Sunday, June 28, 2015

Bluebird Egg Incubation in Progress!

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

Wonderful news!  The female Eastern Bluebird is still incubating her eggs.  That's a good thing because our temperatures have been cool for the last couple of days.

This morning, from a distance of about 20 feet (6 meters), I was trying to photograph the nest box with the female Bluebird inside.  Suddenly, unbeknownst to me while I was pressing the shutter, the male Bluebird landed on the top of the box with a worm in his beak!  If you look carefully at the photo on the upper right, you might be able to see a hint of the female Bluebird's head inside the box.

Apparently, the male Bluebird trusts me.  This does not overly surprise me because he often stations himself in one spot and watches me for quite a long time.  The other day, he perched on the fence and observed me the whole time while I was refilling the bird feeders.

Below is a close-up of the male Bluebird holding the worm.  In the background you can see blooms on a mimosa tree.

The male Bluebird had not flown to the box just to feed himself.  As the photo on the right shows, he flew to the entrance hole of the box and proceeded to feed his mate, who was still inside the box.

So far, so good!  I am hoping that this nesting attempt will be successful without the female abandoning her eggs as the first female did.

I do not know how many eggs are currently in the nest, but on Sunday, June 21st, there were four eggs.  Once I feel that it is safe to check the inside of the box without scaring the female Bluebird away, I will find out how many eggs she has laid.  From that information, and assuming that she laid one egg per morning (as is usually the case), I should be able to calculate approximately when the eggs should start hatching.

A female Bluebird is said to begin full-time incubation of her eggs either on the day she lays her last egg or on the preceding day.  So, once I find out how many eggs are currently in the nest, I can calculate the approximate date on which the eggs should begin to hatch.  In the following paragraphs I shall describe how this calculation is performed.

For instance, if I find that there are currently five eggs in the nest, knowing that the female laid her fourth egg on June 21st, and assuming that she laid one egg on each successive morning, I can assume that she laid her last egg on the morning of June 22nd.  If there are six eggs currently in the nest, the assumption would be that she laid her last egg on June 23rd.  The average number of eggs in a Bluebird clutch is five, but that number can sometimes be as high as seven.

In the former case (if there are five eggs currently in the nest), the female would have begun incubating her eggs either on June 22nd or on the preceding day.  In the latter case (six eggs currently in the nest), the female would have begun incubating either on June 23rd or on the preceding day.

The final bit of knowledge required in order to estimate the hatching date is that the average incubation period for Eastern Bluebirds is fourteen to fifteen days.  Therefore, assuming that there are no more than six eggs currently in the nest, the eggs should begin hatching sometime between July 5th and July 8th.

To the right is a photo of what Eastern Bluebird eggs look like.  I took this photo on June 6th after the first female had abandoned the nest.  She had not been in the area or inside the nest box for several days.  The eggs were due to hatch soon, but she had not incubated them.  These eggs disappeared within a few days after the new female arrived on June 12th.  My assumption at this point is that the new female removed them.

All of the eggs in a Bluebird clutch are said to hatch within one or two days of each other.  Once the hatching date(s) are known, the ages of the nestlings can be calculated, and that is the most important thing to know in the monitoring of Bluebird boxes.

It is important to be able to track the ages of the nestlings in order to avoid opening the box when the chicks are twelve or more days old.  Opening the box during this time period can cause the premature fledging of one or more of the chicks, which could result in their demise.

All for now!  You can follow this blog by entering your email address in the upper right corner of the top of this page.
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. So they could be hatching right now when I'm reading this :-) I do hope everything goes well this time!

    Why don't You, until next year, get one of those tiny webcams that shows what happens live in the nest? They are so tiny now days that they can be placed so the birds won't get disturbed by them and You'll be able to see how they grow every day.

    Have a great day!

  2. Thanks so much for commenting, Christer! Yes, there are now babies in the nest! I checked on July 8th and observed one of them, but there could have been others. I checked again today (July 14th) and observed three, but there could have been a fourth. And yes, I am considering a web cam, but it would not be until next spring. More about the babies on my next blog post.

    Hoping you are enjoying your vacation!