Abstract
The Bicknell's thrush (Catharus bicknelli) is a migratory, Neo-tropical, habitat specialist that is widely regarded as the most vulnerable North American songbird due to the widespread deforestation of overwintering habitat in the Caribbean, recession of spruce-fir forests in North American breeding sites, increased rates of predation as well as migration hazards and pollution deposition at high-elevation. The bird, which is no larger than ones hand, inhabits a several thousand-mile range, spanning from its overwintering sites on the islands of Hispaniola, Cuba and Jamaica to its breeding grounds in southern Canada, Maine and northern New York. Avid birders and recreationists in the Adirondack State Park have long enjoyed the often heard, but rarely seen, presence of the Bicknell's thrush. Previously considered a subspecies of the Gray-cheeked thrush (Catharus minimus), the bird was only recently classified as an individual species in 1995; since then, Bicknell's thrush populations have been monitored for fear that declining montane spruce-fir forest habitat in the Northeast and cloud forest in the Caribbean, combined with various other environmental stressors, will drive populations down.
Bicknell's thrush breeding habitat is categorized as montane spruce-fir forests at, or above, 915m of elevation; nearly a quarter of which is encompassed by the Adirondack State Park, almost exclusively within these forest preserve areas. This effectively protects these areas from any large-scale anthropogenic disturbances (e.g. deforestation, human development). The presence of Bicknell's thrush in the Adirondacks is unique when considering the long history of aggressive and destructive land use in this area. The Adirondack State Park in northern New York State covers 5.9 million acres, with over 60% (or close to 3 million acres) categorized as Forest Preserve, effectively protecting it from development or harvest. Long used for trapping, mining, logging and leather tanning, Adirondack ecology was all but diminished when the Park was created in 1894 to protect the water supply for downstate New York. The Adirondack Park is a prime example of a recovered landscape and, as such, is running on an inverse trajectory of the thrush's Caribbean overwintering grounds.
The destruction of the Bicknell's thrush's various overwintering habitats is by in far the most pressing issues when considering the purported population declines of this species. The widespread deforestation in the Greater Antilles region of the Caribbean is the result of the presence of subsistence farmers clearing forested regions to allow for small-scale agriculture. While this problem is significantly more noticeable in Haiti (<2% of original forest still intact), there seems to be a similar, albeit less aggressive, trend of forest clearing in The Dominican Republic.
Bicknell's thrush habitat in the Adirondack State park all falls within various forest preserve areas, thus preventing any forestry operations from disrupting spruce-fir habitat. However, various other environmental stressors, primarily acid deposition, invasive species introduction and climate change, are all causing drastic shifts in the composition and abundance of montane spruce-fir forests. Increasing global temperatures are pushing spruce-fir bands further up elevation gradients to the edge of their feasible habitat range.
Mercury deposition from the combustion of coal in mid-western power plants is responsible for organismal level contamination in the Adirondacks. Typically, this is in regard to apex predators and aquatic species. However, the detritus heavy and herbaceous diet of the Bicknell's thrush causes these birds to accumulate notable quantities of mercury within their bodies.
Acid deposition, in the form of precipitates, in these areas cause various degrees of direct, and indirect, damage to the high elevation spruce-fir forests. The acid rain itself is unable to kill these trees, however the consistent presence of "acid fog" causes long term damage to needle cuticles, inhibiting photosynthesis and expediting evapotranspiration. Alternatively, acid deposition and accumulation in soils leach vital minerals such as magnesium and calcium while mobilizing previously unavailable and toxic compounds such as aluminum, stunting tree growth and regeneration.
The steady degradation of spruce-fir defenses via acid deposition makes these tree species notably more susceptible to damage from invasive species In the Adirondack Park, the two most pervasive and damaging invasive and pest species are the Balsam wooly adelgid (Adelgis piceae) and the Eastern Spruce budworm (Chroristoneura fumiferana). The Balsam wooly adelgid feeds on, and eventually kills, Balsam firs within the Adirondack State Park. The Balsam wooly adelgid's population is currently kept in check by seasonal cold snaps; cold snaps that are becoming less common due to climbing global temperature. The Eastern spruce budworm is the larval form of an endemic moth. Much like the Balsam wooly adelgid, the populations of eastern spruce budworm are kept down through harsh winter temperatures, as well as insectivorous birds and entomophagous parasites. With these limiting factors disappearing, the eastern spruce budworm and Baslam wooly adelgid populations are beginning to have ramifications for the composition of would be spruce-fir forests.
Lastly, the failure to have continuous policy through the Bicknell's thrush's migratory range stands to negate any major conservation steps taken in the Northeast. Without strong policy, paired with an equally extensive conservation initiative, any changes made to the ecosystem in the breeding grounds would simply be crowded out by the overwhelming, negative momentum, elsewhere.
Currently, the most feasible means of reducing the rate of population decline for the Bicknell's thrush is to educate stakeholders, provide alternate economic avenues for subsistence farmers in overwintering grounds, prevent further deforestation in The Dominican Republic, restore the degraded forest habitat in these areas, continue to build international support and collaboration, establish Best Management Practices, prevent the further spread of invasive and pest species and minimize atmospheric pollution deposition.
This case study was produced through a comprehensive literature review supplemented with interviews conducted with regional experts on the Bicknell's thrush. A survey was conducted in several locations (Canton, NY, Tupper Lake, NY and Lake Placid, NY) to gauge the extent of public knowledge and opinion regarding the Bicknell's thrush. Lastly, GIS analysis of Bicknell's thrush habitat requirements and land classifications of the Adirondacks allowed for inferences to be made regarding the risk of forestry disturbance to Bicknell's thrush within the Adirondack State Park.