Fluid gender, we start to suspect, in marsh bird (5,4)

'marsh bird' is the definition.
'sedge wren' can be an answer for 'bird' (sedge wren is a kind of bird). I'm not certain of the 'marsh' bit.

'fluid gender we start to suspect' is the wordplay.
'fluid' indicates anagramming the letters.
'start to' suggests taking the first letters.
The first letter of 'suspect' is 's'.
'gender'+'we'+'s'='genderwes'
'genderwes' anagrammed gives 'SEDGE-WREN'.

'in' is the link.

Can you help me to learn more?