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