Refurbished Nursery Rhymes: Old MacDonald or Afoul of Vowels

Old MacDonald had a farm With some noisy critters on it. They mostly mooed, or neighed, Or clucked, or brayed, But some stayed Resolutely mute, I'm afraid. It wasn't his equines, Or bovines, Or fowls, Or anything lodged in his barns or corrals, But a couple of rather recalcitrant vowels. The "e" and the "i" and the "o" Answered the call, While the "a" and the "u" Stayed conspicuously AWOL.