There are no special feminine forms to be used before a word starting with a vowel. They exist for the masculine version simply for the sole reason to make the whole conglomerate easier to pronounce (to French tongues) because of the two vowels which would otherwise follow each other. The feminine versions are pronounced ending in a consonant, hence there's no need for a special version.
Ce oiseau (here the two vowels e-o would clash), so it has to becomeCet oiseau (the intervening t provides a nice transition).Cette amie (the "e" at the end is not pronounced, therefore there is no vowel clash).
-- Chris (not a native speaker)
Bonjour Bhairavi !
Thank you for this interesting question.
I've decided to add a note and examples to the lesson to address that excellent point :)
I hope that's helpful!
Bonne journée !
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