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Incubation Costs Calories

Incubation Costs Calories

Incubation Costs Calories

Dr. Jim Kellam, Associate Professor of Biology, Saint Vincent College

This year I began a multi-year study of bird nesting success at Saint Vincent College. With the help of students in March, I erected 52 wooden nest boxes at two sites adjacent to campus: nearby agricultural fields, and around a set of abandoned mine drainage ponds. I started checking the boxes for nests at the start of April, and now that the spring semester is over, I am employing an undergraduate student assistant to check them every three or four days. In the beginning I got worried. In the first week, I only had three boxes with any bird action. Two nests had Carolina Wren nests, and one had an Eastern Bluebird nest. Meanwhile, I had colleagues who were reporting numerous nests in their own boxes and I wondered if my boxes were not placed at good locations. Several weeks later, I am resting easier. Currently out of 52 boxes, I have logged 55 nesting attempts, 81 eggs being incubated, and 10 young being fed. The species breakdown is as follows: Tree Swallow (14 nests with eggs), House Wren (4 nests with eggs), Eastern Bluebird (2 nests with eggs and 1 nest with nestlings), Black-capped Chickadee (1 nest with nestlings), and Carolina Wren (1 nest with eggs). I’m about to get busy measuring and banding all these parents and nestlings for my study!

You’ll note that I have more “nesting attempts” than I do actual nest boxes. This is because many boxes contain partial nests that have not been completed. Some of these may end up being complete nests with eggs soon. Some may represent abandoned attempts due to the birds choosing other locations for their nest. I remember one nest box in particular had a fully-constructed Eastern Bluebird nest in it, but as I checked for eggs, I found my hands covered in little black ants. I left the nest in place for a week to see what would happen. Then, with the ants still there and no further bird activity, I cleaned out the nest box to make it available for another attempt. So far, it remains empty.

There are two other nesting attempts that have sad endings. No, these nests were not destroyed by predators. Instead, I simply found a deceased parent peacefully sitting on its nest. It was a Carolina Wren I found on April 6, and a male Tree Swallow I found on May 16 (see photo). My personal weather station registered a low temperature of 41 degrees on April 6. The low temperature on May 16 was 10 degrees warmer, at 51 degrees. If you live in western Pennsylvania, you will know that our spring has seemed a little slow to get started, but these temperatures are not unusually cold for those dates. Rainfall also has not been above normal. I bring up the weather conditions because when one finds a deceased bird with no external injuries, the explanation is usually starvation.

That’s right. Birds—especially small birds like wrens and swallows—can literally starve overnight. They are very active animals, burning lots of calories per hour. Their small size also puts them at extra risk because the physics of heat loss tells us that smaller birds have proportionately greater surface area over which they can lose heat to the environment compared to larger-bodied animals. And birds need to be able to fly well, which means they can’t store a lot of fat on their bodies like most mammals do.

In a study I did on Downy Woodpeckers many years ago, I would occasionally find a dead bird in its roost hole, but that was during the winter months when it was cold. In the absence of cold weather in April and May, you might think that small birds wouldn’t have a problem maintaining energy balance. In milder temperatures, they are burning less fuel to stay warm. However, scientists have shown that incubation is an energy-intensive behavior for birds to perform. As birds sit on their nest, their heart rate and blood flow is much higher than the normal resting rate. They are warming those eggs and chicks! Meanwhile, if a bird is sitting on the nest for many hours at a time, that means it is unable to recoup the lost calories through foraging. With calories being burned at a faster rate than food is being taken in, the birds continue to be at risk of starving to death. This doesn’t happen often, and it likely happens only to adult birds that are already experiencing some kind of physical challenge like an illness or injury. As much as you and I love nature, we must recognize that it is full of risk and potential danger. The birds manage as well as they can, and the fittest survive to live another day and raise a happy brood.

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Virginia Puglia
Virginia Puglia
4 months ago

I know it is pure speculation, but is it possible the mates of the deceased birds perished while foraging for themselves? Are brooding responsibilities shared by both male and female with these species? With no “relief” from brooding duties by the mate, did the one perish due to duty of incubation? Just wondering your thoughts on this theory.