Speaking of Nature: A look at box occupancy

  • A nest full of healthy little bluebirds, like this one, is something I hope to see in Box 21 in the next week or so. FOR THE RECORDER/BILL DANIELSON

Published: 6/18/2021 5:59:57 PM

A deeper dive into the data leaves me with a better idea of “what” has happened, but no real understanding of “why” it happened. Specifically, I looked for data on box occupancy for the four nest boxes that I have maintained in my yard for the past 16 years. Other data that I examined included the arrival times for tree swallows and house wrens as well as the date of the last snowstorm of the year (an effort to take the weather into account). So here is what I found:

I was quite surprised to find that, although I had monitored the nests in the boxes I placed out in my yard, I didn’t keep written records of who was nesting in which box until 2014. I suppose this is fairly typical; you don’t record data if you have no idea that you might want it in the future. Something “clicked” in my head in the spring of 2014 and the notes started coming more frequently and with greater detail after that.

From 2014 to 2016, the species occupying each box remained stable. Box 21 was “owned” by eastern bluebirds, Box 22 was a home to tree swallows, Box 23 was the property of house wrens and Box 24 was occupied by tree swallows. Then, sometime between the end of the 2016 breeding season and the start of the 2017 season, something happened. I have no idea exactly what it was, but its effects were clear and chaotic.

In 2017, the bluebirds were simply absent. I know they were in the neighborhood, but they no longer showed any interest in the boxes in my yard. One possibility is the fact that my neighbor had also started to offer nest boxes and his habitat may have been more attractive to the bluebirds. Regardless, there was a great shakeup in the real estate market and the tenants were as follows: Box 21 had tree swallows, Box 22 was empty, Box 23 had tree swallows and Box 24 hosted a pair of house wrens.

Things changed again in 2018 with house wrens occupying Box 21, tree swallows in Box 22, house wrens in Box 23 and more house wrens in Box 24. I know that wrens have a habit of “occupying” as many cavities as they can, perhaps in an effort to reduce competition, so I have to admit that I don’t know if the nests also had eggs in them. Personally, I doubt there were active nests with eggs in three nest boxes so close to one another, but I can’t say for sure. The one interesting note that I found in my 2018 journal is the fact that eastern bluebirds did nest in Box 21 after the wrens had fledged and I had cleaned out the box.

It was a “disaster” year in 2019. House wrens occupied Box 21 and 23, while Box 22 and 24 remained empty. I cannot explain what happened that year, which is frustrating. 2020 was another year of change with tree swallows in Box 21 and 22, Box 23 empty and house wrens in Box 24 in a new location (leaning against my garage). Box 24 had been brought up for some maintenance and wrens moved in before I could get the box replaced, but they were very successful that year.

A look at arrival dates showed nothing of particular note. Tree swallows arrived between March 29 and April 9 between 2014 and 2020 with one notable exception of March 9 in 2016. This, you may remember, was the year of the great El Nino. Oddly, there was no change in the arrival of house wrens that year. Between 2014 and 2019 the arrival dates for wrens varied from April 19 to April 27. The only notable exception to that trend was in 2020 when wrens didn’t appear until May 14. This, however, I think I can explain.

Starting in 2015 and moving up to 2019 the dates of the last snowstorm of the year were as follows: April 23, April 3, April 1, April 19 and April 5. However, in 2020 we had a ridiculously late snowstorm on May 9 and the house wrens showed up five days later on May 14. This, at least, makes sense. Little insectivorous birds that hunt for prey in vegetation would be far less likely to find prey when there is snow on the ground. Still, I am just hypothesizing here. No way to tell.

This year, however, things seem to have returned to “normal.” It is important to remember that the location of Box 24 has changed, mostly because the wrens were so successful up against the garage and they have quite happily moved in again. They currently have chicks that are making so much noise that I can hear them from my kitchen window, which is a wonderful sound. Once again there are bluebirds in Box 21 and they are currently working on their second nest of the year. Once again there are swallows in Box 22 and their chicks are almost ready to fledge. Box 23 was moved this year and it has tree swallows with six healthy chicks. The “field” birds are in the boxes out in the open field and the wrens are in a box right up against a building.

So, what might have happened? Birds tend to show great site fidelity in places where they have had success breeding. Often, the same birds will return to the same nest sites year after year as long as things go well for them. Bernd Heinrich noted this in his book as he was able to identify birds that he “knew” who returned year after year. The maximum lifespan of an eastern bluebird is about 10 years and the maximum lifespan of a tree swallow is 12 years. If a dominant pair of bluebirds somehow lost one or both members, then the vacancy may have resulted in chaos. The end of El Nino during the winter of 2016-17 may have proven too difficult for the bluebirds that owned Box 21 and the following four years were predictably unsettled.

So, I find this “return to normal” particularly appealing after our own human lives have been unsettled for the past year. All of the regular species are back, they are all having tremendous success with their eggs and chicks and, as I noted earlier, the bluebirds have even started a second nest. With my summer vacation just a day away, I look forward to many happy mornings sitting in an Adirondack chair and photographing adult birds bringing food to their healthy, thriving chicks.




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