The following is an excerpt from a translation of Chekhov's story "The Student" published in the Norton Critical Editions. Given the prestige of this publication, one would assume what's printed to be, at the very least, grammatically correct.
Is the following sentence correct? If so, how might we parse it to warrant this judgement?
The blackbirds were calling and a creature in the nearby swamps plaintively hooting as if blowing into an empty bottle.
- Anton Chekhov's Selected Stories Texts of the Stories, Comparison of Translations, Life and Letters, Criticism. New York: W.W. Norton & Company, 2014.
I don't why the author doesn't need to use the 'be' verb with the progressive tense in the second clause, like so:
"The blackbirds were calling and a creature in the nearby swamps was plaintively hooting as if blowing into an empty bottle."