Herons' is pronounced exactly the same as herons and heron's: in the International Phonetic alphabet, we can write it as /ˈhɛrənz/ (that's the "h" from hot, followed by the "err" from error, and the "ons" from onions).
For regular plural nouns, the rule for forming the possessive is as follows (quoted from the book Teaching Pronunciation: A Reference for Teachers of English to Speakers of Other Languages):
When the plural form of the noun ends in -s (or -es), an apostrophe is
placed after the plural inflection to indicate the possessive in
writing. Notice that with regular plural nouns, there is no phonetic
difference between the singular possessive and the plural possessive
modifier: the girl̲s̲' books (sounds like the girl̲'s̲ books), the
neighbor̲s̲’ house (sounds like the neighbor̲’s̲ house).
As mentioned in this related question (Genitive Saxon: Do you append apostrophe s ('s) after plurals and words ending with s?) in general, the written sequence s's corresponds to /zəz/, and the sequence s' corresponds to /z/ with only a single /z/ sound.
Usually it is not important to distinguish plural and singular here. If you do need to, you can reword the sentence; in this case, you might say something like "This ball belongs to the [heron/herons]" or "This ball came from the [heron/herons]."