Hurricanes Present Another Threat to Birds Before and During Fall Migration

Magnificent Frigatebird by Greg Homel / Natural Elements Productions

(Washington, D.C., August 8, 2014) Hurricanes Iselle, Julio, and Arthur are the first of what likely will be more powerful late summer and early fall severe storms that billions of birds may face as they wind up their breeding seasons and prepare for perilous migrations that can involve travelling thousands of miles to wintering grounds south of the U.S.

“While human safety is always the primary concern, with hurricanes, the impact such storms bring can be deadly for birds as well. Depending on circumstances, whole colonies of young birds may be wiped out — a whole breeding season gone. Parents tending young may stay with a nest and perish rather than abandon it for safer grounds. Nature can be brutal,” says Dr. George Wallace, Vice President for the Oceans and Islands Division at American Bird Conservancy (ABC), one of the nation's leading bird conservation groups.

Wallace, who conducted his doctoral research on migratory and resident birds in the northern keys of Cuba, including during a period immediately following a major hurricane, says the question about how birds weather storms comes up every year around this time. “It is a complex question that has many variables, but the key bird mortality factors are timing, location, and strength of a storm. Normally, these factors do not all line up in a perfect storm scenario, so while some level of mortality takes place during almost all major storms, it usually does not occur on a massive scale,” he said.

“Because Hawaiʻi has suffered such massive extinctions of native birds, the two major storms threatening  those islands are of great concern. Several bird species such as the ʻAkikiki and the ʻAkekeʻe are already hanging on by a thread so we are keeping our fingers crossed that they escape with minimal impacts,” Wallace said.

Wallace says a study published in 1993 by James E. Wiley and Joseph M. Wunderle, “The Effects of Hurricanes on Birds, with Special Reference to Caribbean Islands,” is one of the most widely referred-to studies on the subject.

Direct impacts from storms come from wind, rain, storm surges, and wind driven displacement of birds from their natural habitat. Such displacements can move birds miles, hundreds of miles or even thousands of miles. Oceanic birds are often driven inland considerable distances by hurricanes in the southern and eastern United States. Dramatic examples include species such as the Magnificent Frigatebird and Black-capped Petrel driven by hurricanes from the Gulf of Mexico and the southeastern coast of the United States as far north as the Great Lakes. Recently fledged birds that have not yet developed good flying skills are particularly susceptible to strong winds, often fatally.

Indirect storm impacts to birds can include loss of food; destruction of nests, nest sites, and roost sites; increased exposure to predation; and changes to microclimates in a bird's immediate environment. Those changes often result from trees being blown down or stripped of vegetation which, among other things, allows more sunlight to the ground and alters foraging habitat for birds.

Strong storms that denude trees and other vegetation can also have a profound post-hurricane effect on birds that feed on nectar, fruit, or seeds. Birds that depend on larger, older trees for nesting, roosting, or foraging can be similarly impacted. Bird species with small populations or that depend on habitat fragments may also suffer serious losses.

For example, populations of the endangered Red-cockaded Woodpecker declined severely following Hurricane Hugo in September 1989. In the Francis Marion National Forest in South Carolina, it was estimated that Hugo caused the death of over 60 percent of the woodpeckers in that location. This was due in large part to the loss of nearly 90 percent of the trees hosting the nesting cavities the birds depend on for breeding, roosting, and protection. Bald Eagles, which often nest in large pines, were similarly impacted by Hugo with over 40 percent of nest sites in South Carolina destroyed.

While birds can easily withstand normal rain events, hurricanes unleash drenching torrents of water and high wind that, apart from potentially fatally tossing birds into hard objects, can also cause them to become soaked to their skin, leading to deadly hypothermia. Additionally, those winds can drive birds far out to sea where they can become weakened to the point they succumb to drowning.

An account by French explorer, clergyman, and botanist, Jean-Baptiste Labat from the late 1600s during his time on Martinique is particularly evocative. “…we saw clouds of ramiers [Scaly-naped Pigeons], parrots, and grives [thrushes and thrashers] coming from Dominica, so beaten by hunger and fatigue that some fell in the sea, others on the sand, others in our fields, and others did not have the strength to stay on the branches where they perched upon arrival.”

Wallace said that while hurricane impacts are largely negative, some good can occur under the right circumstances. “A late storm that hits an area of dry forest such as those that occur in the Caribbean's Greater Antilles can be a boon for insect-eating birds. Such a storm can deliver weeks' or months' worth of moisture and food to a normally drying forest and can improve over-winter survival of both resident and migratory birds and result in migratory birds that are stronger and in better shape to begin a spring migration.”