It's a bad omen of things to come.
During his confinement by Tiberius a like omen had been interpreted as portending his speedy release, with the warning that should he behold the same sight again he would die within five days.
Knowing it was a bad omen for the early foundings of a city to be breached so easily, Romulus killed Remus.
The first sound which he heard outside was an omen.
He makes them see the bad omen with their own eyes.