Birds are among the most easily observed wildlife species in Georgia because they are often tolerant of human presence and observation. Of the 407 bird species that have occurred in Georgia, about 90 are neotropical migrants, which means they breed in the United States and Canada but winter in Mexico, the Caribbean, or Central and South America.
Many of these birds are brightly colored and spectacular birds to observe, and their feats of prolonged endurance and navigation during migration are truly staggering. Imagine the Ruby-throated Hummingbird (Archilochus colubris), weighing less than a penny, flying non-stop over the Gulf of Mexico, a continuous 24-hour flight of about 500 miles. Other species, weighing less than an ounce, routinely migrate more than 10,000 miles a year.
Most of our neotropical migrants are songbirds, such as warblers, tanagers, flycatchers, and thrushes, although there are raptors, shorebirds, swallows and waterfowl that migrate to the tropics as well.
Why do so many birds fly so far each year when others can stay in one place? The primary reason for birds to migrate is to have access to food resources that are only available seasonally. If you are an insect eater, summer in Alaska provides an unimaginable “bug” feast, while winter offers starvation. In order to survive, birds must winter in areas with winter insects. By migrating north they also face less predators and competition than they would in the tropics, and are often able to raise more young each season.
HOW BIRDS MIGRATE?
Blackpoll Warblers (Dendroica striata), weighing less than an ounce, leave the coast of New England on clear autumn nights when winds are favorable and fly straight out over the North Atlantic. Their trajectory would take them straight to North Africa; however, the trade winds blow the flying birds back west, guiding them to landfall in the tropical forests of Venezuela after a 3-4 day flight. This astounding flight is carried out without stopping, sleeping, eating, or drinking. This is the equivalent of a human athlete running 4-minute miles for 80 hours straight. How are these tiny bundles of feathers, muscle and blood capable of such astounding feats of physical endurance? As importantly, how do they know where they are going and find their way?
The constraints of flight:
The ability to fly allows birds access to every corner of the globe. This ability comes at a great price however, as flight places major constraints on birds’ anatomy and physiology. The most obvious hurdle that migratory birds must overcome is that of keeping a very low body weight, while still being able to generate the power to keep them aloft. This remarkable balance is achieved by a number of adaptations, including a simplified skeletal structure, hollow bones, and large keel that anchors powerful flight muscles to the rib cage. Some shorebirds that fly non-stop over 6,000 miles of ocean allow their internal organs including intestines, kidney and liver to atrophy before the flight, further reducing their weight.
Despite all these adaptations, migration still provides a unique challenge that calls for extreme preparation. As the late summer days begin to shorten, hormonal changes are triggered that cause birds to dramatically increase the amount of food they eat. In a relatively short period of several weeks birds can nearly double their body weight in fat, the critical fuel they will burn on long migratory flights.
Today, it is hard to imagine navigating without maps, compasses, and Global Positioning Systems, yet birds have been accurately charting their migratory journeys for millennia without such aids. There are however, a number of environmental cues available to a migrating bird. The simplest form of navigation involves following known features such as rivers, coastlines and mountain ranges. Other birds are able to use the stars, solar position, their sense of smell, and even the earth’s magnetic field to navigate. With this combination of tools, birds are able to navigate with remarkable accuracy, allowing many to return to the same territory year after year.
HUMANS AND BIRDS
Humans have certainly noticed and appreciated birds for thousands of years. This appreciation most likely focused on the culinary and clothing potential of birds. Exploitative market hunting of birds for food and fashion raised concerns about bird populations as early as the 1840’s. This concern was warranted, as evidenced most tragically by the extinction of the Passenger Pigeon (Ectopistes migratorius) and Eskimo Curlew (Numenius borealis). Laws passed in the early 1900’s provided legal protection for all of our native non-game bird species. Now it is illegal to even own feathers, nests or eggs of any of most birds.
Greatly aided by the availability of pocket field guides and affordable optics, the non-consumptive enjoyment of birds has dramatically increased since the days of market hunting. Bird watching is currently one of the most popular outdoor recreation activities in the United States, with 46 million people spending $32 billion annually on bird related sales. This in turn has generated 863,406 jobs nation-wide.
Though many of these species spend most of the year in the Tropics, they are very much a part of temperate ecosystems in the summer. Their ecological and economic value is so great that it is difficult to calculate. Literally tons of insects are consumed by neotropical migrants each summer and many plant species depend on these birds to disperse their seeds and pollinate their flowers. Many of these birds are also sensitive indicators of environmental health.
Anecdotal evidence of bird population declines has recently been confirmed for many species with the analysis of data from several nation-wide bird surveys. The most important is the Breeding Bird Survey that has collected data on birds since 1966 at about 2,000 sites throughout the United States and Canada. Though it is impossible to generalize about all species and regions of the country, most biologists agree that many species, especially in the eastern United States seem to be facing significant population declines. Partners in Flight, a bird conservation organization, has placed 100 species of conservation concern on a Watch List due to low or declining population trends.
Though market hunting is a thing of the past, birds still face a number of threats. The loss and fragmentation of habitat associated with urban sprawl and development is probably the most serious threat to most bird species today. Fragmentation of habitat breaks suitable habitat into smaller and smaller patches, often allowing predators and nest parasites better access to nesting birds. An added complication for protecting neotropical migrants is that by definition, they are highly mobile. To fully protect them, we must preserve not only their breeding grounds, but migratory stopover sites and wintering habitat as well. Because these birds winter in other countries, international cooperation is critical to their preservation.
Beyond habitat loss or alteration, there is an increase in dispersed mortality factors that accompany the growth of human populations and technology. These include a wide variety of problems, from bird collisions with communication towers and buildings, to predation by house cats and mortality from pesticides.
House cats are often considered the largest predator on songbirds in the United States. Several estimates suggest that cats kill hundreds of millions of birds a year in the United States alone. This has led the American Bird Conservancy to start a Cats Indoors program to encourage more responsible pet ownership.
Another example of the challenge of protecting Neotropical Migrants was a massive hawk die-off in 1996 in Argentina, where DDT killed an estimated 20,000 Swainson’s Hawks (8% of the world’s population). DDT had been illegal for over 20 years in the United States, but is still heavily used overseas. This example points out the challenges both of pesticide use, and the need for international cooperation to protect migratory birds.
WHAT IS BEING DONE?
The Georgia Department of Natural Resources is committed to the conservation of all wildlife species, and is involved widely in the conservation of our neotropical migratory birds. The Non-game Wildlife Section of Georgia’s Wildlife Resources division is involved in education, research and monitoring of threatened species, ongoing management of habitat, and acquisition of important habitat for wildlife. The Non-game section has helped establish birding festivals and trails to promote awareness of our neotropical migratory birds. These education efforts include the Pinewoods Bird Festival, and the Colonial Coast Birding Trail. Research projects are currently underway to establish management strategies for some of our most threatened migratory birds, such as Golden-winged, Cerulean and Swainson’s Warblers. We are also involved in a number of surveys to monitor populations of our migratory birds. WRD works with private landowners to help them manage land for our migratory birds. A good example is the collaboration with large landowners to conserve breeding habitat for Swallow-tailed Kites. Critical new habitat is also purchased or protected in order to preserve or create habitat for migrants. Our Game Management section also manages large amounts of land for game species, which often provides excellent habitat for our neotropical migratory species as well.
Prairie Warbler by Deb Zaremba
For more information visit www.georgiawildlife.com