I am a native speaker and teach pronunciation. I think /ir/ is a STRESSED vowel with an /r/ sound and /er/ is an UNSTRESSED vowel or schwa with an /r/ sound. Stressed vowels are generally longer and stronger than the unstressed schwa.
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AnonymousAn interesting tidbit here, I think, is that the short "i" vowel in "bird" has no phonetic purpose: A "silent" vowel. Take the vowel out of "bird" or any of those "vowel-r" examples and you can still correctly pronounce the word!
Not so for my (north) English accent. When I say "bird" you will not hear the "r" and the "i" is a long "oe" sound.