ladybug vs ladybird

ladybug and ladybird both are nouns.

Word	Noun	Adjective	Verb	Adverb
ladybug	Yes	No	No	No
ladybird	Yes	No	No	No

As nouns, ladybug and ladybird are synonyms defined as:

ladybug and ladybird: small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests

Other synonyms of ladybug include lady beetle, ladybeetle, ladybird beetle.

ladybug (noun)	ladybird (noun)
small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests	small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests