Old English bird, rare collateral form of bridd, originally "young bird, nestling" (the usual Old English for "bird" being fugol), of uncertain origin with no cognates in any other Germanic language. The suggestion that it is related by umlaut to brood and breed is rejected by OED as "quite inadmissible." Metathesis of -r- and -i- was complete 15c.
Figurative sense of "secret source of information" is from 1540s. Bird dog (n.) attested from 1832, a gun dog used in hunting game birds; hence the verb (1941) meaning "to follow closely." Bird-watching attested from 1897. Bird's-eye view is from 1762. For the birds recorded from 1944, supposedly in allusion to birds eating from droppings of horses and cattle.