Methods Used in Creating ABC's List of Birds of the United States with Conservation Rankings
A new study on the conservation status of American birds completed by American Bird Conservancy (ABC) is the first ever published to include the full range of bird diversity in all 50 U.S. states and dependent territories. The study finds that more than one third of these birds are in need of conservation attention. This background document elaborates the methodology used to develop the list, which provides the centerpiece of the study.
The list can be used to help inform national priorities for bird species and subspecies conservation, using numeric rankings and color coded categories. Despite the United States having more birders than any other country on Earth, surprisingly, until now, there has not been a comprehensive bird list that both completely and exclusively covers our 50 states and other territories. The American Bird Conservancy (ABC) list provides the most up-to-date comprehensive list of bird species and subspecies for the U.S. and its territories with conservation rankings. The list includes all accidentals (indicated with the letter A) along with well-established introduced species (indicated with an I). Note that we also use the letter P for “presumed” when we do not yet know for certain which subspecies has been recorded in the U.S. (applicable to some accidental or introduced species). Species that are known to nest regularly in the U.S., even in very small numbers, are not considered accidental here.
We hope—in particular—to stimulate increased discussion concerning the conservation status of subspecies, which in general has been poorly documented in the past except for among some of the very rarest taxa. This list puts all bird taxa on the radar screen for conservation regardless of taxonomic decisions regarding species status. We are also especially pleased to include the birds of Hawaii in this list, as Hawaii is home to many of the most unique and threatened American birds, yet these species do not currently figure in most field guides used by birders. We hope that this list will encourage the authors of such guides to rethink this in the future.
Most birders and ornithologists tend to think of bird categorization firstly in terms of families, then in terms of species, and finally subspecies. As most also know, in reality there are multiple complex gradations of forms which in some birds are sharply defined, and in others are clinal or cryptic. In some cases, species are also comprised of clusters of subspecies that can be grouped due to their related ranges and similarity of appearance. These groups may sometimes represent potential future species. In other cases they are on the verge of crossing, or have recently crossed the line into full species status (e.g. the Pacific Wren, which is in fact a complex of seven subspecies).
Using the List
The list is provided in an Excel spreadsheet that is available for your use. It should be cited as: American Bird Conservancy (2012) List of the Birds of the United States with Conservation Rankings. Following sequential numbers and a check box to the left, species names appear in the current AOU sequence followed by subspecies, ranges, conservation scores, population and trend estimates, and notes.
When a species has multiple distinctive subspecies or subspecies groups, the list shows the common name of the subspecies (or of the first-named subspecies in a distinctive group) in black. Subspecies in gray font can be regarded as difficult or impossible to separate from others in that group in the field except by range, but it should be possible (often with some experience and/or good prolonged views) to separate them from other subspecies highlighted by black text (or their groups). Column “G” of the spreadsheet can be cross-referred to the highly abbreviated ABC printed checklist to get the scientific names of subspecies or grouped subspecies mentioned in the printed list. In the Excel spreadsheet, subspecies that are given no common name following a named subspecies are related taxa from an adjacent area. David Sibley's excellent online resource on identifiable subspecies (see bibliography) was helpful in making many of the determinations regarding field distinctiveness, and we have also referred to the eBird taxonomic list and other sources to assist with these designations. Pyle (1997, 2008) also identifies some subspecies “groups” that represent “geographically concordant groups of subspecies with shared characters”. These are not always distinctive in terms of field characters, but we still think that they are of interest, and highlight them by surrounding them with a black outline.
Note that where no name appears in the “Subspecies/Race” column, it is because only one subspecies occurs in the U.S., or because the species is monotypic. Note that species that do not have an entry in the field entitled “Scientific Name of Subspecies Recorded in the U.S.” are also monotypic. Species common names are repeated but grayed-out in rows where there are additional subspecies—for ease of use if you wish to re-sort the database but still read the full species names. Rows are also numbered so the list can easily be restored to its original sequence.
In addition to including subspecies, we have begun to identify some distinct ecological types or “habitypes” which are listed in the “Notes” section. These are the latent subspecies and species of the future, and while not yet separated taxonomically, they can be recognized due to their unique ecological traits. For example, habitat preference, timing of breeding, or a specific nesting or molting strategy. Examples include populations of the Swainson's Warbler that nest in Appalachian rhododendron forests as opposed to bottomland swamp forests, and those Marbled Murrelets that nest on the ground, rather than in trees in Pacific Northwest forests. These “habitypes” most closely match the concept of “Distinct Population Segments” recognized under the Endangered Species Act, yet there is not a perfect correlation as some may not be entirely geographically isolated (note that we also identify some species with disjunct populations/population segments in the same “Notes” field). Geographically concentrated hybrids or morphs, such as those found among some gulls and murres are also noted, but require additional thought and work from a conservation perspective.
Note ranges refer to breeding distributions unless otherwise specified. The range descriptions for non-accidental species and subspecies primarily refer to the U.S. range (e.g. “widespread” refers to widespread within the U.S.), unless otherwise indicated. The words eastern, western, northern, and southern refer to portions of a species' range rather than areas of the country. For accidental and introduced species, the global range of the named subspecies is typically provided. Due to space restrictions, these range descriptions are necessarily brief and included to provide an extremely general guide only.
Bird taxonomy has remained in a state of almost continuous flux since ornithologists first started naming species. Recent developments in field identification techniques as well as scientific advances have often served to further complicate an already rather confusing picture for most non-specialists (in the relationships of large gulls for example). While a Bald Eagle is still clearly a Bald Eagle, it is now far less clear what a Red Crossbill, Herring Gull, or elepaio is. As conservationists, it is important to us at ABC that the broadest diversity of forms are given adequate conservation attention, and elevation to species status clearly confers a greater level of conservation significance. In its new list, ABC recommends the elevation of all forms where strong evidence exists for recognition as full species, especially those already included in major lists published by other scientific authorities, or recently recognized through genetic or other studies. Our underlying basic philosophy being that if a population is largely and consistently genetically isolated (whether sympatrically or allopatrically), and/or can be reliably and consistently separated based on significant phenotypical differences, it should be considered for full species status. The presence of small stable hybrid zones, or occasional but regularly occurring hybrids across populations in our view is not discounting for species status for the hybridizing populations, since many birds that are clearly recognized as separate species are able to hybridize, and related taxa may be genetically closer in one part of their range than in another.
Most bird taxonomy is still decided based on the underlying principle of the biological species concept, and it seems likely that as taxonomy evolves into the future, far more bird species will be recognized than is the case today. ABC welcomes efforts to begin the process of establishing quantitative thresholds for determining species limits such as those proposed by Tobias et al in the recent paper “Quantitative criteria for species delimitation” published in Ibis. We hope that ultimately such thresholds will create a transparent process through which the reasons for species and subspecies decisions can be made clear to all members of the bird community, no matter what their level of expertise in the fields of taxonomy and genetics. We hereby lay down a challenge to bird taxonomists to implement such a quantitative system that can enable those interested in birds at all levels, to support a single recognized global list of bird species, subspecies, and other groupings (such as habitypes and population segments). We hope that this current ABC list will contribute to the discussion that will eventually lead us towards this goal. We also intend to continue to update and improve this list over time and will incorporate new determinations based on such an approach, or other new advances in science.
The ABC list primarily follows the most recent American Ornithologists Union (AOU) checklist (53rd supplement) for sequence and bird names with changes to reflect additional species splits. Subspecies primarily follow Pyle (1997, 2008), again with a few departures based on some recent publications and ABC's own views. Pyle's treatment (which itself often relies on a variety of other sources) frequently results in the recognition of fewer subspecies than the 5th edition of the AOU checklist (published in 1957) which is the most recent comparable subspecies checklist to this ABC list in terms of its scope and geographic coverage. For Hawaiian birds, we have also referred to the Hawaii Biological Survey's complete Hawaii checklist.
We have created six annexes for U.S. territories that have (or have recorded) additional species not yet found in the 50 states. We also include an annex for species that appear to now be extinct in the 50 states, despite their frequent inclusion in other lists (those that appear to have a more realistic chance of still surviving are coded PEX, meaning Potentially Extinct, in the main list); one for species that are not accepted by the American Birding Association (ABA) but have potentially credible records, or species that have been accepted at some point by ABA or AOU but the records are old and of potentially uncertain origin (e.g. Bumblebee Hummingbird); and finally one annex for introduced or escaped species that are either regularly encountered, or have gained or appear to be gaining a breeding foothold. Note that species and subspecies listed for U.S. territories are only those that are not also found in the 50 U.S. states, and each subsequent territory list only includes taxa not listed for previous (meaning Excel tabs to their left) nearby territories. No subspecies common names are provided in these annexes, and distinctive subspecies therefore have their scientific names bolded, and in one case, two are grouped in a box to indicate their similarity to each other. We use the abbreviations EX to mean Extinct, and the abbreviation EIW means Extinct in the Wild throughout.
Full species conservation scores and data were provided by Partners in Flight (PIF)/Rocky Mountain Bird Observatory, and the U.S. Shorebird Plan except in the cases of new splits in which scores are calculated based on the population and range of the species concerned. Also, scores for Hawaiian species and those for U.S. territories were calculated by ABC. Population data for waterfowl were obtained from BirdLife International (where PIF does not have comprehensive data as yet). A full explanation of how the conservation scores are calculated can be found in the PIF Handbook on Species Assessment. Justifications for individual landbird species scores can be found by downloading the data from the Rocky Mountain Bird Observatory website.
In summary, the Combined Score is the sum of the scores for Population Size, plus the highest of the two Distribution scores, plus the highest of the two Threat scores, plus Trend.
Subspecies scores were developed by ABC primarily by assessing the range extent of subspecies as compared to the “parent” species. To develop subspecies population scores we used range proportion in relation to total population size since subspecies population sizes are generally not well known (except for some of the very rarest species, where we referred to published data, although all these subspecies score 5, as the upper threshold for changing to a 4 is 50,000 individuals). We recognize that basing population scores on range size could introduce some bias, as not all species are uniformly distributed across their ranges, but there is not currently sufficient population data to systematically address this question at the present time across all taxa. Species also face differential threats across their ranges, and we hope that in future it will be possible to factor more of these differences into the threat scores. Some trend and threat scores for subspecies were altered from the full species based on published data and expert opinion. In the related issue of Bird Conservation magazine (see checklist home for a sample download), we also provide a list of declining migrants on page 26. This list includes the 25 fastest declining regular medium to long-distance migrants determined using a combination of Breeding Bird Survey and Christmas Bird Count long-term trends.
Conservation Ranking Categories
Secure (4-8): Species/subspecies as a whole appears to have no immediate major conservation issues and can be regarded as safe at the present time. While some of these might be showing declines or likely declines/uncertain population trends (trend scores of 3 or higher), these are offset by their large ranges and populations. If these declines were to continue, some of these may later be upgraded to Potential Concern.
Potential Concern (9-12): While these species/subspecies can also be regarded as currently safe, they typically have smaller populations or ranges, or slightly higher threats or more negative population trends than Secure taxa, and need somewhat closer monitoring, especially those with a total score of 11 or 12. Local conservation actions may also be appropriate for some of these species, especially those that congregate in large numbers at key sites.
Vulnerable (13-16): These are species/subspecies that deserve conservation attention. They typically have limited ranges, smaller populations, higher threats or significantly higher declines (or a combination of these) and they should be considered in conservation planning and action for regions and habitats where they occur. These actions can often be delivered at a habitat/landscape scale to benefit multiple vulnerable taxa rather than through individual species/subspecies programs.
At-Risk (17-20): These are birds that need more urgent conservation attention and many of the highest risk taxa should be considered for ESA listing if they are not already listed. Those at the top end of this rank are most in danger of extinction, and those at the lower end should receive specific conservation attention to ensure they do not go in that direction.
The terms Potential Concern, Vulnerable, and At-Risk do not suggest necessarily that there is concern that a species may be threatened with extinction. Lower scoring species may instead face population declines or range contractions that could negatively impact their conservation status, while some are more vulnerable to threats due to low abundance or small ranges but still have stable populations at present. Those species in the very highest scoring categories (the highest scoring At-Risk species) are those currently considered at risk of global extinction.
While some taxa are currently regarded as Secure, we should still address threats that may reduce their populations, as some of these may later need to be upgraded to higher concern categories. Also, human-induced threats (e.g. collisions) typically affect birds in all threat categories.
Interpreting Conservation Rankings
You may be wondering why some species familiar to you receive the conservation rankings that they do. For example, anyone living in Delaware might wonder how a species such as the Snow Goose could be ranked above "Secure". We think this is a good example, so here's the explanation:
The Snow Goose has a global population that is increasing, giving it a Trend score of 1 - the lowest on the scale of 1 through 5 - but its PIF population estimate of 3.9 million is in a similar range to birds such as the Canada Warbler and Evening Grossbeak. So, while the species may appear locally abundant, there are in fact many more species that are more numerous; hence its Population Size score merits a 3 (see the Partners in Flight Handbook for precise population sizes for each numeric category).
If you look at the range map for Snow Goose in any field guide, you will also see that these 3.9 million birds concentrate in a relatively small area for both breeding and wintering, making them more vulnerable to threats than more widespread species, and giving them a score of 3 for Breeding and Wintering Range.
While Snow Geese are themselves a threat to parts of the Arctic tundra, they also depend on it for nesting, and the tundra is one of the ecosystems most likely vulnerable to climate change. Large numbers of geese winter in saltmarshes, which are also vulnerable to climate change due to sea-level rise, while others depend on agricultural fields, which could be turned over to crops less favorable to geese. It therefore gets a score of 2 for Threat, not high, but also not the lowest.
For all these reasons, the Snow Goose has a Total Score of nine, putting it one over the maximum score for “Secure” and just into the “Potential Concern” category. However, if its population continues to grow as recent data suggest, it can soon be upgraded to Secure.
ABC would not recommend species-specific conservation attention for birds at the low end of this ranking category. In fact, Snow Geese may need to be controlled to boost populations of some more vulnerable species whose Arctic nesting habitat they can impact through over-grazing, but it is a bird we should keep an eye on, since there are enough factors to suggest that its conservation status could change negatively fairly rapidly if threats were to increase and its population trend were to begin to head downward. There are many other species that are at the low or high ends of categories, and if you wonder why a bird is ranked the way it is, you can check the full scores from the spreadsheet. As mentioned elsewhere, a strong negative population trend will almost always bump a species into a higher category regardless of its abundance. We'd also definitely like to hear from you if you think we have any birds ranked incorrectly.
Area Importance Scores
An important component of the list from a conservation standpoint is the “Area Importance” score (column Q). This score enables conservationists to determine how significant the U.S. population of a species is to its global survival. For example, it may be more important from a conservation perspective to invest in conservation in the U.S. for a species that has a significant portion of its range here, than one that may be rare in the U.S. but common in Mexico (for example). U.S. Area Importance has been assessed using a number of sources, but primarily the Handbook of the Birds of the World and the National Geographic Guide Sixth Edition which contains some excellent new subspecies maps.
Since this list and the associated checklist were produced primarily for conservation awareness purposes, we emphasize the scoring of species that are proven to nest or winter regularly, even in small numbers, and those whose regular occurrence during migration may potentially be conservation-significant for the species concerned, even if the total numbers occurring are small (e.g. Sharp-tailed Sandpiper). Our use of the term “accidental” on the list is therefore generalized to include species that do not appear to fit these criteria well, and whose occurrence is either sporadic (as opposed to irruptive), vagrant, highly peripheral to their core populations, or includes just a small fraction of their migratory populations. It differs somewhat from the American Birding Association's (ABA's) definition of the same term, and covers species from the ABA's categories “rare”, “casual”, and “accidental”, without being a perfect match for all birds covered by those ABA terms. Note that some seabirds are likely to occur more regularly in the U.S. EEZ and Hawaiian waters than records suggest, due to the minimal observer coverage in pelagic areas.
The Area Importance Scores, from 0 to 5 are assessed as follows:
0 = Accidental or Introduced (A or I on the spreadsheet). Note that the Red-crowned Parrot has an introduced (or part naturally occurring) population with potential conservation significance. The Mariana Swiftlet also has a tiny introduced Oahu population that may eventually have conservation significance if it were to grow and the wild population were to decline.
1 = 5% or less of range or population in the U.S. for some portion of its life-cycle – species at range periphery, vast proportion of population occurs elsewhere. These taxa are indicated by a colored circle with a white center rather than a solid circle on the spreadsheet.
2 = 5 - 35% of the population in the U.S. for some portion of life-cycle – population depends on the U.S. but needs the majority of its conservation applied elsewhere.
3 = 35 – 65% of the population in the U.S. for some portion of life-cycle – population is almost equally dependent on the U.S. and other areas.
4 = 65-95% of the population in the U.S. for some portion of its life-cycle – population is heavily dependent on the U.S. and needs most of its conservation applied here.
5 = 95% or greater of range or population in the U.S. for some portion of its life-cycle – species is endemic or functionally endemic to the U.S. and marginally significant numbers occur elsewhere.
We anticipate future upgrades to this first edition of the list, and we welcome general feedback as well as any specific additions, corrections, or suggested changes. Note that American Bird Conservancy has a much broader geographic focus than the area covered by the current list and we may expand the list to cover additional areas at a later time.