In Spring Skies: A River of Birds
"I never for a day gave up listening to the songs
If, like Audubon, you enjoy the morning songs of birds awakening to a new day, and find pleasure in seeking them out at their hidden perches, then Big Bend in the spring is the right place to be. For years people have flocked to Big Bend National Park in the spring, drawn by the opportunity to see an incredible diversity of birds that passes through in the annual migration. Over the years observers have reported 446 species in the park. Of this total, nearly 190 (42 percent) are migrants headed north to their breeding grounds. Many of these species represent a special group called “neotropical” migrants, referring to birds that nest in the United States and Canada, and winter south of the United States. Within this group are the “vagrants,” birds off course and wandering far from their normal range or migration routes. During the spring of 1996 two species listed as “hypothetical” on the park list visited the area, much to the delight of the lucky few who were able to see these rarities. For two days in March a violet-crowned hummingbird visited the Barker House near Rio Grande Village. A Mexican species whose breeding range extends into the United States normally only to southeast Arizona, the hummingbird seen here represents one of only four sightings recorded in Texas. Not quite so far out of range but never before fully documented in the park, a Swainson’s warbler seen May 4th in Boot Canyon provided a number of detailed reports for park records.
Even more intriguing were the observations of two individual flame-colored tanagers, another Mexican species that had previously been recorded in the United States only in southeast Arizona. These two individuals were well documented with photographs and represent the first-ever Texas record. With potentials like these, it’s no wonder that people come from all over the world to tap this “river of birds,” add new species to their lists, and marvel at the phenomenon of migration.
Migration may be defined as an extensive, seasonal movement between breeding regions and wintering regions. Birds do not hold exclusive domain in the area of migration. Certain insects migrate, as do some mammals, like the Mexican long-nosed bat found in Big Bend. For most people though, the word “migration” brings to mind birds. Approximately half of the world’s 9,000 bird species are migrants. In North America, approximately 600 of the 800-plus species recorded are migrants. We tend to think of the birds in the backyard as “ours,” but migrants really are a shared gift. While many people take notice of the seasonal changing of bird communities in their backyards, few pause to consider what a tremendous feat those migrant birds have accomplished.
Some species travel thousands of miles, crossing over large bodies of water or vast expanses of arid land. Along the way there are many dangers that can result in death. Predators, extreme weather, and obstacles such as power lines, all take their toll. The very act of migrating requires a tremendous expenditure of energy. Migrants can lose anywhere from 26 to 44 percent of their body weight during the passage from south to north. Many fall exhausted and dying after fighting strong head winds and storms. Even if a migrant bird makes it to the breeding area there is still no guarantee of success.
Birds that migrate give up their territories each year, and must reclaim them at a cost of energy. Not all individuals are successful in securing a territory and finding a mate on the breeding grounds. Those that do find a mate still have trials to overcome. A study of nesting American redstarts in New Hampshire found that up to 70 percent of redstart eggs and nestlings were lost to predators, even in relatively intact forest habitat. Given these odds and the perils of the passage, it’s a wonder that birds migrate at all. That they do is an indication that there are advantages to this behavior. Some are immediately obvious. Birds that migrate take advantage of better climates for living. This is especially true for long-distance migrants. In moving between areas, migrants exploit different habitats for food. A variety of habitats means a greater variety of food and increases the probability of a more nourishing diet throughout the year. Some migrants change their diet to take advantage of seasonally abundant food sources in the two areas. The Nashville warbler, a fairly common migrant through Big Bend in the spring, is mainly an insectivore during the breeding season. When it arrives on wintering grounds in central Mexico and Guatemala, the warbler will switch to feeding on nectar from flowers just coming into bloom. While abundant food and hospitable climates are easily identified benefits of migration, others do not become obvious until we look at a map.
Most of the spring migrants that pass through or stay to breed in Big Bend spend their winter in Mexico and Central America. Only a few, like the Swainson’s hawk, yellow-billed cuckoo, and cliff swallow, winter as far away as South America. In fact, almost 50 percent of the migrant land birds of North America winter in just five countries: Mexico, the Bahamas, Cuba, Haiti, and the Dominican Republic. Compare the land mass area of these five countries to that of Canada and the United States. The combined area for the five countries is approximately 836,000 square miles. That of Canada and the U.S. is 6,230,000 square miles. Wintering birds are more tolerant of close association and often form mixed flocks of several species. When these same birds are ready to breed, their tolerance level goes way down. By migrating to a North American breeding ground they find more room and expend less energy establishing a territory than if they stayed in the crowded wintering ground. Other benefits of geography have less to do with available space and more with available light.
We all are aware that summer days are longer. Most of us even participate in the annual ritual of turning the clock forward in spring to “save” one more hour of precious daylight. Migrants traveling north in the spring are practicing the same principle. The longer days of the northern summer provide longer working hours for adult birds to forage for food. Even though the summer season is short the longer daily period of food gathering allows birds to raise their young more quickly. This is very important as birds are most vulnerable to predators and parasites while they are in the nest, either as brooding adults or nestlings. Given that the breeding season is short and the time required for migration long, most neotropical migrants have only enough time to raise one brood. If the brood is lost to predators, storms, or disease, then the entire season is a disaster. Despite the many perils involved and the possibility that either the adults or the young will die, migration continues. For millions of years migration has been a successful response to changing environmental conditions. This success story though, may be changing.
Since 1966 the U. S. Fish and Wildlife Service has conducted Breeding Bird Surveys (BBS) across the country. Over the years the accumulated data have provided a fairly accurate picture of the status of breeding birds in the United States. In the first years of the survey observers witnessed huge migration fronts and a healthy diversity of species. Beginning in the 1970s and accelerating through the 1980s and 1990s, declines began to be documented in many populations. In Texas surveys, neotropical migrant species declined by 54 percent between 1980 and 1989. In an effort to pinpoint causes many researchers have taken to the field, often following the migrants to their winter homes. Though the problem is complex and much is still not known about migrant species biology, several potential factors have been identified.
Initially much of the blame for the decline was attributed to deforestation on the wintering grounds. There is no question that forest removal in Latin American countries is occurring at a rapid rate. Some estimates for the felling of mature tropical forest are as high as 74,000 acres per day. For species that rely on mature tropical forest for wintering habitat the impacts are bound to be negative. The summer tanager, a migrant and breeder at Big Bend, winters in tropical lowland forests from Mexico to South America. Since 1980 summer tanager populations have declined by 17 percent. The olive-sided flycatcher, a migrant through Big Bend, winters in temperate and cloud forest areas at higher elevations in the Andes. This habitat is also under siege and populations of olive-sided flycatchers have declined 22 percent since 1980. Not all neotropical migrants, however, utilize mature forest for wintering habitat. Some prefer secondary growth, scrub lands, and coffee plantations. For these species, the felling of mature forest may actually benefit their survival. To further complicate the picture, recent studies have shown that many species can tolerate a very high level of disturbance on their wintering grounds. Since the size of a breeding population is directly related to the number of young fledged the previous summer, perhaps more serious problems exist on the breeding grounds.
Habitat destruction on the northern breeding grounds, particularly in the forested areas of the United States, has fragmented suitable habitat into areas that are dangerous and unsuitable for nesting. Consider the sequence a breeding bird follows upon arriving in the north. First it must find suitable habitat, then attract a mate, and finally construct a nest, lay eggs, and raise young. If the habitat has been fragmented it takes longer to find a good area, decreasing the time available to raise young and increasing the chances of starvation. Fragmentation of habitat on the breeding grounds also makes it easier for predators (jays, owls, rodents, and house cats) and brood parasites (birds which lay their eggs in the nest of other birds, like the brown-headed cowbird) to find a nest. Brown-headed cowbirds parasitize over 100 different species and have driven some, like the black-capped vireo, to the brink of extinction. Recall that most neotropical migrants have time to raise only one brood. A breeding pair that loses its eggs or young to predators, or spends its season raising a brown-headed cowbird, has lost a year. If this happens to many individual pairs in a population, the whole declines. Many researchers point to these events as the most important factors in the decline of neotropical migrant populations.
Another factor is the importance of stop-over areas along the migration route. Migrating birds, particularly long-distance migrants, need suitable habitat in which to rest and refuel before continuing on. Disturbances in these areas could cause serious impacts. Researchers acknowledge that habitat destruction at all three points, breeding, wintering, and stop-over points, is cumulative and critical. Hunting, persistant environmental contaminants, and other unknown factors further complicate the issue. Add to this the political boundaries which birds do not recognize but that we cannot ignore. The situation is grim, but not hopeless.
Most populations still number in the hundreds of thousands to millions and extinction is not imminent for all. If we act now, there may still be time to effect a change. That we need to act now may be hard for some people to understand because, after all, these are only birds. But, there is a bigger picture. Dick Cunningham, devoted birder and former Chief of Interpretation of the Western Region, National Park Service, perhaps said it best: "Birds provide an excellent barometer of environmental change. They remind us that change, especially accelerated, unnatural change, can be destructive to all types of life. This is as true for human beings as it is with all the other organisms with which we share this planet. Thus changes in the status of migratory birds provide clues to the quality of the earth’s environment.”
As understanding of this concept dawns, and with the alarm bells sounding, efforts are beginning. Concerned individuals are planting native vegetation around their homes to provide habitat for wildlife, participating in research, and organizing activities that promote education and understanding. On a larger scale, programs like the Partners in Flight project seek to create partnerships between the key players involved: state and federal agencies, non-governmental organizations, private landowners, and other nations. The focus of the project is to change current land management practices based on a single species concept to ones that foster an ecosystem approach and managing for biological diversity. Such practices would benefit many life forms, including neotropical migrants and, in the long run, us.
Most people enjoy birds, whether they be casual watchers in the backyard or hard core birders chasing elusive rarities around Big Bend National Park. Every year bird watchers spend millions of dollars on books, equipment, and travel in their quests for new species on the list. Every day, somewhere, someone raises his or her head to watch a passing flock or to listen to the song of a hidden bird. It is appropriate then, that birds serve as the rallying point for people from all over the world to join together in search of solutions to environmental decline. We can only hope that we are in time and that the river of birds that rushes through spring skies doesn’t diminish to a trickle.
Did You Know?
The Big Bend area of Texas derives its name from its location on the bend of the Rio Grande. The river creates a natural border that separates the United States from Mexico. More...