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The ABA Checklist Committee considers an exotic bird to be established in one or more regions of the ABA Area when the following eight criteria are met:
1) The species is recorded in the form of a published photograph or a specimen archived in an ornithological collection. This criterion ensures that species identification can be confirmed independently.
2) There is a more-or-less-contiguous population of interacting or potentially interacting individuals, rather than a scattering of isolated individuals or pairs. Most exotics present within the ABA Area are limited to metropolitan areas. For persistence, it is vital that exotic birds in these areas are not isolated from each other but rather occur in sufficient proximity to allow interaction and therefore gene flow. Some exotics are found in the ABA Area as a single interacting population, while others occur in several populations that are isolated from each other.
3) The population is not currently, and is not likely to be, the subject of a control program where eradication may be a management goal that is likely to succeed. Some exotics (e.g., Mute Swan) present a clear danger to native species or habitats, or to agriculture or commerce, in some areas, and listing these species as established may create a conflict between some birders and land management personnel.
4) The population is large enough to survive a routine amount of mortality or nesting failure. We cannot provide a numerical threshold for determining when an exotic species is established. The reason for this should be obvious: No single number would be adequate for populations as varied as large, long-lived parrots with low reproductive potential and small, short-lived finches with high reproductive potential. Demographic characteristics such as habitat preferences, lifespan, reproductive output, dispersal frequencies and distances, and genetic viability will be considered separately for each species. Members of the CLC will critically review each species based on the documentation provided and will make a judgment based on the best available evidence. Much attention will be given to factors such as population size, distribution, and, particularly, evidence of successful breeding. However, we recognize that some number of individuals is preferable as a baseline to judge when a species may be established. The FOSRC prefers that populations ideally contain at least "several hundred individuals," and the CLC agrees that in almost all cases, populations numbering only dozens of individuals may be too small to be considered established. Additionally, information should be provided to indicate that there is little or no evidence that ongoing releases play a substantial role in population maintenance. For gamebirds whose numbers may be artificially supplemented from time to time, evidence should be provided that these releases are not necessary to maintain population size or persistence.
5) Sufficient offspring are being produced to maintain or increase the population. Such criteria will vary from species to species, according to factors affecting the population, both natural (competition from other species; effects of hurricanes) and artificial (recapture for the pet trade; culling by hunters). Certainly, a species whose numbers are increasing and whose range is expanding is a better candidate for establishment than a species whose numbers and range are stable. Species with declining numbers and/or contracting range should have a much greater evidentiary threshold to meet before being considered established.
6) The population has been present for at least 15 years. Previous CLC criteria used a 10-year persistence threshold. As we have seen with several exotics, 10 years is an insufficient period to judge the likelihood that an exotic will persist. Accordingly, we have increased the persistence criteria to 15 years. The CLC readily acknowledges that 15 years may also be insufficient in some cases to determine establishment; populations of many exotics follow a "boom and bust" cycle over several decades—the population of Crested Mynas at Vancouver became extirpated more than 100 years after its introduction. With long-lived species (e.g., Amazona parrots) or when gamebird populations are regularly subsidized, one could argue that persistence should be for 30 or more years for genuine trends in the population to become obvious. Our point here is that like numerical criteria, no simple formula of the number of years for persistence can apply to all species. Flexible persistence criteria ("at least 15 years") and lack of numerical criteria will allow Committee members to exercise their own judgment in potentially uncertain or controversial cases, but only in the context of strong biological evidence and with the intention that the final judgment be a conservative one.
7) The population is not directly dependent on human support. Although somewhat subjective, this criterion is meant to exclude from consideration those exotics that rely on direct human support for their ongoing survival and/or persistence (reliance on bird feeders; periodic releases of additional individuals). For instance, the Monk Parakeet population at Chicago, Illinois is wholly dependent on bird seed provided by humans during the winter months, and this population therefore is not recognized by the CLC as established, despite its size or persistence.
8) A publication, ideally in a peer-reviewed journal or book, describes, how, when, and where the above seven criteria have been met. A publication will streamline the voting process by clearly presenting evidence of establishment. In the absence of a publication, the CLC may still vote on a motion to add an exotic to the ABA Checklist if such evidence has been gathered by a Committee member or other interested individual. In the latter two instances, a detailed analysis of the issue must be published in a suitable scientific source if the species has been determined to be established.
Note that the CLC has not mentioned any threshold for geographic range occupied in the ABA Area. Again, this will vary considerably between species, and the CLC will vote on each species on a case-by-case basis. As an example, during 2006, the CLC considered adding the Black-hooded Parakeet (Nandayus nenday) to the ABA Checklist based on a large and increasing population along the central Gulf coast of Florida. This species met all eight of the above criteria as an established exotic, but was nonetheless rejected because two CLC members were concerned that its geographic range (perhaps 150 square miles) was not sufficiently large to confirm establishment.
The CLC has chosen to "grandfather in" the 17 species presently found on the ABA Checklist that exist in the ABA Area wholly as exotic populations (species with both native and exotic populations, such as the Canada Goose or House Finch, are considered natives). The 17 exotics species presently on the ABA Checklist are the Mute Swan, Chukar, Himalayan Snowcock, Gray Partridge, Ring-necked Pheasant, Rock Pigeon, Eurasian Collared-Dove, Spotted Dove, Budgerigar, Monk Parakeet, Green Parakeet, White-winged Parakeet, Red-crowned Parrot, Red-whiskered Bulbul, Spot-breasted Oriole, House Sparrow, and Eurasian Tree Sparrow. (The European Starling is a native vagrant based on a specimen from Shemya Island, Alaska). If a CLC member or any other birder believes that one or more of these "grandfathered" species should be removed from the main part of the Checklist, then data should be gathered and published so that the Committee can vote on a motion for removal. The CLC readily acknowledges that some exotics currently on the ABA Checklist do not meet one or more of the above criteria, and that these species likely would be rejected as established species should the new criteria be applied to them.
The CLC hopes to eventually determine the states or provinces in which establishment has been attained for each of the 17 exotics that are on the main list of the ABA Checklist (we cannot determine establishment of a species on a more local level). The criteria—or more accurately, the lack of criteria!—used to determine establishment varies among the local records committees so substantially that the CLC feels it is necessary to produce its own list based on the above eight criteria.
Exotic species that become extirpated will be moved from the main list of the ABA Checklist to Appendix: 1, Extirpated Exotics, a list that currently contains four species. For species with greatly declining populations (e.g., the Budgerigar in Florida), we choose to wait until the population is completely extirpated before we vote on removing the species from the main part of the Checklist in the (unlikely) case that the population rebounds. The ABA Recording Standards & Ethics Committee has ruled that extirpated exotics cannot be "counted" on lists submitted to the ABA.
In addition to the 17 exotics currently on the ABA Checklist, literally dozens of other species have been observed within the ABA Area. In Florida more than 100 exotic birds have been documented by photographic or specimen evidence. At some future point, the ABA CLC intends to compile a list of all exotic species or species of uncertain provenance that have been recorded within the ABA Area.