|Female Common Eider by Lilia D'Alba|
We evaluated all bird species on the basis of five independent
characteristics of sensitivity to climate change, one measure of
exposure, and three assessments of conservation status based on factors
other than climate change.
We assessed sensitivity of birds to climate change based on five basic traits that demonstrate adaptability from temporal, spatial, ecological, and evolutionary perspectives. Each trait was scored as zero (low sensitivity) or one (high sensitivity). In addition, we scored the exposure of each species’ habitat to climate change impacts on a scale of zero (low climate exposure), one (medium), or two (high). Scoring was based on the expert opinion of the science committee and outside experts.
Our approach provides a general picture of potential impacts to our nation’s birds. Although we have a fairly good understanding of the inherent traits that make various species sensitive to climate change, the mechanisms are extremely
complex and yet to be fully revealed. Despite uncertainty about how birds and their habitats will change, we provide this first assessment as a basis for future analyses and as a starting point for guiding bird conservation in response to a changing world.
Five Basic Sensitivity Traits
Migration Status: A species was considered to have high sensitivity if it is a long-distance migrant that traverses many habitats during migration, using day length as a primary cue for timing its migrations. These species may experience a mismatch between food availability and the timing of arrival at critical stopover areas or on their breeding grounds.
Breeding Habitat Obligate: A species was considered to have high sensitivity if it was categorized in the 2009 State of the Birds report as an obligate of a single habitat type. This factor indicates species that are less likely to be able to adapt to a different habitat type. Seabird species received a high vulnerability score if they forage only in coastal or pelagic waters.
Dispersal Ability: We defined species with poor dispersal ability as those that lack the ability to shift when restricted by geographic barriers, narrow elevation requirements, or high site fidelity, whose specialized behaviors may make them unable to move in response to changing conditions, habitats, or resources. High sensitivity species include most island species, continental species such as lekking grouse, and species with island-like distributions, such as alpine, saltmarsh, and highly colonial breeders.
Niche Specificity: Species were scored as having high sensitivity if they are highly specialized on limited resources such as food, nest sites, or microhabitats that are likely to be disrupted or depleted due to climate change.
Reproductive Potential: We evaluated species whose life-history traits, including combinations of low annual reproductive effort and long generation time, may limit their ability to adapt to climate change events. A high sensitivity score was given to species that only raise one or fewer young per year.
Habitat Exposure: Species that are restricted to “sub-habitats” at highest risk of disappearance or severe degradation due to climate change were evaluated under this measure. Species restricted to habitats most susceptible to climate change were considered to have high exposure; those species restricted to habitats of medium susceptibility (especially due to increased drought conditions) were considered to have medium exposure; and species using the least susceptible habitats were considered to have low exposure.
Overall Vulnerability: The summed scores for the five sensitivity traits and the measure of habitat exposure give a composite score of vulnerability to climate change. We categorized species as showing High Vulnerability (vulnerable score
of four or more), Medium Vulnerability (vulnerability score of two or three), or Low Vulnerability (vulnerability score of zero, one, or two).
Species of Conservation Concern
All of the birds of the United States have been assessed for conservation need, but climate change threats were rarely considered. These assessments were based on species protected by the Endangered Species Act, the USFWS list of Birds of Conservation Concern, and the American Bird Conservancy/Audubon Watchlist. If a species is on any of these lists, we included them as Species of Conservation Concern. We then compared the vulnerability scores of birds to the current assessment of their conservation status.
This report calls attention to how climate change may heighten threats for birds that are already of conservation concern. Additionally, many species that were not previously considered of conservation concern may now be of concern because of
the threat of climate change to their populations.