There are approximately 250 species of neotropical migratory birds, most of which are songbirds. They breed in North American forests during our summer and spend winters in Central and South America in search of insects, nectar, and fruits. These songbirds play a major role in maintaining the health and stability of forested ecosystems by dispersing seeds, pollinating flowers, and consuming massive amounts of insects that if unchecked could lead to defoliating outbreaks. They are also enjoyed by millions of people.

Although songbirds are arguably the most watched and beloved of wildlife, they have experienced a significant decline in recent years (Terborgh 1989, 1992; Finch 1991; Hagan and Johnson 1992). This decline is concerning because bird populations are indicators of ecological integrity and are highly sensitive to adverse environmental change (Maurer 1993). This article reviews two important factors ¤ roads and habitat fragmentation ¤ in the decline of neotropical migratory songbirds.

Why are neotropical migratory songbirds declining?

Songbirds require large amounts of continuous forested habitat for survival and successful reproduction in both their wintering grounds in Central and South America and their summer breeding grounds in North America (Robbins 1979; Whitcomb et al.1981; Robbins et al. 1989) Although much of the birdsø tropical habitat has been degraded, studies suggest that conversion of large tracts of North American forest is the leading cause of their decline (Terborgh 1989; B"hning-Gaese 1993). Much of North Americaøs forested area has been logged, converted to agriculture or suburban landscapes, and left inhospitable for songbirds.

More subtle causes of habitat loss include the construction of roads and power lines. These linear barriers also have been correlated with a decline in neotropical migrant songbirds (Berkey 1993; Boren et al. 1999; Ortega and Capen 2002). Whether by forest conversion or the construction of roads and power lines, fragmentation subdivides habitat into smaller and smaller parcels. The result is an increase of edge habitat, or the boundary between intact forest and surrounding impacted areas. Small forests with large amounts of edge habitat are a hostile landscape for nesting neotropical migratory songbirds. In these areas, songbirds face two great threats: 1) the loss of eggs and nestlings to predators and, 2) parasitism by cowbirds.

Nest Predation

Nest predation is thought to be a leading cause of declines in neotropical migratory songbirds (Wilcove 1985; AndrTn and Angelstam 1988; Yahner and Scott 1988). Forest edges comprise ideal habitat for many predators that would not typically invade a forest ecosystem, and many opportunistic predators concentrate their feeding efforts along these edges. When roads, power lines, or pipelines are constructed through forests, small mammalian predators such as raccoons, opossums, skunks, and feral cats use these linear avenues to access songbird breeding grounds and prey upon their eggs and young. Additionally, egg-eating birds such as American crows or blue jays also focus their hunting along forest edges.

Brood Parasitism

The Brown-headed Cowbird (Molothrus ater) also thrives along forest edges and may pose an even greater hazard to songbirds than that posed by predation (Brittingham and Temple 1983; Temple and Cary 1988). Cowbirds are an obligate brood parasite, which means they lay their eggs in the nests of other birds and rely on the host parents to rear their young. This can greatly reduce the reproductive success of parasitized songbirds because the host parents dedicate much of their time feeding the fast-growing cowbird nestling while neglecting their own young.

Cowbirds are native to the northern Great Plains and evolved in close association with bison; they expanded their range as European settlement brought domestic cows and grain throughout North America. Songbirds did not evolve with cowbirds and have only recently been exposed to nest parasitism. With hundreds of millions of cowbirds now living throughout the summer breeding range of songbirds, they will continue to be a great threat.

Other Factors

In addition to fragmentation and edge effects, roads and other linear barriers contribute to the decline of songbirds in others ways. Songbirds are very sensitive to noise and will avoid roads with a large volume of traffic (Reijnen et al. 1995, 1996). With millions of miles of roads in North America, this renders ineffective a huge amount of potential summer breeding habitat. Songbirds also can be attracted to less-traveled roads for gravel to aide in digestion, for insects and worms on roadsides, and to take dust baths (Noss 1995). This can lead to collisions between birds and vehicles (e.g. Novelli et al. 1988). It is estimated that a million vertebrates are victims of road kill every day in the United States; many of these are songbirds. Additionally, worms contaminated by road pollution can be fatal to the birds that feed upon them (Noss 1995).

Conclusions and Solutions

Neotropical migratory songbirds are beloved and provide priceless ecosystem services, however, a severe decline of songbirds has been documented. Many causes for this decline have been identified. Edges created from roads, forestry, agriculture, and suburbanization have resulted in a number of ecological changes for songbirds, including greater susceptibility to nest predation and brood parasitism. Habitat fragmentation has created a population sink in many of the areas where songbirds once thrived.

To reverse songbirdsø decline, it will be necessary to preserve critical summer breeding habitats and, where possible, protect and restore large tracts of intact forest. Conservation efforts should be focused on a regional scale because small nature preserves alone will not be sufficient to preserve songbirds (Askins 1995). Maurer and Heywood (1993) recommend decreasing timber harvest on remaining tracts of extensive forest on public lands. In urban areas, Hennings and Edge (2003) suggest increasing forest canopy and reducing street density within a 100-meter radius of streams. Successfully protecting and restoring large continuous forest tracts, reducing forest edges, and improving urban/suburban habitats should help slow songbirdsø decline.

--- Adam Switalski is the Science Program Coordinator for Wildlands CPR.

References

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