A changing landscape may have dire implications for birds

07/29/04

PORTLAND - In their desire to get close to nature by building lakeside cottages and homes in the woods, Americans may very well be hastening the decline of many native bird species that breed in forest habitats.

The development boom in the nation's rural areas is putting increasing pressure on forest ecosystems, and the resulting decline in native vegetation and the increase in human activity - ranging from all-terrain vehicle use to predatory pets roaming the woods - is putting more and more native birds at risk, according to research presented Wednesday, Aug. 4, at the annual meeting of the Ecological Society of America in Portland, Ore.

The research, conducted by a team of scientists from the University of Wisconsin-Madison and the U.S. Forest Service, details broad patterns of birds' response to housing growth and land cover change in the lower 48 United States.

The bottom line, according to UW-Madison researchers Anna Pidgeon and Chris Lepczyk, is that as rural forested landscapes are developed and parsed by roads and openings for new houses, many native bird species are at risk as deep forest breeding habitat is perforated.

Using data from the Breeding Bird Survey, a broad-based effort to monitor bird populations across North America, the researchers looked at changes in the abundance of species. Comparing that data to U.S. census data and the National Landcover Dataset, a satellite survey of land cover in the U.S., the Wisconsin and Forest Service team was able to sketch a broad picture of human pressures on native forest bird species.

"We have found in the Midwestern United States that as land cover becomes more human dominated, the number of species declines," says Lepczyk, who led a team that examined the roles of land cover and housing density on 137 species of birds, native and exotic. Of those, 37 species were affected negatively by humans, while 13 species had positive relationships and 23 species exhibited a mix of adverse effects and benefits from human shaping of the landscape.

"This mirrors nationwide results previously reported by Audubon," says Pidgeon.

Now, Pidgeon has expanded the study to begin to examine how the pressures of housing growth and land cover change have altered bird populations during the past 30 years across the 48 contiguous United States. "We are seeing geographic clusters within the U.S. where some species populations are increasing, which we suspect is due to increases in both generalist and exotic species," says Pidgeon.

While human population has grown significantly across the continental United States since the 1970s, Lepczyk says the number of houses sprouting up in previously undeveloped areas is likely having a greater impact than the raw number of humans inhabiting the landscape.

"Houses, we think, may represent a better indicator of impact (on native bird species) than human population," Lepczyk explains.

The study results portray an increase in exotic bird and generalist species, such as European starlings, pigeons, crows and jays, as a consequence of increased housing density on the rural landscape.

What clearly puts some native forest species at risk is the outright loss of wooded habitats as roads and lawns replace native vegetation, says Pidgeon. Not only does such development shrink available breeding habitat, but it also opens corridors for bird predators such as raccoons and skunks. Lawns also provide foraging areas for brown-headed cowbirds, parasitic birds that lay eggs in other birds' nests.

"Roads provide access and increased edges that nest predators including jays and crows use," Pidgeon says, "and we know from the work of others that an increase in predation accompanies an increase in housing density" as the domesticated animals that accompany humans, cats and dogs in particular, exact a heavy toll on native forest bird species.

What's more, human activities, such as the growing use of all-terrain vehicles and the replacement of native vegetation with exotic and ornamental plants, reduces cover and food resources for native birds.

Native species like house wrens, robins and catbirds can benefit from human changes to the landscape, but species like the scarlet tanager and some warblers depend on large, contiguous tracts of forest to successfully reproduce.

Humans, according to Pidgeon and Lepczyk, can benefit some birds by establishing feeding stations, sources of water and nesting boxes. But the increased density of housing, especially in northern deciduous forests, is having a net negative impact.

"Whole species, like the Cerulean Warbler, could be in jeopardy if we don't preserve enough large tracts of mature deciduous forest. The $64,000-question is how much is enough?" says Pidgeon

Source: Eurekalert & others

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