Question

ｇｍ

画像下線部のカンマがついている理由がわかりません。

次の英文の下線部を訳しなさい
It was at lunch on a cold Sunday in late January that we first
heard the noise. It sounded like a tapping; slightly metallic
Reluctant to leave the table, at first we speculated on the
possibilities. But when the tapping became persistent, or rather
insistent, we went to investigate. And so we came upon the
blackbird, pecking at the window.
(法政大)