Marshside is within easy striking distance for me so I go a few times a year. Referring to Annabel's maps: The best time to do the Redshank path is probably at high tide when the birds congregate closer in, otherwise they can be spread out all over the mudflats and it's possible you won't see anything near by, although in May I think there will still be loads of Skylarks out on the salt marsh. However, if you have to choose only one walk, then I'd do the Cossens circuit or Rimmer's Route. There are often lots of birds at the back of the reserve which you won't see from any hide. Keep an eye open for Hares, too.