Ecology and Vulnerability

Blackburnian Warbler

Background
The blackburnian warbler is a forest bird that breeds along the eastern seaboard from the Maritime Provinces to Massachusetts and New York and west to Alberta. It ranges as far south as Georgia but, as a long-distance migrant, winters from Costa Rica to Peru and Venezuela ^1,8. In Massachusetts, these birds are found at higher elevations in the northwest corner of the state ¹. This species primarily breeds in mature southern boreal coniferous forest, but is also found in mixed forests, particularly ones containing eastern hemlock, and occasionally in primarily deciduous forests ⁸.  Because these birds live exclusively in the interior of forests, they can be harmed by forest fragmentation that limits the amount of forest interior and affects their abundance and survival ¹⁴.  Blackburnian warblers eat a wide variety of insects from canopy foliage and have shown declines when canopy is lost following disturbance ⁵. In some regions, they specialize on the eastern spruce budworm (Choristoneura fumiferana) just after their arrival to the breeding grounds ¹¹.

This species is stable to declining throughout its range; declines are related in part to habitat loss from deforestation on both the wintering and breeding grounds ⁸, particularly given their limited wintering area ¹².

Climate Impacts
Climate-induced habitat loss may impact blackburnian warblers. For example, the eastern hemlock and Fraser fir that the birds often breed in at the southern portion of their range are threatened by wooly adelgids (Adelges tsugae), a tree pest that appears to be increasing due to climate change ⁹. The trees have already nearly disappeared from the hemlock forests of New Jersey and southern New England as well as high-elevation forests in the southern Appalachians because of wooly adelgids ⁸. Climate change may also threaten the montane spruce-fir forests that the birds use. Boreal forests are expected to become less common because of climate change - possibly becoming locally extinct in the Northeastern U.S. under severe climate projections ¹⁰.

Habitat fragmentation effects are also a concern as these forests are reduced in response to climate change, particularly upslope. Range reduction is expected for species that inhabit montane spruce-fir forests at the southern edge of their range ¹⁰. Models of future distribution considering habitat and climate changes predicted that landscape capability (suitable climate and habitat) for the blackburnian warbler would decrease by 71% of their 2010 Northeastern range by 2080 ², a loss of ~1%/year.

Climate-induced changes in the timing of seasonal events could also have serious consequences for this long-distance migrant. Studies have shown that birds are arriving earlier to their breeding grounds across the northern U.S. ^6,13. Climate variability could worsen these timing problems, since late spring storms and extreme weather events can kill migrating birds ^3,15. However, many long-distance migrants, including the blackburnian warbler, have not shifted their spring arrival dates as much as short-distance migrants ⁷. As a result, there are mismatches between migration dates and food resources for many long-distance migrants ⁴. The spruce budworm, which the blackburnian warbler sometimes heavily relies on for food, is likely to emerge earlier in spring in response to temperature-driven changes in its host tree, the white spruce (Picea glauca) ¹¹.