by Dr. Reneé Carleton, Professor of Biology, Berry College
Not Just A Western Problem
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 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?
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!
- 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
What put in Eastern Bluebird nest to protect them thru winter cold, snow. Think have 3 male babies returning
Hi there. We have an article here about how to winter-proof your birdhouse so birds can use it as a winter roost/shelter.
Please add the capability for us to send your information on to other birders
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 firstname.lastname@example.org
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.
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?
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.
Do the Eastern bluebirds in upstate ny Adirondacks leave during the winter, and if so where do they go!
Hi Robert, Populations of Eastern Bluebirds in the northern part of their range, including the Adirondacks, are entirely migratory, spending winters in the southeastern United States or Mexico. You can see a range map and learn more here.
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.
July 2020: We live in western suburb of Boston and have a clutch of ten eggs in a house we’ve had for 5 years or so (the birds keep coming bac). What are the chances all will survive and if we put out mealy worms, will it help?
Hi Jen, Bluebirds typically have a clutch size of 3-5 eggs, so it’s likely that more than one bird is laying eggs in this nest (unless it is another species, such as a House Wren or chickadee). Extra large clutch sizes often means that some will not hatch because they can’t be incubated properly. The best thing to do is to let this nest continue on naturally. You can put out food in your feeders if you like, though be sure the feeders aren’t too close to the nest boxes as it can attract predators to the nest box. If you’d like to reduce competition, we recommend installing another box on your property – though each species has their own requirements for spacing (i.e. Eastern Bluebirds need 300 feet between their nest and another Eastern Bluebird nest, but they don’t mind other species nesting closer). You can view these species preferences on the species pages of our Right Bird, Right House tool.