The protocal which is used is based on the criteria of the IUCN. That means when a species is not seen over a period of 50 years in its known or suitable habitat it could be declared extinct. From time to time there are researchers and academics who made surveys to refind thought to be extinct species. Sometimes over a longer period. And first when all the efforts to find this species again were failed the IUCN declared this species officially extinct.
EXTINCT (EX) A taxon is Extinct when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon's life cycle and life form.
The bird species that are treated as Extinct by BirdLife are taken from a comprehensive study undertaken by the Committee on Recently Extinct Organisms (e.g., MacPhee and Flemming 1999, Harrison and Stiassny 1999). All recently extinct bird species are treated, subject to three criteria:
1) They must have a formal name and hence be a valid taxonomic entity. Therefore four apparently distinctive but as yet undescribed Mascarene forms are discounted: a fody from Réunion and a petrel, a bulbul and a babbler from Rodrigues (Cheke 1987, Cowles 1987). The petrel is known from travelers' reports as well as subfossil bones, the bulbul and the babbler from bones only, and the fody (see Foudia bruante below) just from historical accounts (Cheke 1987, Cowles 1987).
2) They must be considered a full biological species. Knox and Walters (1994) give a list of extinct subspecies, largely based on Greenway (1967); a thorough update of this list is still required. Thus, for example, the following are discounted: Anas (platyrhynchos) oustaletti, Turnix (varia) novaecaledoniae, Halcyon (cinnamomensis) miyakoensis, Psitticula (echo) equus, Hemiphaga (novaeseelandiae) spadicea, Lalage (leucopyga) leucopyga and Acrocephalus (familiaris) familiaris, for which the prevailing published opinion is that they are valid at the subspecific level only, along with a number of other names which are almost certainly synonyms (see list of hypothetical taxa below).
3) The species must have been documented to have survived past (the admittedly arbitrary) ~AD 1500. Such documentation includes paintings and reports, which Banks et al. (1993) recognize as being valid historical types. All species known only from pre-1500 subfossils are excluded, recognizing that this excludes a number of New Zealand (Holdaway 1999), Pacific (Steadman 1995) and Hawaiian (Olson and James 1982) extinctions from the last two millennia. No moa species survived as late as 1500 (Holdaway and Jacomb 2000, contra WCMC 1992) nor Aepyornis maximus or Cygnus sumnerensis (Baillie and Groombridge 1996).