In large parts of the U.S., over half of our songbird species are in decline. Scientists say habitat destruction is largely to blame.
Across the United States, more than 250 species of neotropical migratory birds fly south for the winter.
Beginning in the early 1970s, scientists began to notice that many species seemed to be in decline
What was going on?
Scientists have concluded that the decline of neotropical migratory song birds in the United States is closely linked to four issues closely linked to human population growth and habitat destruction:
- Tropical Forest Destruction
The population of Latin America and the Caribbean has doubled in the last 35 years, and with it has come unprecedented destruction of tropical rainforests. As populations have exploded, more landless peasants have colonized forest areas and cleared vegetation, with slash-and-burn cycles becoming progressively shorter. At the same time, logging over wide areas and the rapid expansion of commercial farming has accelerated the disappearance of forests and fueled the rapid destruction of once-lush bird habitat. In the Peten region of Guatemala, for example, 77 percent of the land was covered in dense forest in 1960. By 1990, that number had fallen to just 29 percent.
- Pesticide Use Overseas
Neotropical migratory birds are being killed by the heavy use of insecticides, herbicides and fungicides, which are used to boost crop productivity to feed increasing numbers of people in the developing world. In some cases, birds are poisoned outright by chemical application, or by consuming grain and insects that have been sprayed. In other cases, the pesticides accumulate and concentrate within the birds, resulting in deformed chicks or eggshells that are so thin they break before hatching.
- Suburban Sprawl and Forest Fragmentation
As the population of the United States has grown from 76 million in 1900 to over 280 million today, cities and suburbs have sprawled outward. Fairfax, Virginia, for example, a suburb of Washington, D.C., saw 69 percent of its forest converted to homes and businesses between 1980 and 1995. As human populations have risen, and forests have fallen, primary predators such as wolves, bobcats, and cougars have been wiped out, while the ecological niche of meso-predators such as feral cats, raccoons, possums and foxes has expanded. The result has been massive predation of Neotropical songbirds, which tend to nest in the open and near the ground rather than in tree cavities or higher up in the forest canopy.
With forest fragmentation has come an invasion of native and non-native birds that compete with deep-forest species for food and nesting sites. One example is the brown-headed cowbird. Cowbirds were once confined to the forest edges of mid-western prairies where they fed in grasslands grazed by roaming bison. Today, however, because of widespread forest fragmentation, parasitic cowbirds can be found all across the United States. A single cowbird may lay as many as 20 eggs in a breeding season - one or two eggs per songbird nest. Because Neotropical migrants tend to build open cup-shaped nests, and raise only a single brood a year, they are particularly susceptible to cowbird parasitism.
- Intensive U.S. Farming Practices
As American farmers make increasingly intensive use of their lands, bird populations suffer. Post-to-post cultivation has wiped out edge-row thickets where many songbirds used to nest, while many farmers now cut hay three times a year where they used to cut just once. The result is that hedgerow and ground-nesting birds like the northern bobwhite, the eastern meadowlark, the vesper sparrow, and the grasshopper sparrow are in rapid decline.
What about good ole Window Glass?
Bird-window impacts kill a lot of birds, but generally not birds that are in trouble, as they tend to be "bird feeder birds" found near buildings, rather than grassland and forest birds which are generally found farther from buildings, and which are generally the birds that are in decline.
Birds are adapted to deal with loss of life from hurricanes, storms, freezing, and most impacts, and so episodic loss tends to be compensated for by greater nest survival. Loss of habitat either in their wintering grounds, their feeding ground, their nest grouns, or at essential links in the migration change are something different. To make a human analogy: murder has no impact on population numbers, and neither do traffic accidents. Shut down all the grocery stores (or gas stations), however, and we are all going to be toast in very short order. Ditto for birds.
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