Blog #422 Nature Can Be Brutal: Hurricanes During Migration

Hurricane Ian, courtesy of NOAA.

Each year in late August, September, and October, billions of birds face perilous migrations as they travel thousands of miles to wintering grounds south of the U.S. during hurricane season. Such storms' impact can be deadly for birds and humans, as whole colonies of young birds may be wiped out and their habitats destroyed. Nature can be brutal.

This week’s Category 4 Hurricane Ian ripped through the Gulf of Mexico, hitting the subtropical barrier islands of Sanibel, Captiva, and Fort Myers Beach with its full force, leaving massive destruction at the height of the fall bird migration. Sanibel is home to the J.N. "Ding" Darling National Wildlife Refuge, part of the largest undeveloped mangrove ecosystem in the U.S. The refuge is world famous for its spectacular migratory bird populations and Florida’s iconic shorebirds and wading birds. Sanibel also hosts a large resident Osprey population as well as migrating Ospreys from the north.

How do birds weather such storms, and what long-term impacts do such storms have on resident and migratory birds? These are complex questions, but the key bird mortality factors are a storm's timing, location, and strength. Normally, these factors do not all line up in a perfect storm scenario, so while there is ample bird mortality during almost all major storms, it usually does not occur on a massive scale. To look deeper into the effects of hurricanes on birds, the seminal 1993 paper by James E. Wiley and Joseph M. Wunderle, entitled “The Effects of Hurricanes on Birds, with Special Reference to Caribbean Islands,” is a good starting point.

A juvenile Osprey from Salt Point awaiting migration, courtesy of Cynthia Sedlacek.

Wiley and Wunderle looked at the direct impacts of the storm’s wind, rain, storm surges, and wind-driven displacement of birds, sometimes up to thousands of miles from their natural habitat.

Hurricane winds can drive birds far out to sea where they can weaken and drown. Oceanic birds are often carried far inland to the southern and eastern U.S. Dramatic examples include the Magnificent Frigatebird and Black-capped Petrel driven by hurricanes from the Gulf of Mexico and the southeastern coast of the U.S. north to the Great Lakes. Unfortunately, recently fledged birds that have not yet developed good flying skills are particularly susceptible to strong winds, often fatally.

Ospreys and other birds can easily withstand normal storms by sheltering in the vegetation. High hurricane winds however damage almost everything in their path, ripping up trees and bushes, and fatally tossing birds into hard objects. The torrents of drenching rain can also give the birds deadly hypothermia.

There are profound indirect post-hurricane impacts from the denuding of the landscape, destroying the trees and vegetation needed for food, nests, roosting, shelter, and protection from predators. Birds eating nectar, fruit, or seeds are particularly hard hit. Birds that depend

Many Osprey nest trees were lost on Sanibel to Hurricane Ian, courtesy of YouTube.

on larger, older trees for nesting, roosting, or foraging, such as the south’s resident Ospreys can be similarly impacted. Bird species with small populations or that depend on habitat fragments may suffer especially serious losses. For example, Hurricane Hugo (September 1989) destroyed over 40 percent of the endangered Red-cockaded Woodpecker’s nesting trees in South Carolina.

How can birds avoid storms? Birds are sensitive to changing air pressures and try to avoid storms by flying when the wind is favorable, just after the passage of a low-pressure system. Air in a hurricane moves counterclockwise. After the system passes to the east, it often brings in winds from the northwest, which the birds take advantage of to avoid hurricanes.

Low-pressure systems with hard rain and wind cause them to seek shelter in trees, hedges, and other places. Birds seek warm places to shelter during storms to avoid hypothermia brought on by the colder temperatures that accompany the rain. Flying, especially in rain, takes a lot of energy, and birds typically rest to conserve their energy in colder temperatures. Large storms such as hurricanes generate long-distance, low-frequency infrasonic waves (<20 hertz) that cannot be discerned by the human ear. These long-distant auditory warnings give most birds and other wildlife ample time to prepare or evade the storm’s wrath. Presumably, these sounds warn Ospreys and other birds of the approaching storm so that they can avoid its path.

How do larger birds travel with hurricanes? Larger birds such as geese, herons, gulls, and raptors, able to maintain flight in wild gale winds, can utilize hurricanes during migration. The birds get into the end of the hurricane’s spiral and, by chance, move with it toward the eye of the hurricane. When the storm reaches land, some of the birds may fight the winds to escape the storm while others may travel with the eye until the hurricane dissipates. The majority of seabirds, with their superior powers of navigation, find their way back to shore quickly if they are not too weakened from having flown for so long without food.

Captiva Island Nest Cam view of Osprey on its nest during Hurricane Ian, courtesy of Mary Ann Steggles.

Hurricanes can be beneficial in some circumstances. Severe storms hitting dry habitats such as those that occur in the Caribbean's Greater Antilles can be a boon for insect-eating birds. Such storms deliver weeks' or months' worth of rain and food to a normally drying forest, which improves the over-winter survival and condition of resident and migratory birds.

What should NYC expect after Hurricane Ian hits Florida? Forecaster details the latest. - silive.com