monitored. Comparative temperature monitoring took place during the summer of 2002 .
The photo on the left shows the monitoring setup. The picture was taken with the camera pointed in a Southerly direction. The three houses were installed on an open, steep hillside on identical metal poles: 1) a wooden Stokes Western Birds Nest Box (Stokes Birdhouse) purchased from Home Depot, 2) a white plastic box that was made into a birdhouse, and 3) a stainless steel Museum Birdhouse. The white box also had a long temperature probe attached that measured the outside air temperature. The entrance holes of all the houses faced away from the sun but the houses themselves were exposed to direct sunlight with no shade during the entire day. The entrance holes were blocked with hardware cloth to prevent any birds from entering or nesting in the house during the course of the monitoring. The relative position of the houses was changed during the monitoring to insure that there were no positional effects, and if fact, there weren't any. The houses were arranged in a formation such that no house would provide shade for any other.
Monitoring took place during July and August, the hottest part of the summer. On several days during this period temperatures were in the high 90's. These are close to extreme conditions for temperate climates. In addition, the decision to locate the houses on an open hillside with no shade was to purposely simulate extreme conditions. Normally, it would always be advisable to locate a birdhouse where it can receive at least some shade at the hottest time of the day. Most actual uses of the Museum Birdhouse, or the wooden Stokes Birdhouse for that matter, should result in less temperature buildup than recorded in these test conditions.
It's worth noting why the Stokes Birdhouse was chosen as representative of wooden birdhouses. The Stokes Company has a good reputation as a purveyor of many bird related products. In fact, the Museum Birdhouse Manual cites a Stokes book on bird behavior as being one of the best sources available for the backyard birder. So the Stokes house was picked because it represents the way wooden birdhouses ought to be built. The intent was to compare the Museum Birdhouse to one of the best commercially available wooden birdhouses.
The features of the Stokes wooden birdhouse and the white box remained unchanged during the monitoring. The Museum Birdhouse was changed in numerous ways and the results compared to the other two houses, e.g., the size and shape of the vents on stainless house were changed as was the finish on the outside walls and the roof. The rationale for changing the finish was an application of the principle that reflective objects absorb less heat than less reflective ones. In one condition a quarter inch of white foam insulation was applied to the roof of the stainless house.
Temperatures were recorded with remote data loggers made by Onset Computer Corporation. The photo on the left shows one of the loggers. The data loggers can be adjusted to allow temperature to be recorded every so many seconds for up to a month. After intervals ranging from one to four days, the monitors were removed from the birdhouses and the data downloaded into a computer for analysis. Each house had its own identical logger so temperatures were simultaneously monitored under identical wind and cloud cover conditions. The photo shows the wire temperature sensor (small dark ball above the word HOBO) on the outside of the logger's case. This arrangement made the sensors more responsive to rapid temperature changes. Click here to see the loggers installed in each of the three houses.
The results of the monitoring were representative of the data shown in the graph below. The net effect of the final and best design (numerous narrow slotted vents and a roof more polished than the sides) was that the stainless steel Museum Birdhouse was only about a single degree warmer than the Stokes wooden house during the hottest part of an 87 degree day. When temperatures were near 100, the Museum Birdhouse was two degrees warmer. In most respects the stainless house behaved very much like the white box. Interestingly, during non-peak morning temperatures the stainless house was several degrees warmer than the wooden house. It's not clear what causes this. There could be a complex interaction between reflectance, emissivity, the sun's angle, and the unique properties of stainless. Regardless, this may actually be beneficial since for most portions of the nesting season warmer inside temperatures during the cooler parts of the day may assist in egg incubation.
The best available information indicates that eggs as well as newly hatched birds can tolerate temperatures up to about 107 degrees. In none of the monitoring did the stainless or the other two houses reach temperatures this high. All three houses performed well even under what might be considered extreme testing conditions.
In summary, the stainless steel Museum Birdhouse performed about the same as the Stokes wooden birdhouse when temperatures were in the 80's. At higher temperatures, the stainless house was only a degree or two warmer than a wooden house but still substantially below a level that is dangerous to newly hatched birds. The extreme testing conditions more than likely resulted in temperature buildups higher than would be experienced under actual use for all three birdhouses tested. For any birdhouse, an effort should be made to face the entrance hole away from direct sunlight and the house should be located where it receives at least some shade during the very hottest part of the day.