Bluebirds Nesting Too Close

Eastern Bluebird - Photo Earl Harrison

Fifty-one seasons ago I read that nestboxes should be spaced at least one hundred yards from other boxes in order for our Eastern Bluebirds to nest in peace. My first season in 1968 is a long time ago and you would expect that by now, I would have seen everything there is to see as I have religiously monitored my nestboxes every season.

Until 2018, the closest distance between two successful bluebird nests on my trails was recorded in 2006 when two pairs of bluebirds nested 70.7 yards apart within the Panhandle Road Grid in the Delaware Wildlife Area. The grid was set up for Tree Swallows and consisted of six rows of nestboxes spaced with 25 yards between boxes and rows of boxes. When four boxes are spaced at 25 yards to make a square, the diagonal is 35.36 yards. The bluebirds had nested in boxes two diagonals apart for a distance of two times 35.36 yards, or 70.7 yards. I assumed that their hunting territories were separate and opposite.

June 18 and 22 were the first-egg-dates for the bluebird nests in 2006 and both nests became successful by fledging four and two, respectively. At the time, I was very impressed since both bluebird families had followed successful Tree Swallow families in the same boxes.

Other bluebirders have reported bluebird pairs nesting less than one hundred yards apart, but there were tall houses standing between their nestboxes. Our homes can effectively separate territories so bluebirds can comfortably nest in the front and backyards while being mostly unaware of their close neighbors.

On July 7, 2018, I was monitoring nestboxes at the Olentangy Environmental Control Center located on the Delaware County border with Franklin County in Central Ohio. The OECC is a sewage treatment facility for Delaware County and its eastern border is the Olentangy River. The nestboxes were installed to attract bluebirds and swallows in 1983 before I used measuring tape to layout locations. Most of the boxes had been “stepped off” and spaced to attract Tree Swallows.

Box 9

For hunting, bluebirds flew to the tree line in the background. The unlucky family lived in the nearest box, Box-9.

On July 7, I opened Box-9 to count three bluebird eggs. The next box, Box-8, held a bluebird nest made of pine needles that told me that the nest had been built during rainy weather. Wet pine needles keep their shape to make nest building possible, while wet grass loses its shape and collapses like shoe strings.

The second nest did not concern me at the time since “sister nests” do occur. Nine days later, I had a different reaction when I found four eggs in each of the two nests. I had never found bluebirds nesting so close together. I began to think like a scientist and ultimately planned to watch both nests to count the adults involved, etc.

On July 26, I found four nestlings in Box-8 and three nestlings and one unhatched egg in Box-9. On August 1, I returned to gather data. I selected a location about 50 yards away where I setup my spotting scope and a canvas director’s chair. I lined up my scope with both boxes and set its magnification at 15x so both boxes would be in focus so I could tell the sex of any bluebirds that landed on either box. I numbered each line in my composition book to represent minutes and started my observations at 9:33 AM. I recorded data until 10:25 when rain forced me to abort the project and retreat to my car.

Once the rain subsided, I returned with my 100-ft. measuring tape and measured the distance between the two nestboxes. The boxes are 94 feet apart, or 31.3 yards.

So, what did I observe at the site during 51 minutes? At Box-8, adult bluebirds landed and entered the nest cavity to feed young thirteen times. The male attended to his family eight times while the female fed her nestlings five times. Most food items were large enough to be visible with bare, green caterpillars being the most common.

Box-9 was another story. After no bluebirds landed on the box for 42 minutes, I had a strong feeling of doom. When a female bluebird finally landed on the front of the box with a large green caterpillar in her bill, I started to think positively, but not for long. The female leaned her head into the entrance, leaned back, and swallowed her intended food item. Her behavior told me that her nestlings were dead.

After I had loaded my equipment in my car, it was time for the last observation of my investigation. I opened both boxes to count four live nestlings in Box-8, and I found three dead nestlings in Box-9. The deceased had been dead for a day or more since I could smell a weak odor of death. Since I have salvage permits from the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife, I secured the unlucky nestlings in a sealed plastic bag and transferred them to my home freezer. They will be presented to the Ohio Wesleyan University Zoology Museum. At the least, their salvaged wings and tails can be used to teach feather emergence, etc.

The salvaged nestlings showed no evidence of violence so the reason for their deaths will remain a mystery. Nonetheless, as I checked Box-8, the parents were extremely vocal as they circled above me. They were a very active couple. Did they keep the other pair from feeding their young? I will encourage museum curators to weigh the remains.

Well, I guess that spacing nestboxes one hundred yards apart for nesting bluebirds remains a good management standard.

Conserve on through all the good and bad news!