Department of Integrative Biology, University of Colorado Denver
On the morning of June 23rd, 2014, I got a call from one of my collaborators, David Augustine. It was Monday, and I was on the way to my field site in northern Colorado. David told me that the site had been hit by an intense storm the night before, with hail stones large enough to break windows on cars and houses. He suggested that I check all of the nests I had been monitoring. I would come to find that the storm had caused high nest mortality. Although the 2014 hail storm was devastating for the birds nesting there, it provided a valuable opportunity to study an infrequent but important source of mortality in grassland breeding birds. It also offered a rare chance to demonstrate how remotely sensed weather data can be used to estimate hail-related avian mortality.
My graduate research project involves monitoring songbird nests. Most of the species I study nest on the ground, with little protection from inclement weather. The most common cause of nest failure is predation, and I had set out to study how habitat patterns affect predation probability. While storms can kill birds, documenting weather-related mortality is much more difficult than documenting predation. That is because predation is common and widespread, while the timing and location of severe storms are challenging to predict. The zone of highest intensity often occurs where birds are not being monitored, and the evidence of mortality would disappear too quickly for direct ad hoc assessment. Heavy rain and small hail appear not to be fatal for ground-nesting birds, so studies must focus on large hail, a highly unpredictable phenomenon.
When I got to my field site that Monday, I gathered my crew and we spent the next eight hours checking every nest that we knew about. Of 107 nests under observation, we found that 69 had been destroyed by hail. We found crushed eggs, dead chicks, and dead adults. I was responsible for monitoring nests in only one half of the study area, while another crew monitored nests in the other half. I was amazed to learn that almost none of their nests had been destroyed in the hail storm.
In examining Doppler weather radar data for the June 22nd hail storm, we discovered that the base reflectivity pattern matched up very well with the pattern of nest mortality. Probing further, we found that advanced storm metrics including azimuthal wind shear and maximum estimated size of hail were even more precise in explaining where nests had failed and where they had not. Mortality probability increased with hail size, but wind shear augmented hail impact, explaining more variation in mortality probability than hail size alone. Collecting ground-truth data on hail-related avian mortality remains a significant logistical challenge, and our study demonstrates that remotely sensed weather data can serve as an accurate and more feasible metric of hail-related mortality. In combination with breeding bird density surveys, these weather data can improve annual mortality estimates for birds breeding in hail-prone areas.