Broadsides do not work on gulls (3,5)

I believe the answer is:

sea birds

I'm a little stuck... Click here to teach me more about this clue!

'gulls' is the definition.
The definition and answer can be both animals as well as being plural nouns.
Maybe there's an association between them I don't understand?

'broadsides do not work' is the wordplay.
'not' indicates named letters should be taken away.
'work' is an anagram indicator.
'broadsides' with 'do' taken away is 'brasides'.
'brasides' with letters rearranged gives 'SEA-BIRDS'.

'on' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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