Romanis eam* legionibus Gaius Marius in secundo consulatu suo proprie dicavit. erat et antea prima cum quattuor aliis: lupi, minotauri, equi aprique singulos ordines anteibant; paucis ante annis sola in aciem portari coepta erat, reliqua in castris relinquebantur; Marius in totum ea abdicavit. ex eo notatum non fere legionis umquam hiberna esse castra ubi aquilarum non sit iugum.
* sc. aquilam
(Pliny the Elder, Nat. Hist. 10.16)
The eagle was assigned to the Roman legions as their special badge by Gaius Marius in his second consulship. Even previously it had been their first badge, with four others, wolves, minotaurs, horses and boars going in front of the respective ranks; but a few years before the custom had come in of carrying the eagles alone into action, the rest being left behind in camp. Marius discarded them altogether. Thenceforward it was noticed that there was scarcely ever a legion’s winter camp without a pair of eagles being in the neighbourhood. (tr. Harris Rackham)
Crambe Repetita 