The Language Vorlin (2006)
In most regional varieties of American English, the sound represented by the letter R is more of a semi-vowel or vowel than a consonant. It is a relatively rare sound among the world’s languages.
J.C. Catford describes the production of this phoneme in A Practical Introduction to Phonetics (second edition), from which I quote:
The term ‘rhotacized’… has been applied to the very peculiar sound represented by ir in the word bird in a common American pronunciation of this vowel. This vowel was formerly described as ‘retroflexed’ but this is not a correct description… This very strange American vowel involves not only a concavity or ‘sulcalization’… of the tongue in the neighbourhood of the uvula, but also some slight degree of pharyngalization.
If you would like to hear specimens of this vowel, find a good clear recording of an American network TV or radio newscast and listen for words such as word, swerve, turn, heard. It is not an easy sound to produce. Children in Ohio and Indiana, where this phoneme is most extreme, often have difficulty learning to pronounce words like “girl” and “world.”