David M. Larson
Golden-winged Warbler (Photograph by Dennis Cooke, CC BY-NC 2.0)
One of the great technical advances of recent times that has allowed for fine-tuning of our understanding of many aspects of bird behavior has been the invention and improvement of geolocators. Often deployed on birds captured on their breeding or wintering grounds, these recorders can provide data on the location of birds for up to a year. They can be useful in helping to plot the annual movements of individual birds and the differences in migratory patterns of subpopulations of a species (Larson 2014). The geolocators are small and have no transmitter capability, so the birds have to be recaptured in order to read the data.
An important skill in science is the ability to recognize the existence and importance of serendipity in data collection. When you combine geolocator data with serendipity, you may discover some interesting phenomena. And this is the crux of the article described here.
Streby et al. (2015) were carrying out a fairly standard migration study on Golden- winged Warblers when serendipity hit. Several of their geolocator-carrying warblers arrived on the breeding grounds in eastern Tennessee between April 13 and April 27, 2014, following migration from eastern Colombia. The researchers captured five of the birds on May 3–9 and read the geolocators. The data revealed that rather than stay on territory, the birds departed on April 26-27 and flew toward the Gulf of Mexico. Between April 27 and April 30, a huge storm system swept through the breeding-ground region, producing 84 confirmed tornadoes, 35 human fatalities, and over a billion dollars in property damage. After the storm passed, on May 1-2, the five tracked warblers returned to their breeding grounds and resumed defending territories. Without the geolocator data, the researchers would not have known that the warblers had flown some 700 kilometers from the breeding grounds apparently to avoid this oncoming storm. The researchers had evacuated the storm path as well.
Although each of the warblers carried out this evacuation using a different route, their general flight paths corresponded to their normal fall and spring migration routes via the Gulf of Mexico coast and, in one case, along the Florida coast to Cuba.
Golden-winged Warblers are obligate migrants, meaning that migration occurs on a regular schedule and is considered to be innate or genetically determined. This migration pattern is in contrast to facultative migration, which is triggered by acute weather changes or food availability. Although obligate migrants can delay, alter, or reverse parts of their migration path because of weather or other external stimuli, it was not clear that obligate migrants could undertake facultative migrations. Yet this is exactly what appears to have happened with these five warblers.
It seems inescapable that these male Golden-winged Warblers performed a brief facultative migration of over 1500 kilometers in order to avoid the oncoming storm system. They then returned to defend their breeding territories throughout the remainder of the season. The trigger for their exodus remains speculative. At the time of their departure the weather patterns onsite were not unusual; temperature, atmospheric pressure, wind speed and direction, cloud cover, and precipitation were all unremarkable. The storm system was still 400–900 kilometers distant. The remaining environmental clue that seems most likely to have provided the trigger was infrasound, frequencies below the range of human hearing. Infrasound can propagate at over 100 decibels thousands of kilometers ahead of such storms, and the frequencies involved are well within the range of bird hearing.
The most reasonable explanation for this behavior of the warblers is that they heard the sound of the approaching storm system and decamped to avoid personal risk. They returned when the coast was clear and continued on with their breeding season. Serendipity came into play because the birds involved were carrying geolocators that were recovered, revealing unsuspected and unexpected behavior.
References
- Larson, D.M. 2014. Gleanings: Bugging the Birds: Tracking Individuals Through Migration. Bird Observer 42 (6): 377-9.
- Streby, H.M., G.R. Kramer, S.M. Peterson, J.A. Lehman, D.A. Buehler, and D.E. Andersen. 2015. Tornadic Storm Avoidance Behavior in Breeding Songbirds. Current Biology 25: 98-102.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon s Birder s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, Vice President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.