The Appalachian Trail above Newfound Gap is crowded with birds on a June morning. The forest generally behaves as tho it has been doing this for a very long time without human help or interference, with veeries and black-throated blue warblers calling from the rhododendron thickets, the occasional ovenbird somewhere down the slope repeating its accelerating phrase. That characteristic of the Great Smoky Mountains is that they are old, independent, and seemingly unaffected by events occurring at lower elevations. However, the information gathered from point count surveys and banding stations conducted throughout the park doesn’t quite match that sense of permanence.
The southern Appalachian songbird ranges are shifting. Not abruptly, not all at once, but demonstrably – elevation bands that supported certain breeding species a few decades ago are migrating upward as temperature gradients alter. Species that used to peak in abundance at mid-elevation areas are now found at their maximum concentrations somewhat higher up the slopes. It’s the kind of change that’s easy to overlook in a single season and impossible to detect across thirty years of normalized point count data. That kind of long-term record has been accumulated by the Monitoring Avian Productivity and Survivorship stations, including the long-running banding operation at Tremont in the western side of the park. It is the methodical, unglamorous task of showing up at the same locations in the same seasons and counting what’s there.
Adriaan Dokter’s work at the Cornell Lab of Ornithology explores the same general subject from a completely different viewpoint. Dokter and the BirdCast team have developed techniques for identifying mass songbird migration events in the atmosphere and modeling where birds are moving, in what numbers, and on what timeline using weather surveillance radar networks throughout the continent—the same systems that track precipitation. A billion birds traveling the eastern United States on a clear October night shows up in the radar data as something unique from weather. That signal, evaluated over numerous years, reveals trends in migratory timing, route variations, and population-level changes that no ground-based survey network could capture at continental size.
The question of why the patterns appear the way they do is what links the radar work to the ground surveys in the Smokies. Wildfire smoke is one component of that problem that has recently garnered significant scientific study. The recent fire seasons in Canada propelled smoke plumes at historically unheard-of concentrations across most of the eastern United States. For breeding songbirds, smoke presents issues that go beyond the apparent respiratory concerns. The frequencies that territorial songs and mate-attraction calls occupy are muffled and distorted by dense particulate matter, which alters how sound moves thru the atmosphere. The reproductive success of a male black-throated blue warbler may be hampered in ways that don’t manifest right away but build up over the course of a season if his song isn’t carrying as it usually does.
The Smokies findings are especially significant at this time because of the convergence of these pressures: range shifts from temperature change, auditory disturbance from smoke, and the baseline stress of habitat fragmentation at the forest margins. Due to its elevation gradient, the park has species assemblages that have been driven out of lower-elevation habitat throughout much of the region, making it one of the biggest remaining blocks of contiguous eastern deciduous woodland in the United States. The birds who reach the summits of the ridges have nowhere higher to go if the park’s own conditions are changing quickly enough to cause additional upslope movement.
