[Dominic]

Indeed, my chickens are on 75% stuff from the garden at weekends and Probably 90% mash during the week.
They had a few not quite ripe cherries that had split or been nibbled by bugs yesterday.

[hillfooter]

... Modern compound feeds provide 3.5% calcium which is plenty for even the most productive hybrid layers so there's only real need to supplement calcium to older birds...

This would be true if all backyard chicken keepers took your fine advice   and only fed their chickens on commercially produced mash or pellets.

For people like me who encourage their chickens to eat plenty of weeds, grass, bugs and worms each day, or people who regularly feed kitchen scraps, they also need to have shell grit available so the chickens can regulate their own calcium intake.

I actually think that calcium deficiency is a fairly rare condition in backgarden poultry.  It's only likely in situations were they are kept confined on sterile or artifical surfaces such as concrete.  Birds that are free ranging will generally find enough for themselves unless the ground is devoid of limestone and gravels.  Coupled with the likeihood that they will also have access to compound feed which is usually 4% calcium which is plenty enough for a star commercial layer which is producing 320 eggs a year.  Most backyard poultry would produce not much more than half that.

Calcium deficiency is much more likely to occur in commercial cage layers or possibly barn raised birds if they are not fed a proper compound diet and these are all young birds, less than 80 weeks old.  Birds also pickup minerals and vitamins from picking in their droppings though i don't advocate encouraging this.

Most backyard chx live a natural lifespan of typically upto 6 years or so and older birds are less efficient at absorbing calcium which is where oystershell comes in.  Being large particle size it is retained in the gizzard where it is used to grind up food and the slower release of calcium that produces, benefits older birds.

Chx as they age often suffer from poor eggshell quality (thin shells, chalky surfaces, porous or cracked shells, wrinkled or deformed shells, calcium pips et al).  Whilst some of these conditions might indicate lack or poor absorbtion of calcium in the majority of cases there's some other cause which can't be fixed by feeding excessive calcium which Lindeggs reference reports can cause severe damage to a chx kidneys.  Particularly if a nutrional deficiency is suspected it's easy and worthwhile to increase calcium and vit D3 initially in such cases.  It's an easy option to try which is why there the usual insistance on feeding more calcium but don't persist beyond two weeks if no improvement is noted.

Rough chalky shells with pips are usually a sign of excess calcium not a deficiency.

I offer oystershell ad lib to my birds which are in or beyond their second season.

Best wishes
HF