Severe storms have the potential to kill large numbers of birds -- by hail, lightning, high winds, floodwaters and other associated phenomena -- although the animals are often capable of detouring around such disturbances or successfully taking refuge. It’s difficult to quantify bird mortality associated with tornadoes, the world’s most ferocious storms, because they’re fairly unpredictable and highly localized. Nonetheless, at least one well-documented event suggests tornadoes can occasionally result in mass bird deaths -- particularly for migrating birds caught over open water, a situation known to increase avian vulnerability to bad weather. Twisters, furthermore, certainly modify bird habitat.
1993 Louisiana Tornado
On April 8, 1993, a broad northerly frontal system set off a tornado over the barrier island of Grand Isle, along Louisiana’s Gulf Coast. In the days following, massive numbers of dead birds washed ashore along the island beaches and those of nearby Elmer’s Island and Port Fourchon. Researchers inventorying the carcasses estimated that perhaps as many as 40,000 birds representing 45 species, mostly neotropical migrants, may have been killed by the storm event. Indigo buntings probably suffered the greatest mortality; three species of conservation concern -- the Cerulean and Swainson’s warblers and the seaside sparrow -- were notably represented among the dead. Gauging the full effect was complicated by the prevalence of gulls, crabs and other scavengers swiftly processing dead birds; the likelihood that many carcasses were lost at sea; and the difficulty of locating carcasses of diminutive species such as the ruby-throated hummingbird.
Discussing the 1993 mass kill in a report published in the Journal of Field Ornithology, D.A. and M.G. Wiedenfeld noted that the high mortality likely resulted from the tornado’s confluence with the spring migration of neotropical warblers and other songbirds trekking northward from the American tropics. Such birds typically take off from the Central American and Yucatan Peninsula coasts at dusk, making the long, trying journey across the open waters of the Gulf of Mexico overnight and arriving on the U.S. Gulf Coast sometime the next day. The tornado’s outbreak in the early afternoon coincided with the peak arrival time for weary migrants along the coast, maximizing its lethality.
Wiedenfeld and Wiedenfeld theorized that most of the birds killed off Grand Isle were probably blown back out to sea by strong winds and ultimately drowned. The prevalence of dead birds of a given species found clumped together, the researchers noted, further suggested that incoming migrants of a kind were mostly affected by the localized, extraordinarily intense gusts of the tornado itself. A number of birds native to the area that were not found among the dead -- such as northern cardinals and mockingbirds -- are resident and terrestrial, which bolsters the idea that the great brunt of the mortality was taken by migrating flocks approaching from sea. The seaside sparrow was one of the few nonmigrants significantly counted among the dead, but it frequents coastal habitats and was likely blown offshore by the storm. Notable sex and age patterns in the dead birds -- for example, nearly all of the killed indigo buntings discovered were male -- seemed to reflect natural segregations of migration timing within a given species, although the researchers noted that individuals of a particular sex or age might also be better or worse at surviving a storm event.
While tornadoes may occasionally kill birds outright, they undoubtedly wield a much greater overall impact as modifiers of habitat. A few studies -- one in the Ozark National Forest of Arkansas, another in a Cross-Timbers woodland in Oklahoma's Tallgrass Prairie Preserve -- suggest that tornado damage to forests may influence bird distribution and diversity. A tornado ravaging a mature, closed forest opens up so-called “edge zones" where woods abut nontimbered patches; birds that require interior-forest habitat might decline in such gutted woodlands, while edge-zone specialists might increase. Tornadoes tend to kill bigger trees in forests they assault, which creates a preponderance of snags. These standing dead trees become important nesting sites for a variety of birds, from cavity-roosting species such as woodpeckers to large raptors such as bald eagles.
- Southwestern Naturalist; Effect of a Tornado on Birds in a Cross Timbers Community; Daniel J. McGlinn, et al.
- Journal of Field Ornithology; Large Kill of Neotropical Migrants by Tornado and Storm in Louisiana, April 1993; David A. Wiedenfeld, Melissa G. Wiedenfeld
- The Southwestern Naturalist; Effects of Tornado Damage on Forest Bird Populations in the Arkansas Ozarks; John W. Prather, Kimberly G. Smith
- The Migration Ecology of Birds; Ian Newton
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