Temperatures: According to Cornell, the optimal range for bird egg development is 96.8 °F to 104.9 °F (36 °C to 40.5 °C). If egg temperatures are lower, embryonic development slows. Higher temperatures can be lethal for the embryo. (Birdscope, Summer 2002, Vol.16 No.3, Cooper and Chu). Bluebird eggs and nestlings cannot survive temperatures exceeding 107 °F (41° C) (Conley Black). Prolonged excessive heat can severely impact nestling health due to dehydration and heat stress.
Temperatures inside a nestbox can reach 120 °F, and are often at least 10 degrees higher than the outside temperature (Cudsworth et al). Some research (?) indicates that if temperatures outside are 100-104 °F, the percentage of eggs that hatch drop, and nestlings under nine days old can die from heat stress or dehydration. (Up to 6-9 days old, the nestlings are as sensitive as eggs to cold/heat.)
Use a high/low thermometer during full sun in your nestboxes to see if they hit 107 degrees F.
Kate Arnold of Paris TX notes that "if the nestbox has ventilation holes or slots, and particularly if it's in the shade during the afternoon, the eggs and chicks will survive. They get water from the bodies of insects brought to them by their parents.... The only time I ever lost eggs to heat, it was over 100 degrees for days in a row, and the eggs were in a thin nestbox in full sun. I now use thicker wood on all my nestboxes, and all but one are shaded from about noon on. The one box is in full sun, but it is made of 1" wood and I have never had heat-related losses in it."
Nestbox design and location are critical to managing heat. The TX Bluebird Society box has excellent ventilation and a large roof overhang, and is a good design for hot climates. See plans.
In hot climates, place nestboxes where they will be shaded in the afternoon (during the hottest part of the day, which is usually 1-3 p.m.), but far enough away from large branches to avoid allowing climbing predators to jump onto the box.
Hanging boxes in the shade of trees protects them from sun, and also from vandals and some predators.
Ventilation is necessary.
Slots at the top of the box are more efficient at removing air in a nestbox than round holes.
If possible, position boxes so breezes will blow through the box from side to side to carry away summer heat.
Additional ventilation in summer can be provided by a 3/8" opening at the top of each side of the roof overhang. By installing a 1/2" thick x 3/4" wide x 7" long (or whatever length of box) wooden strip over each vent opening, using a screw at the rear as a pivot, the strip can be moved up to winterize the box, where it can be fastened in the front with a second screw. (Sialia 1993, p.92)
Roof Overhang (for shade): In hot climates, roofs should overhang the sides by at least 2" and 4" in the front.
Nestbox Color: According to Keith Kridler, a dark colored nestbox can be 18 °F hotter than an identical light colored nestbox. Never paint boxes in a hot climate a dark color. Painting the exterior of the box white or light tan (before nesting season begins) can help reduce interior temperatures.
Umbrellas: Folks have connected a small umbrella to the post to protect the nestbox from direct sunlight, securing it with cable ties. Monitor the box to verify that the adults accept the addition. If high winds or storms are imminent, remove the umbrella. Second Roof: Make a second 3/4" styrofoam roof (custom-cut) with push-pin spacers elevating the styrofoam from regular roof (works like a cooler) - holes are punched in styrofoam sides to allow tie-on with elastic, or you can attach it to the box with a bungee cord. Make sure the extra roof is wide enough to shade the sides of the box as the sun crosses the sky. You can also apply this to the west side of the box.
Bruce MacDonald of Canada uses 1/4" Luan heat shields for the roofs with the 1/2" nylon spacers.
Heat Shields and Screens can be very effective at controlling temperatures inside the box.
TBN Experiment: Check out D. Shiels' experiment with heat shields - interior temperatures were as much as 6.3 degrees F lower than outside temperatures. Plans for Shiels' design are available on the Texas Bluebird Society website.
Shelly Harris Screens: Shelly Harris from Oklahoma has been working on heat shielding of bluebird nestboxes that are not in shade. She found window screening sold at Home Depot reduces heat from the sun's rays by 90%. All you need to do to see the effectiveness of the screens is to stand directly in the hot sun for a minute. Then put the screen up over your face about 5" away from your face and you can immediately feel a reduction in heat. See photo album of box with shielding materials.
She uses it as "shades" over the roof, south, and west sides of nestboxes that are in full sun. The screen shades are mounted to be approximately 1" - 2" from the surface of the nestbox, both top and sides. It is important to allow free flowing ventilation so the box does not retain heat. The screening is not expensive, doesn't ravel, and is easy to work with.
She devised a hook mounted system that allows the screening to be mounted to the nestbox in a few seconds (after a few ceiling hooks are installed on the nestbox). The screening is attached to two pieces of square 3/8" X 36" square Poplar doweling. The screen/solar shades can be rolled up and several easily carried in a tote bag pocket while checking nestboxes. Bluebirds have shown no hesitancy in approaching or inspecting these nestboxes. The screening has withstood 60+ mph winds for two nights of thunderstorms in Oklahoma without any sign of weakness or dislodging.
Harris did some preliminary heat testing and had very good results. The shielded nestbox internal temps were equal to the temp. of a nestbox in full shade. Plus, the ventilation was not compromised. The nestbox tested in full sun was 4 - 6 degrees hotter on a sunny, 84 degree day. The tested nestboxes have good ventilation and all are of the same design and made of wood.
Babies hatched: S. Harris has had a pair of EABL's nest and lay a clutch of 4 eggs in the original heat shielded box. Three eggs hatched on 7/4, and one egg was infertile and was removed on 7/7. As of 7/22/03, the temperature in the heat shielded houses has never raised even a single degree above ambient air temperature. On a 106 degree day, the temperature in the nestbox was 104.
Charcoal Super Solar Screening 36" X 84" (Super Solar Screening by Phifer, Home Depot SKU #178467 - solar screen is also available from metroscreenworks.com)
3/8" X 36" Square Poplar Dowels (Squares)
Staple Gun and Staples
Pliers (if you choose to use them)
Hand Saw (or something to cut square dowels)
Scissors to cut screen to size
Zinc Screw Hooks size 8 X 2-9/16"
Zinc Screw Hooks size 10 X 2-1/6" 2 1/2"
Deck Screws (just set in surface of wood. Do not penetrate housing thickness, to hold spacer dowels away from nestbox surface for screening to rest on)
Drill (to make pilot holes for screws in nestbox) - not necessary, just makes it easier
Another, possibly simpler approach by Shelly: Use square doweling, some 2.5" screws, screening cut to length, and some clothes pins or small clamps or clasps of some type. If the doweling "bridges" were installed at every high point of the nestbox (meaning top, and edges of each side to be covered), then the screening could be cut to length and clothes pinned, stapled, or clamped in place on the two end bridges of each side to be covered. Maybe this would make it even easier.....and less expensive since no screw eye hooks would need to be purchased. The only thing to be very careful of is to not screw through the entire thickness of the nestbox when mounting the square dowel bridges.
Variation by H. Priest of MA: Haleya used Shelly's solar screening with terrific success. However, in the Northeast, neither Home Depot nor anyone else carries the solar screening, and she was unable to order it online due to shipping restrictions. She was able to get it through Aubuchon.com. You can also try metroscreenworks.com (order by the roll).
She cut out the desired size, stapled one end to a piece of 1/2" by 1/2" pine the length of the screen, and then then stapled the top of the screen directly onto the edge of the roof, letting the wooden edge hang. To prevent it from blowing off in strong winds, tie some of the screens to the poles.
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