A completely useless creature that does nothing but cause nuisance and aggravation.
Originally coined in the US Navy by sailors tasked with washing decks clean of the massive amount of avian excrement created by sea birds that shit all over everything.
Has come to define any living creature, human or otherwise, that is without redeeming qualities.
Sailor: "I just got assigned to swab the fucking launchdeck... The goddamn shitbirds flew over last night and completely destroyed it."