The phrase at large means freely or in a state of freedom. In this sentence, it describes birds flying freely without any restrictions.

১. "At a stitch" is not a standard English idiom; perhaps it refers to "in a stitch" (laughter).
২. "Random" would need to be "randomly" (adverb) to modify the verb "fly."
৩. "At large" is also used to describe a criminal who has not yet been captured.

অতিরিক্ত তথ্য: The word "large" in this idiom comes from the Old French meaning "free."