This spring has been hot. And now, in places like eastern Virginia, daytime temperatures have been over 100 degrees Fahrenheit in June. Such highs are expected in this area in August, but not this early. In the Williamsburg, Virginia, area at New Quarter Park, the Historic River Chapter, Virginia Master Naturalists who have adopted Bluebird Boxes on the 13-box trail are concerned about a number of nests that have or will be hatching out this week.
"There are three new bluebird nestlings in my box at New Quarter Park," said Shirley Devan, volunteer bluebird box monitor, on June 22. "I checked today in the extreme heat. Still one egg. Mom popped her head out of the box when I parked. Then she flew out when I opened the car door. I wanted to get photos but the nest is too high and with the extreme heat I didn't want the mom off the babies too long. They cannot thermoregulate yet and they could bake in the box!"
Ideal development requires that bluebird eggs be maintained between 96 and 105 degrees Fahrenheit. Females can sense when eggs are outside this range.
Bluebird Box Trails: How They Got Started
Nationwide bluebird box trails have been popular since the 1970s when the bird species’ population numbers began to drop due to the chemical DDT, which softened shells and prevented them from hatching. Also of concern was the loss of habitat, trees with holes in them for nests. Stands of such trees were being cut due to rapid suburban development, especially in the eastern United States. But many golf courses, suburban developments, and parks helped the birds by adding trails of wooden boxes. The bluebirds have taken to them and population numbers have climbed.
And Now, the Extreme Weather Challenge: Is it Too Hot for Cavity Nesting Birds Like Bluebirds?
According to the Bluebird Monitor's Guide to Bluebirds and Other Cavity Nesters, the incubating female tries to keep the eggs at around 92-95 °F. Sometimes when it's really hot, as it has been this spring, the females don't sit on the eggs as much, or they fan the eggs with their wings, or the eggs hatch faster. On the other hand, they may sit on eggs or nestlings more often to draw heat away. If at all possible, the authors recommend, the inside of the nestbox should not exceed 100 °F.
Beating the Heat: Bluebirds and Boxes
Some research indicates that if temperatures outside are 100-104 degrees Fahrenheit, the percentage of eggs that hatch drops, and nestlings under nine days old can die. Wooden boxes apparently get hotter than PVC boxes, and all of the boxes at New Quarter and along most older bluebird trails are wooden. Ventilation holes are key. In hot climates, roofs should overhang the sides by at least 2" and 4" in the front. Some people tie small umbrellas on the top of the box for shade.
Nestlings and Thermoregulation
When bluebirds are born, the nestlings are naked of feathers, blind, and helpless. Until they are about a week old and can control their own body temperatures, mothers will spend time brooding them and keeping them either warm or shaded. Then, at just over a week old, the nestlings are able to thermoregulate their body temperatures, thereby decreasing the risk of mortality due to excessive heat.
As nestlings enter their second week of life, they demand constant feeding as they are in their period of greatest weight gain. In Virginia temperatures are forecast to continue to be high, so parent birds will be seen by the monitors at New Quarter Park going in and out of boxes often to feed surviving nestlings. Bluebirds eat and feed their young insects, so nest box monitors might consider helping out by supplying meal worms and shallow pans of water for cooling and drinking.
Sara E. Lewis - Sara E. Lewis is a writer and publications consultant. As a freelancer, she writes and produces marketing materials and especially enjoys ...
