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Terms & definitions

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Data quality and data derivation

The risk of extinction for each species is evaluated against the IUCN Red List Criteria which set thresholds by which species are assessed as globally threatened, based on population and range size and rates of decrease in these. The quality of these data varies between species, and therefore in BirdLife's World Bird Database, data quality and data derivation, are coded to indicate the probable accuracy and reliability of the data. This information is shown in the additional data tables. The terms used and their definitions are shown below.

Data quality

Population Size and Trend

  • Good = Based on reliable and complete or representative quantitative data
  • Medium = Based on reliable but incomplete or partially representative quantitative data
  • Poor = Based on qualitative information, but no (or potentially unreliable/unrepresentative) quantitative data

Range Size

  • Good = Boundary largely defined by localities/areas in which the species is known currently (>1980) to occur, and/or by areas of suitable habitat at appropriate elevations where it is thought that it is highly likely to occur
  • Medium = Boundary largely defined by localities/areas in which the species is known currently (>1980) to occur, or to have occurred in the recent past (>1970), and/or by areas of suitable habitat at appropriate elevations where it is thought that it is likely to occur
  • Poor = Boundary largely defined by localities/areas in which the species is known to occur, or to have occurred, and/or by appropriate buffers and/or geographic features where it is thought to occur

Data Derivation - This refers to the method by which the data was obtained, which contributes to its reliability. The definitions for data derivation are based on the Guidelines for using the IUCN Red List Criteria, modified specifically for population and trend estimates. Derivation is not coded separately for range estimates.

Population Size

  • Observed = Based on complete population counts
  • Estimated (directly) = Based on surveys or direct estimates
  • Estimated (indirectly) = Based on an explicit calculation of estimated AOO and estimated population density derived from related species
  • Inferred = Based on indirect evidence in units of number of individuals e.g. numbers captured for trade
  • Suspected = Best guess based on circumstantial evidence in units other than number of individuals e.g. a best guess based on intuitive understanding of the species' ecology and range

Population Trend

  • Observed = Based on complete population counts at beginning and end of trend period
  • Estimated = Based on surveys, interpolated or extrapolated through trend period
  • Projected = Based on estimated trends extrapolated into the future
  • Inferred = Based on indirect evidence in units of number of individuals e.g. change in number of individuals in trade
  • Suspected = Best guess based on circumstantial evidence in units other than number of individuals e.g. rate of habitat loss