Translation:The birds fly.
to fly : (verb)날다.
Root(the part that has meaning):'날-' / Ending(the part that changes):'-다'
When the verb/adjective whose final consonant of last syllable of the root is 'ㄹ'; it is ALWAYS dropped when the following ending is starting with vowel (아/어/았/었/ㅂ니다-/ㄹ-/ㄴ- ...).
Therefore 날 + ㅂ니다 >> 나+ㅂ니다 >> 납니다.
lead : (noun)납
to lead : (verb)이끌다/끌다/인도하다/안내하다/연결되다/이어지다/이르다/지휘하다/선두를 서다
This is sadly not true. The final ㄹ is generally dropped when followed by ㅂ, ㄴ or ㅅ. It is NOT dropped when followed by a vowel (this includes 아/어/여/았/었/였). 날다 is 납니다 in formal polite speech (ㄹ is followed by ㅂ, so it disappears), but 날아요 in informal polite (ㄹ is followed by 아 so it stays on it's place)