How Does Drought Affect Eastern Bluebirds?

Photo © Melissa Ivey

by Dr. Reneé Carleton, Professor of Biology, Berry College

Not Just A Western Problem

Paired Up
Paired Up

A lovely pair of Eastern Bluebirds surveys their territory.

We often think about drought as a problem afflicting western North America, but drought also impacts eastern states. Reduced or complete lack of rainfall for weeks or months can result in thirsty lawns, dying crops and other vegetation, and tough times for many animals. A critical reduction in those valuable resources for breeding birds is an obvious consequence.

Recent and recurring droughts here in Georgia prompted us to investigate how drought affects Eastern Bluebird reproductive success. My students and I maintain 40 nest boxes on our Berry College campus, so we had some data available. But Eastern Bluebirds breed throughout most of the eastern half of North America, where droughts are unpredictable in occurrence and duration. In order to see how drought affects the entire breeding range, we needed more data. NestWatch to the rescue! Thanks to the dedication of citizen scientists from more than 35 states and 3 Canadian provinces, NestWatch provided us with more than 26,000 Eastern Bluebird nesting observations spanning 7 years.

My colleagues and I examined drought impacts not only during the nesting period (egg laying through expected fledging date), but also impacts when drought conditions were in place 30 days and 60 days prior to clutch initiation. In other words, we wanted to see if there were critical periods in which drought affected reproduction. We also wanted to find out if drought severity played a role. We combined North American Drought Monitor drought severity data and a vegetation greenness index with NestWatch data to evaluate drought effects at each nest box location during individual nesting periods.

Drought Decreases Success
Drought Decreases Success

This 7-year study documented a decrease in hatching rate and fledging success under drought conditions. Clutch size was unaffected.

Drought Decreases Hatching and Fledging

We found that drought conditions, regardless of severity, did not affect clutch size. So, even though clutch sizes of Eastern Bluebirds typically decrease as the breeding season progresses, drought conditions present during laying (or up to 60 days before laying) do not result in females laying fewer eggs. However, we found that drought does have negative impacts on the hatchability of those eggs and the survival of nestlings. The number of eggs hatching and nestlings successfully fledging decreased with increasing drought severity. We also found that drought occurring 30 and 60 days prior to the expected hatching and fledging dates also decreased reproductive success. In other words, when drought occurs during incubation and when pairs are feeding their broods, Eastern Bluebird parents produce fewer surviving offspring and this gets worse as it gets drier.

Of course, there is more to this story than we explored. For example, how does drought actually cause the decrease in hatching success and nestling survival we found? The exact mechanism is unknown due to the large scale of our study. For instance, does drought eliminate much-needed prey items, or possibly increase embryo mortality?

From Backyards Like Yours
From Backyards Like Yours

A female Eastern Bluebird carries nesting material to her chosen nest box.

Thanks to You

Fortunately, Eastern Bluebird populations are in good shape, thanks in part to the nest boxes we provide, but what about other species of birds, especially those in decline? Adequate food resources and habitat are critical for the survival and reproductive success of breeding birds. The more we know about factors that negatively impact these resources, the better we can predict the consequences on birds that rely on them. And while we can’t control drought occurrence, we can continue to examine its effects on birds. Thanks to NestWatch, the contributing citizen scientists who monitor nesting birds, and supporters of the Cornell Lab and ornithological research, scientists like myself can further understand the impact of environment on bird population health. Without your efforts, this research wouldn’t be possible. Good work!


Reference:

  • Carleton, R. E., J. H. Graham, A. Lee, Z. P. Taylor, and J. F. Carleton. 2019. Reproductive success of Eastern Bluebirds (Sialia sialis) varies with the timing and severity of drought. PLoS ONE 14(8): e0214266. https://doi.org/10.1371/journal.pone.0214266

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10 Comments

  1. C Driver says:

    What put in Eastern Bluebird nest to protect them thru winter cold, snow. Think have 3 male babies returning

  2. Derick Driemeyer says:

    Please add the capability for us to send your information on to other birders

    • Holly Faulkner, Project Assistant says:

      Hi Derick, I’m not sure what you mean. If you’d like to share this post, please feel free to copy the URL in your address bar. If you have additional questions, please email us at nestwatch@cornell.edu

  3. Tom Hendricks says:

    I’ve had a nest box in my side yard here in Tifton, Georgia for years. This past year was the best ever… clutches of 5, 3, and 2. One in the last clutch did not hatch. The other 8 fledged.

  4. Darlene Pollard says:

    I was thrilled to have bluebirds for two years, and each time they had 3 broods in the same birdhouse. They came back this year, layed 6 eggs, all hatched, and the rain started. The nest got wet, babies died, and I haven’t seen a bluebird since.
    What can I do to increase the chances of them staying in 2020?

    • Holly Faulkner, Project Assistant says:

      Hi Darlene, While there’s no guaranteed way to entice them back to your yard, you can try providing mealworms at your feeders, and then see if there are any improvements you can make to your bluebird house. First, check out our Features of a Good Birdhouse infographic for tips on how to ensure your nest box is the best it can be, and check out our Eastern Bluebird nest box plan if you’d like to build a new one. We also have Mountain Bluebird and Western Bluebird nest box plans.

  5. Robert mills says:

    Do the Eastern bluebirds in upstate ny Adirondacks leave during the winter, and if so where do they go!

  6. Genny says:

    We have 2 bluebird boxes that have produced no less than 3 but more often 4 clutches per season. This year, 2019. We had 3 clutches, with 4-5 eggs in each, and none hatched. We were devastated.

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