In a general sense a threatened bird species is one which is declining in abundance or occurrence and is headed towards extinction. However, just as 'oils ain't oils', threatened birds aren't just threatened birds.

There are different categories of threat, or status, for species based on the factors which are affecting them and the speed or dimension of their decline. These threat categories are determined mainly by the International Union for Conservation of Nature (IUCN). As a general guide species are sorted into one of the following categories:

Extinct

Extinct in the Wild

Critically Endangered

Endangered

Vulnerable

Near Threatened

Least Concern

These criteria are used by governments on local, state, national and international levels to evaluate the health of populations of species, and to determine where to direct valuable conservation resources. One thing of note is that in these lists the conservation 'currency' is taxa, not species. What this means is that organisms below species level (such as sub-species) can also be listed and be subject to conservation effort. This allows for the greatest diversity possible to be conserved, such that future generations of organisms can evolve.

One thing to bear in mind is that there are a number of threatened taxa lists around the world, and a taxon in one particular region may have differing status on different lists. For example, in Australia the Federal Environmental Protection and Biodiversity Protection (EPBC) Act was established in 1999 to protect species and environmental issues of national importance. Accordingly, as part of that, there are lists of threatened taxa. The same applies for each state, such as the Victorian Flora and Fauna Guarantee (FFG) Act 1988.

If we consider the Grey-crowned Babbler we can see the difference in the way a taxon or species is treated depending upon the jurisdiction it falls under. Within Victoria, the FFG Act recognises this species as Vulnerable due to declines in population size as a result of land clearance, habitat fragmentation, and a reduction of habitat quality. However, this species is not listed under the Federal EPBC Act as requiring protection, as the species is still common in other parts of Australia (e.g. New South Wales, Queensland, Northern Territory). This allows species of greater concern nationally to receive more conservation attention, with the knowledge that Victoria is working to reverse the declines. So the system is designed to work on a complementary basis, such that conservation resources are allocated to minimise threats to species survival as best as possible.