Bluebirds and Extreme Heat

The Williamsburg area weather forecast calls for a high temperature of 106 degrees today with a "feels like" temperature of 111 degrees Fahrenheit. 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. I have read that ideal development requires that bluebird eggs be maintained between 96 and 105 degrees Fahrenheit. Females can sense when eggs are outside this range.

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, 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. 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 are wooden. Ventilation holes are key. In hot climates, roofs should overhang the sides by at least 2" and 4" in the front. I read that some people tie small umbrellas on the top of the box for shade. Perhaps we should give this a try!

I took this photo of a group of nestlings on Tuesday, June 22. Apparently, one egg in that clutch did not hatch. These birds were about one week old at the time the photo was taken. When they are born, as I've shown in previous posts, 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. These nestlings are entering their second week of life, and will demand constant feeding as they are in their period of greatest weight gain. We'll see the parents going in and out of this box often.

I'll post updates on the fate of our heat wave babies in the days ahead.