Posted by: Sandy Morrissey | September 18, 2016

Weather Affects Bluebird Nesting Season

Weather always seems to be in the news lately and now it seems to have made a dent in our bluebird population this year.


Bluebird male fluffed up to stay warm in April

This was our 6th year of banding bluebirds and keeping close track of their nesting activity. Although we made a quantum leap in the number of nest attempts (going from a previous high of 115 to a whopping 137 nest attempts), our success rate crashed, going from an average in previous years of 80% success to a dismal 60%. This means we banded and fledged fewer bluebird babies. One good statistic that shows this is the average number of babies banded per nest attempt. For all previous years, the average was 3.3. The average in 2016 was 2.4.

Here’s what I think happened (though I have no scientific proof). We had an extremely mild winter. I don’t think the bluebirds ever migrated. They were fat and ready to lay eggs by early April. By the end of April, we had 56 nests with eggs (as opposed to an average of 34 in previous years).

However, if you recall, April turned extremely cold, with several days of snow and wind chills of 10 degrees. At the time I wondered how the eggs would survive this, and as time went on I learned. Many of those nests failed. The eggs were abandoned. After we removed the failed nests, the bluebirds built again and laid more eggs. Most of these nests succeeded at the usual 80% rate.


Nestbox with bluebird eggs, surrounded by snow

But we missed out again when it was time for the second broods. In the cases where the first clutch of eggs failed because of cold weather, the females were then actually laying their third batch of eggs. For many of our bluebirds, this was too much to ask, and they didn’t lay the third clutch of eggs.

 Additionally, we had many nests with abandoned eggs at the end of the season. I surmise the females laid the eggs with normal instincts, but by then it was late in the season and they lost interest along the way.

 As I said, I’m making this all up, but it seems to me the best explanation of our unusual bluebird season. If I am in the least bit right, then we can expect Climate Change to wreak havoc on all our nesting birds. Food supply has to be in sync with the nesting cycle. Almost all baby birds mainly eat insects and there are very few insects around when the temperature dips below freezing. Extreme heat presents other problems. While one fluky-weather nesting season doesn’t constitute Climate Change, it sure shows how unusual weather can affect a whole nesting cycle. Scary.



  1. Great blog! Thank you for all you do for the Bluebirds!

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