Roman omens showed the will of the gods
Don't ignore the gods
The Roman omens were very important and with good reason!
When, after the victory in Troi, Odysseus forgot to bring a sacrifice to Poseidon (Neptunus), the god would hunt him for ten years. The lesson that Homeros taught the Greek and Romans as well with his Odyssee was, that their anger can last very long, if you don't obey their will.
So it was very important to know the will of those whimsical gods. And, as always, there were the priests who would interpret these Roman omens. Augurs and haruspices were specialists, but everyone could observe deviations from what was considered normal and report these omens to the authorities.
More prodigies and bad omens after a disaster
In his book Ab urbe condita (History of Rome) Titus Livius starts the description of every year with the omens that had been seen. After the lost battle at the Trebbia he reports numerous omens All these Roman omens are to be taken very seriously. The Romans did:
"The senate immediately proclaimed a sacrifice festival of nine days for the
stone rain in Picenum and for the other omens the decemviri
(ten-men) were instructed to consult the Sibylline books"
Bus see also Cassius Dio 47.40 in the midst of the proscriptions in the 2nd Triumvirate and the measures taken by the aediles.
Altes Museum Berlin
These books were obtained by king Tarquinius Superbus from Sibyl of Cumea and kept secure in a vault beneath the temple of Jupiter on the Capitoline hill. In times of great danger these Sibylline books were consulted by the decemviri in order to establish the right measures to be taken. The Sybils were considered wise women, who would utter prodigies and give advice not only when asked for but also at their own initiative.
A consul wouldn't order his army to attack without having consulted the will of the gods first, while the rightness of his appointment after his election to consul, had to be approved by the gods before he was installed in office. This taking of the auspicies was done by the augurs. The first to have auspices taken were Romulus and Remus in order to decide which of them was to be king.
Ignoring the auspices will bring bad luck
Gaius Flaminius was the first consul who shirked this obligation. He knew what the augurs could do to prevent him from being appointed. Needless to say he lost the battle.
It's obvious that before a battle the auspices had to be taken. But it was not very convenient to have to sit and watch the birds for several hours. Therefor the pullarius would bring a cage with the sacred chickens. When it was time to consult the gods, the chickens were released and fed. The more eagerly the chickens ate, the more favorable the signs were. If they refused to eat or even flew away was considered a bad omen. At both the battle against the Samnites (293 bc) and the battle of Drepanum the chickens refused to eat. Both consuls attacked anyway. The outcome of both battles was very different.
The entrails tell the truth
Yet another way of learning the will of the gods was practiced by the haruspices. A haruspex would read the entrails of a sacrificed animal.
Haruspices were also very competent in interpreting lightning.
Even thunder and lightning were prodigies
Lightning and thunder were important because the Romans feared these omens so much that when Pompeius feared to loose a debate in the senate he claimed to have heard a thunderstruck and de meeting was ended immediately. See also Livius (23.31).
The haruspices were also specialists in reading the lightning. Like with birdwatching they divided the terrain they were observing in sectors and depending of the direction of the lightning and the sector hit they claimed to know wich god was sending a good or bad message.