It's a reference to the story in Livy and several other authors that when the Gauls attempted an assault on the Capitoline (the rest of the city having been taken already) the geese alerted the garrison with their calls. But geese were by no means sacred, they were one of the most commonly-eaten birds, either caught by hunting or trapping, or else kept in confinement and force-fed (to fatten their livers). Geese were important in augury, but so were all birds, that's sort of the point of augury. The goose was also a sacrificial animal (Ovid thinks Isis in particular received geese as offerings, which might go back to the common supposition among the Romans that geese were sacred to the Egyptians) but there wasn't anything especially sacred about them--Ovid actually jokes that despite the heroism of the Capitoline geese the animal is still eaten and offered as sacrifice. Other authors complain what a nasty bird the goose is, since its calls are irritating and it eats everything during migration time