There is a difference between species names and collective nouns: Species names can come in plural and in singular, but collective nouns never come in singular, like the collective nouns, חטה (wheat), שעורה (barley), פשתה (flax), ענב (grapes).

And moreover, the noun שֵעָר (hair) is a collection of hairs and in construct state is שער ראשו- the hair of his head, but שערה (hair) is a singular noun (and the noun שַעַר with a פתח vowel refers to hair that is not typical). And from this, the sages of blessed memory teach that שֵעָר is never less than two hairs.