The statement that there are 21 breeds/types of cockatoo is not quite correct.

According to most modern taxonomic authorities, there are 18-20 recognized living species of cockatoos. Some key facts about the number of cockatoo species:

• BirdLife International currently recognizes 19 extant cockatoo species.
• The IUCN Red List also identifies 19 existing cockatoo species plus 2 extinct species (the Kangaroo Island cockatoo and Paradise parrot).
• Other sources list anywhere from 18-20 living cockatoo species today.
• The exact species count differs based on how certain subspecies are classified.
• There are numerous additional cockatoo subspecies and color mutations.
• But clear genetic evidence supports 18-20 distinct species within the scientific family Cacatuidae.
• The lower estimates tend to group some geographic variants as subspecies rather than separate species.

So while there is some continuing debate, most experts today agree on 18-20 scientifically defined living cockatoo species, with several more extinct and many recognized subspecies. The diversity occurs within this approximate range, not the higher counts over 20 that were proposed decades ago.

Why do a lot of courses suggest there are 21 species of cockatoo?

Modern taxonomic classifications generally identify fewer species today based on newer genetic evidence. Here’s some more context:

• In the past, some geographic subspecies were elevated to full species status, inflating the counts.
• With recent genetic analysis, many of those variant populations appear to be the same species, just isolated regional forms.
• For example, the Tanimbar corella was previously classed as a distinct 21st species – it’s now grouped as a subspecies of the little corella.
• Similar reclassifications of the Goffin’s cockatoo and others brought the species count down.
• Outdated sources may still list 21 or more species based on outdated taxonomy.
• But today, most authoritative organizations like BirdLife International, IUCN Red List, and ornithological societies recognize fewer species – in the 18-20 range.

So, the higher 21 count appears to be from outdated taxonomic classifications. With modern genetic testing reorganizing relationships, current experts accept fewer total species – making the 18-20 range the accurate figure.