Our hens are on pasture and exposed to wild birds, moved regularly but some weeks not enough (time). I like many get soft shells. Our hens have organic pellets, seaweed meal, kitchen waste, oyster shell and all the bugs they can eat.
Like pigs, it is illegal to feed chickens kitchen waste...
You could certainly get the vet to check faecal samples for egg counts, or post mortem any that die to get a better idea as to requirement for worming.
Worms have often developed due to increased in intensive farming practices, so in the old days there would have been fewer around and with low stocking densities and rotational grazing plans there would have been less disease due to worms. But equally there were fewer medications for anything and higher losses and generally lower production would have been the norm.
I wouldn't have thought any soft shelled eggs would be normal with feeding layers pellets and grit, so personally I'd be consulting your vet to see what is going through your flock.
Agree with fsmnutter, more extensive ground, less stocking density, less production in previous years tended to mean that there would be less exposure to worms in years past and less need for wormers. The people that did have higher stocking/set stocking would have just suffered more loss as there weren't the drugs to help too.
The good news is that if you specifically follow a stocking regime geared towards minimising worm contamination, you will likely need less wormer. Learning all about worms and about ground management can really help in that regard. There's a fab vet and poultry judge called Victoria Roberts who produces books. Her website is vicvet something - you could always check out the blurb on her material and see if any of the publications suit your needs. I met her at a vet conference, she's great.
You'd probably want to get the chickens worm-egg-counted a few times in the first year (or couple of years, depending on what you find) just to get a picture of what is going on on your ground but then after that, if there are not many worms, you might find it cheaper just to worm-egg-count at "a key time/or key times" of the year to keep an eye out rather than worming.
Specialist poultry worm egg counts and poultry postmortem examinations are available from various places in Scotland, England and Wales. I know because I work for one of the places! (SAC Consulting in Scotland).
Really great to see people so enthusiastic about reducing medication needs in production animals, also good solid replies, you're all fab.
Hopefully free chicken online talks soon - I'll try to remember to stick a lot of stuff about worming in them, and I'll post dates online as soon as I work out when I am going to do them...
Obviously contact with wild birds means there will never be no worm or disease risk unfortunately, but there is a lot you can do to minimise risks and still use less in the way of medication if that is the production type you are aiming for. Always take advice from a vet though, when changing your prophylactic health regime, even just a quick run-by on the phone - you need to avoid an accidental parasite breakdown at all costs or your ground could get very "poultry sick" quickly if you do something wrong by mistake. A quick chat is sometimes all it takes to avoid a problem.
C