Monday, June 22, 2015

The Case of the Disappearing Eggs

Text and photos © 2015 Adele Wilson, author of Bonding with the Barn Swallows

Unhatched Bluebird Eggs

Bluebird pair, May 12, 2015
If you are following this blog, you know that a pair of Eastern Bluebirds began nesting in Box #1 on May 10th.  The photo to the right shows the male and female Bluebirds on May 12th.  The female is holding straw to carry into the box.

The female Bluebird laid five eggs, one per day, beginning on May 19th.  However, as of June 6th, I had not seen the female around for a couple of days.  I did not observe her presence on the following days, either.

The five eggs never hatched.  It had become obvious that the female Bluebird had abandoned her nest without having incubated her eggs.

[Note:  At the end of this post is a timetable giving significant dates and observations.]

Tree Swallow Eggs

In the meantime, a pair of Tree Swallows had begun nesting in Box #2 on May 19th and the female had begun laying eggs on May 28th.  By June 3rd when I checked the box, I saw at least five Tree Swallow eggs.  There could even have been additional eggs that were being obscured by a feather.

Male on fence, female peeking out of box
Unlike the Bluebirds, the Tree Swallows were diligently incubating their eggs and protecting their nest.  Each day from my porch I would observe the male Tree Swallow guarding the box while perching on the fence several feet away from it.  The female would be inside the box, often peeking out of the entrance hole.  When the female went out to catch food, the male would take over incubation duty.

On June 9th, I checked the Tree Swallow box again.  The female was inside the box, but flew from it when I was about 5 feet (1.5 meters) away.  I opened the door and held a mirror above the nest, but all I saw was a heap of feathers.

Tree Swallows typically use feathers to cushion and protect their eggs; so I was not surprised.  However, I decided not to tamper with the feathers for fear of alarming the birds.  After all, from my observation on June 3rd, I already knew that the nest contained at least five eggs.

The New Bluebird Nest  

New female Bluebird, June 12, 2015
The male Bluebird continued hanging around the Bluebird box, even after his mate had left the premises and abandoned the eggs.  Fortunately, as described in my previous post, a new female Bluebird arrived at the box on June 12th.  What an exciting day that was!

The new female began carrying straw into the box, a sign that she was building her own nest on top of the old nest, that is, the nest that had been built by the previous female.

On June 15th, I decided to check the Bluebird box.  I opened the door, held up a mirror, and was shocked by what I saw.  All five unhatched eggs laid by the previous female were missing!

I was dumbfounded.  Had the eggs been removed by a predator – perhaps a bird, a snake, or even a human?

It had been a clean removal.  There was no evidence of crushed eggs.  There were no eggshells in the nest or on the ground below the box.  Eggshells or crushed eggs would have been evidence of a raccoon, sparrow, wren, or possibly a cat, having preyed upon the eggs.
I frantically began researching Bluebird nesting behavior.  I discovered that, when a new female Bluebird arrives at a previously built nest that contains unhatched eggs, she will either build a nest directly on top of the old eggs or remove the eggs and continue adding to the previously built nest.

The fact that the old, unhatched eggs had disappeared from the nest in such a clean, sanitary way therefore led me to consider the possibility that the new female had removed them.  At any rate, it was just as well that the old eggs had been removed because it freed me from the decision of whether to remove them or not.

Barring the possibility that a human had been responsible for the removal of the eggs, the only other type of predator that could have removed the eggs so cleanly would have been a rat snake.  Both of our bird boxes are mounted on 3/8-inch (0.95 cm) diameter poles, 4 to 5 feet (1.2 to 1.5 meters) above the ground.  Both boxes’ entrance holes are 1 9/16 inches (4 cm) in diameter.  The poles do not have predator guards; so predation by a rat snack could have been a possibility.

However, despite the fact that three different species of birds have nested on my porch each summer since 2011, I have never seen a rat snake on or near the premises.  I will “never say never”, though.

I concluded that when the new female Bluebird arrived on June 12th and began adding straw to the old nest, she decided to do some thorough house cleaning.  She must have removed the old female Bluebird’s unhatched eggs in preparation to lay her own.

That seemed like reasonable explanation for the old Bluebird eggs disappearing.

Checking the Tree Swallow Nest – the Surprise

On June 15th, just a moment after I had checked the Bluebird box (Box #1) and discovered that the unhatched Bluebird eggs were missing, I walked over to Box #2 to check the Tree Swallow nest.  As usual, the male Tree Swallow was perched near the box and the female was inside the box peeking through the hole.

But there was something very different occurring this time.  The female Tree Swallow inside the box was acting unusually jittery and skittish.  She was extremely alarmed that I was approaching her nest.  The photo to the left shows the way the female would usually look when she was peeking out of the box.  But on this day, I could see her twitching her head and scurrying about on the inside of the box.

I decided not to try to open the door on the Tree Swallow box.  The female Tree Swallow would have certainly flown from the box, but I didn’t want to upset her any further.

A day later, on June 16th, I decided it was again time to try to check the Tree Swallow box.  Based on my calculations of egg-laying dates and incubation times, the Tree Swallow eggs should have started hatching by that date.

To my dismay, the Tree Swallows were nowhere near their box.  Strangely, neither the male nor the female Tree Swallow had been around all day.  Each day for the past three weeks both the male and female Tree Swallows had been fiercely guarding their box.  But that was not the case today. 

I walked to the box, opened the door, held up a mirror, and saw what to me was a ghastly sight.  The nest was completely bare – no feathers, no eggs!!!
There was no evidence of the eggs having been pierced or crushed, and there were no eggs or fragments of eggs on the ground below the box.  The bottom of the nest was clean.  It had been a sanitary removal, identical to the removal of the eggs in the Bluebird box.

My conclusion was that whatever entity removed the eggs from the Bluebird box also removed the eggs from the Tree Swallow box.

Again, I thought about the possibility of a rat snake preying upon both sets of eggs – those in the Bluebird box and those in the Tree Swallow box.  The thought made me cringe.

The Aftermath – Thoughts and Theories

I became extremely depressed.  Now that the Tree Swallows had left the premises, there would be no birds around to swoop at would-be predators.  There are a number of feral cats roaming around, and the Tree Swallows had been doing a good job of keeping them at bay.

The photo above, from May 20th, shows the Tree Swallow box in the background with the Bluebird box in the foreground.  Although the photo does not show it well, the Tree Swallow box is about 75 feet (23 meters) from the Bluebird box.  We had tried to mount the Tree Swallow box at a closer distance, as is often recommended by Bluebird and Tree Swallow organizations, but we were unable to do so.  A distance of 6 feet (1.8 meters) from the Bluebird box had been thwarted because of a rock bed directly under the ground surface, and other locations were restricted by our landlord due to grass-mowing concerns.

Besides, the Tree Swallows had been able to happily nest in their box during the time period when the original female Bluebird had laid her eggs.  It was only a few days after the new female Bluebird arrived that the Tree Swallow eggs went missing.

New female Bluebird, June 14, 2015
Perhaps the new female Bluebird is more vigilant and cautious than was the previous female.  My theory so far is that the previous female was relatively inexperienced at nesting, while the new female is perhaps a little older in age and more aware of the hazards than can exist when nesting close to other birds.  Perhaps, therefore, the new female Bluebird had not only removed the old eggs from her box, but had also, in her desire to have no other birds nesting near her own nest, removed the eggs from the Tree Swallow box.

I have observed some different behavior by the new female that leads me to believe that she is indeed more experienced, and I will cover this behavior in a future post.

On June 18th, I removed the Tree Swallow nest from Box #2 and thoroughly cleaned the box with a 10% bleach solution from a spray bottle.  The box ended up being soaked, and on top of that we had a fair amount of rain afterwards.  I then decided to have my neighbor take the Tree Swallow box down for a couple of days so that I could bring it inside to dry out.  On the morning of June 21st, I remounted the box.

Time will tell if there is an egg predation problem occurring in this neighborhood.  If the new female Bluebird lays eggs and the eggs remain undisturbed, it will lessen the possibility that a rat snake or human preyed upon the eggs in both nests.  You can follow this blog to find out what becomes of the new Bluebird nest and whether other birds decide to nest in the now-empty Tree Swallow box.  All you have to do is enter your email address at the top right of this page.


May 10th – Bluebirds arrive at Box #1
May 19th – Female Bluebird lays the first of 5 eggs
May 19th – Tree Swallows arrive at Box #2
May 23rd – Female Bluebird lays last egg, incubation supposedly begins
May 28th – Female Tree Swallow lays the first of at least 5 eggs
June 6th – Female Bluebird has not been around for a couple of days, 5 unhatched eggs still in nest
June 12th – New female Bluebird arrives
June 15th – Discovery that unhatched Bluebird eggs had been removed from Bluebird nest
June 16th – Discovery that Tree Swallow eggs had been removed from Tree Swallow nest
If you have found this story interesting, you might want to check out my book, Bonding with the Barn Swallows.  Its 117 photos include closeups of the baby Barn Swallows that were hatched on my porch during 2011 and 2012.  There are also photos of the parent swallows guarding the nest and feeding their young.  As an extra bonus, the book includes photos of five different juvenile Barn Swallows, just ten days after fledging.  You will be amazed at their varied markings.  The book describes how one special male Barn Swallow communicated to me by his body language on the utility wire and how, only two days later, I discovered what he was trying to tell me.  To find out more about the book and read a preview, just click on the image to the right.



  1. How odd!
    My guess is a snake, as You say the others would have mnade a mess while trying to get to those eggs and You would most likely have seen marks around the entrance.

    I hope the Blue bird eggs will hatch and that the parents will be able to defend them.

    Have a great day!

  2. Hi Christer,

    Thanks so much for commenting. I am still hoping it wasn't a snake, but perhaps I am in denial. The new female Bluebird seems to have quite a courageous and assertive personality -- even more so than the male. The two Bluebirds have now started guarding and defending the second box as well as their own box. This is getting very interesting ...

    Hoping your shoulder pain calms down. Wishing you the very best for the rest of the week!