Migratory Birds
Roadless forests provide undisturbed nesting, stopover, and wintering habitat for migratory birds protected under the Migratory Bird Treaty Act.
Overview
Many migratory songbirds nest only in the deep interior of mature forests. Wood Thrush, Cerulean Warbler, Ovenbird, Acadian Flycatcher — these species evolved in landscapes where the forest stretched unbroken for miles, and they rely on conditions that exist only well away from forest edges. A 2024 meta-analysis of edge effects on nest success in eastern North American forest birds found that nest success rises steadily from the forest edge out to roughly 250 meters, after which it levels off. Forest interior more than 250 meters from any edge supports significantly higher nest success than habitat closer to edges. Roads remove this interior condition: they don't just take the land they occupy — they convert the forest on both sides into edge habitat for hundreds of meters (Akresh et al. 2024).
Two mechanisms explain why interior matters. The first is nest predation. Predators that follow forest edges — Blue Jays, raccoons, snakes — find nests near roads and clearings more easily than they find nests in deep interior. The second is brood parasitism. The Brown-headed Cowbird is a native North American species that lays its eggs in the nests of other birds, leaving the host parents to raise cowbird chicks that out-compete the host's own young. Cowbirds historically lived in western grasslands and rarely reached eastern forest interior. Forest fragmentation by roads has changed that — cowbirds prefer edge habitat where forest meets opening, and where cowbirds reach, forest songbirds that didn't evolve defenses against them lose nests at rates as high as 95 percent. Wood Thrush nests in fragmented Illinois landscapes have shown cowbird parasitism rates approaching 100 percent, with nest predation rates of 50 to 96 percent on top of that. For wood thrush and similar species, fragmented landscapes may be population sinks — habitats that drain population from larger, intact tracts elsewhere (Akresh et al. 2024; Cox et al. 2012; USDA Forest Service 2006).
The legal framework for migratory bird protection is the Migratory Bird Treaty Act of 1918, which prohibits taking, capturing, killing, or possessing migratory birds, their parts, nests, or eggs, except as permitted by regulation. Executive Order 13186 directs federal agencies to evaluate the effects of their actions on migratory birds under NEPA, with particular attention to species of concern. The roadless condition has functioned as a practical complement to these legal protections — by preserving the interior forest conditions that interior-nesting species require, the Roadless Rule has helped maintain breeding habitat for species that are declining across most of their range. More than half of North American migratory bird species are currently experiencing population declines; the Connecticut Warbler, for example, has declined by 62 percent since 1966. The fragmented landscapes that road construction creates are documented as a major contributor to these declines (BLM 2008; Hallworth et al. 2021).
What the research shows
Forest interior more than 250 meters from any edge supports higher nest success. A 2024 meta-analysis of edge effects on nesting success in mature forest and shrubland birds of eastern North America found that nest success rises steadily from 0 to roughly 250 meters from the forest edge, then levels off. Core areas more than 250 meters from edges support significantly higher nest success than habitat closer to edges (Akresh et al. 2024).
Roads concentrate nest predators near edges. Predation by edge-following predators — Blue Jays, raccoons, snakes — is concentrated near roads, clearings, and forest edges. Distance to unpaved road was the strongest predictor of nest survival in a study of 463 nests across 17 songbird species. Nest predation, not parasitism, accounts for 75 to 100 percent of nest failures in most studies (DeGregorio et al. 2014; Akresh et al. 2024).
Brown-headed Cowbirds parasitize nests in fragmented forests. Brown-headed Cowbirds lay their eggs in the nests of other songbirds. Cowbirds prefer edge habitat and forest-grassland boundaries, and forest fragmentation by roads expands their access to nests in species that didn't evolve defenses. Wood Thrush nests in fragmented Illinois landscapes have shown cowbird parasitism rates of 89 to 100 percent, with reduced reproductive output that may make these landscapes population sinks (Robinson 1992, as cited in WA DOT; USDA Forest Service 2006).
Many migratory bird species require large interior forest tracts. Minimum forest patch size for breeding viability varies by species but is large for many: 350 hectares for Ovenbird, 500–900 hectares for Acadian Flycatcher, and over 500 hectares for Kentucky Warbler. Fragmentation reduces patch size and breeding success simultaneously, and many migratory bird species are sensitive to both (USDA Forest Service 2009).
More than half of North American migratory bird species are declining. Population declines are widespread among North American migratory birds. The Connecticut Warbler has declined 62 percent since 1966; the Cerulean Warbler is declining at roughly 4.4 percent per year; the Golden-winged Warbler has lost an estimated 22 percent of its Great Lakes habitat and 43 percent of its Appalachian habitat since the 1960s. Habitat fragmentation on breeding grounds is documented as a major contributor (Hallworth et al. 2021; NRCS 2016).
Sources
Show all 9 sources
Peer-reviewed research
- Akresh, M. E., McInvale, S. L., & King, D. I. (2024). A meta-analysis of edge effects on nesting success in forest and shrubland birds of eastern North America. Journal of Field Ornithology, 95(2), 12.
- Hallworth, M. T., Bayne, E., McKinnon, E., et al. (2021). Habitat loss on the breeding grounds is a major contributor to population declines in a long-distance migratory songbird. Proceedings of the Royal Society B, 288(1949), 20203164.
- DeGregorio, B. A., Weatherhead, P. J., & Sperry, J. H. (2014). Power lines, roads, and avian nest survival: effects on predator identity and predation intensity. Ecology and Evolution, 4(9), 1589–1600.
- Cox, W. A., Thompson, F. R., III, Root, B., & Faaborg, J. (2012). Declining Brown-Headed Cowbird Populations Are Associated with Landscape-Specific Reductions in Brood Parasitism and Increases in Songbird Productivity. PLoS ONE, 7(10), e47591.
- Mumme, R. L., Schoech, S. J., Woolfenden, G. E., & Fitzpatrick, J. W. (2000). Life and Death in the Fast Lane: Demographic Consequences of Road Mortality in the Florida Scrub-Jay. Conservation Biology, 14(2), 501–512.
Government and technical reports
- NRCS. (2016). Healthy Forests for Golden-winged Warbler — Working Lands for Wildlife FY17-21 Conservation Strategy. USDA Natural Resources Conservation Service.
- Bureau of Land Management. (2008). Migratory Bird Treaty Act — Interim Management Guidance (Instruction Memorandum No. 2008-050).
- USDA Forest Service. (2009). Multiscale Habitat Suitability Index Models for Priority Landbirds in the Central Hardwoods and Appalachian Bird Conservation Regions. Northern Research Station.
- USDA Forest Service. (2006). Gallatin National Forest Travel Plan Final Environmental Impact Statement — Issue 14: Migratory Birds.