distance between centres of lines；distance between centers of tracks；midway between tracks

a bird in the bush

a bird in the hand

A bird in the hand is worth two in the bush

a bird of Jove

a bird of Juno

a bird of Minerva

a bird of paradise

a bird of passage

a bird of peace

a bird of prey

a bird of wonder

a new strain of bird flu

adjutant bird

altricial bird

amidostomosis of aquatic bird

an early bird

animal bird manure

anseriform bird