There is no considered immediate threat of survival of a species that is categorized as least concern. This does not mean that the species faces no threats; populations may even be on the decline in species in this category. The population status of species must be reevaluated on a regular basis in order to be listed in any catetory on the IUCN Red List.
A species that is considered to be near threatened by the IUCN Red List may face extinction at some point, but do not currently meet the criteria to be categorized higher in the list. The future survival of birds in this category may depend on conservation actions to reverse any threats they face.
A species is categorized as vulnerable when it is believed it faces a high risk of extinction in the medium-term. Measures need to be taken to change the current circumstances facing the species and its prospects for long-term survival. Bird species are usually put into this category when they face loss or destruction of habitat. A 20% population loss over 10 years or three generations will place a species in this category.
The conservation status of a species in an indication of the likelihood that the species will go extinct. The status of endangered indicates that a species faces a high risk of extinction in the near future. It is the second-most critical conservation status as categorized by the IUCN Red List. The Endangered Species Act of the United States may categorize species differently than the IUCN list.
Critically Endangered birds face a high risk of extinction; it is the highest risk category assigned by the IUCN Red List for wild species. There are several criteria that can result in this status, including a population of less than 50 mature individuals, or a declining population of less than 250 adults with extreme population fluctuations. The Red List does not consider a species extinct until extensive research has been conducted.
A bird is considered extinct if there is no reasonable doubt that the last individual has died. This is the case when exhaustive surveys in known and/or expected habitats have failed to record a single individual of the species. Surveys are done over a time frame that corresponds with the bird's life cycle. Extinction can be caused by a number of factors, including loss of habitat, disease, and climate change.