Chapter 1 - Introduction: Just Passing Through - Migrant Use of the Connecticut River Valley
Locating a migrating Black-and-white Warbler in transit from its Central American or Caribbean island wintering grounds to its breeding territory in New England can be a challenge. It is one of nature’s most wondrous events that this tiny, 1/3-ounce package of feathers, bones and muscle flies 3000 miles in the spring to its breeding grounds. But until it reaches its breeding ground, it is on the move for many weeks, stopping only long enough in any location to feed and rest, before it begins the next leg of its northward journey (Figure 1).
Incredibly, in any given year the opportunity to breed occurs only after the individual has survived fall migration to the wintering grounds, survived the season on the wintering grounds, and ultimately makes a successful spring migration back to the breeding the grounds. Upon arrival on the breeding grounds, the male Black-and-white Warbler establishes a one-acre territory, finds a mate and builds a nest. The pair mates, and soon the female lays four to five eggs. The eggs are incubated for about 10 to 12 days, then the chicks hatch. The chicks remain in the nest for another 8 to 12 days while the parents attend to the chicks' voracious appetites and protects them from predators and cool spring nights. Although these statistics vary some among Neotropical migrant species--- warblers, vireos, thrushes, flycatchers, and tanagers--- they all exhibit similar patterns of breeding biology. Few may realize that the annual adult survival rate for a group like the warblers is only 36-67 percent. An even higher percentage of the young raised in a given season will perish before their first breeding season from thunderstorms, late season freezes and snowstorms, drowning while crossing expanses of ocean, food shortages, exhaustion, and predation. Because these bird populations have adapted to environmental change and challenges over millions of years, they produce enough offspring each year to offset the losses that occur, allowing their population sizes to remain stable.
The population of any wildlife species, including those of the avian world, starts to decline when more members of their species die in a period of time than are replaced by breeding. When this occurs over a number of years, the population struggles to maintain itself--- not enough breeders are being hatched into the population to produce the offspring needed to create a stable population. Eventually the species is designated as threatened or endangered, depending on how steep the population decline has become. Extinction is the final stage of this downward spiral. Ever increasing, growing human populations and expanding land use activities are contributing to these downward population trends. Vast amounts of land were impacted by agricultural clearing and deforestation in the 19th century. These ongoing activities were joined by urbanization and suburbanization of the 20th century, and the more recent phenomenon of land use "sprawl." The result? Large tracts of forest habitat have disappeared. In the context of evolutionary adaptation, two centuries is a short time.
Because the evolutionary process of adapting to changing environments requires the passing of genetic material from parents to offspring, across generations, there simply is a not enough time to adapt to large scale changes that occur over short periods of time. From the perspective of evolutionary time, wide-scale changes in land use that result in habitat loss over the course of a century--100 years-- is abrupt, sudden change. In one year a tract of forest provides the habitat for a pair of Black-and-white Warblers to establish their one-acre territory and raise four offspring. In the very next year, the same tract of forest is transformed into a housing development. A bird, like the Back-and-white Warbler, that has evolved as a forest breeder over thousands of years, does not have the genetic map to become a bird of the suburbs in 12 months. Take this event and multiply it across an entire region where Neotropical migrant forest birds nest, and the impact on required habitats and bird population stability is profound.
The breaking up of large tracts of habitat into smaller pieces of land is referred to by landscape ecologists and conservation biologists as habitat fragmentation (Figure 2) .
There is a large body of information on the impact of habitat fragmentation and habitat loss on Neotropical migrant bird breeding grounds. From this foundation, more attention is being given to the habitat requirements of birds as they migrate between the wintering and breeding grounds. Our understanding of stopover habitat, as it is called, is playing a larger role in the overall conservation of Neotropical migrant birds.
This increased attention to stopover habitat is an important step in understanding the the complete lifecycle of migrant birds. The act of migration is a very stressful and energy demanding period for a bird as it flies thousands of miles in a relatively short amount of time. From the start, a spring migrant's hormones are peaking and its biological clock is ticking as the imperative to breed pulls it north. With that, there are oceans to cross, food to find, storms to avoid, predators to evade, freezing cold that challenges the maintenance of body temperature, and competitors vying for the same territory and mate upon arrival. And, all this occurs before a single egg is laid. The stress of migration is now compounded with the increasing impacts of habitat loss and fragmentation. Consider what has changed. Three hundred years ago this same group of species migrating along the East Coast flyway encountered large regional expanses of unbroken salt marshes, forests, and other habitat types. Natural events such as hurricanes and fires would influence these habitats, creating habitat diversity, but the random effects of a storm or fire occur on local levels. With such large expanses of unbroken habitat available, suitable stopover sites were available almost everywhere.
At the start of the 21st century, the Gulf and East Coast landscapes have been undergoing dramatic alteration for over 300 years, and are now home to some of the densest populations of humans in the United States. Large areas of habitat have been fragmented and over the course of centuries, the fragmentation process continues as human land use activity expands. The distances between suitable stopover habitats has grown--- separated by agricultural fields, roads, cities and suburbs---and more birds are required to make use of fewer remaining areas. They are left with fewer and fewer options for finding shelter from storms, avoiding exposure to predators, and finding food supplies for refueling depleted body fat. Bird researchers on the Gulf Coast have outlined the decisions a migrant must make; with fewer stopover habitats available, the critical nature of these decisions increases (Figure 3).
This habitat loss increases a migrant’s vulnerability while it competes for fewer suitable stopover habitats and the critical resources they contain. Larger numbers of birds perish en route or arrive on territory in poor breeding condition. Loss of habitat, and the associated bird mortality that occurs, is happening faster than can be compensated for by the surviving birds' reproductive rates. The population declines below the numbers needed to off-set the mortality associated with the the migrant lifecycle, and we begin to label those populations as of concern, threatened, or endangered.
For the managers of the Silvio O. Conte National Fish & Wildlife Refuge (SOCNFWR), this is a serious conservation issue. Unlike many refuges and preserves that are defined by real estate related property boundaries, the SOCNFWR is an ecosystem-based refuge. The SOCNFWR is defined by the naturally occurring boundaries of the Connecticut River Watershed. The watershed stretches from Long Island Sound to the Connecticut Lakes on the U.S.- Canada border. It runs the 420-mile (675 kilometer) length of the Connecticut River and encompasses 7.2 million acres (2.9 million hectares) within Connecticut, Massachusetts, Vermont, and New Hampshire. For southern New England, and the Connecticut River Valley (CRV) in particular, loss of habitat to sprawl has become a serious wildlife conservation issue. Massachusetts loses 40 acres (16 hectares) per day--25 square miles (65 sq. kilometers) of open space every year--- and the rate is accelerating. In the face of these land use pressures, SOCNFWR managers needed to know how important the CRV is as a migratory highway, and what stopover habitat types within it need protection. These questions led to the creation of the SOCNFWR Bird Stopover Habitat Survey. A major challenge was to design a way to sample migrants (whose movements are not nearly as predictable as birds on territory), and to do it over a 7.2 million acre ecosystem.
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