Learn Farsi: قوها سفید هستند، با این حال جوجه‌های آنها خاکستری هستند. - Swans are white; nevertheless, their chicks are gray.

It literally translates to "with this state" or "with this condition" (با = with, این = this, حال = state/condition). It works exactly like "nevertheless" or "even so" in English, introducing a contrast to the fact that was just mentioned.

Since swans and chicks are animals, shouldn't the verb be in the singular form است instead of the plural هستند?

In Farsi, inanimate plural subjects must take singular verbs, but animals fall into a middle ground. You can use either the singular است or the plural هستند for animals. Using the plural, as we do here, is very common and sounds a bit more formal, or simply treats the animals more as active individuals.

Why is there an extra ی in جوجه‌های آنها?

The word for chick is جوجه and its plural is جوجه‌ها. Because we need to connect "chicks" to "their" using the ezâfe linking vowel (which sounds like "e"), and جوجه‌ها ends in the long vowel "â", we must add a consonant to bridge the gap. The letter ی acts as this bridge, creating جوجه‌های آنها (jujeh-hây-e ânhâ).

The word for gray, خاکستری, looks like it contains the word for dirt or dust. Is that right?

Yes, exactly! It comes from خاکستر (khâkestar), which means "ash." That word itself is built from خاک (khâk), meaning "dust" or "earth." So literally means "ash-colored.".