The start and the concept

Another 'research' project for 2012 is combining Hens in a Fruit Cage. The theory is that the hens will eat the insect pests on and around the fruit plants, provide copious amounts of fresh fertiliser on the fruit cage ground, keep the grass and most of the weeds down and hopefully make me a saving of the very expensive hen feed of Organic Layers Pellets (Costs around £15+ per sack compared to about £6 for the ordinary).
The fruit cage will be built over the next month (so maybe a bit late for most fruits this year but will get some in) and it will be like an extension to the existing hen run (we have to have a fenced hen run due to Foxes). The idea is we let the hens in most of the year and then shut them out when the fruits start to look appetising!
What I need to research is which fruits will best survive the hens' natural urge to peck at anything green or shooting. Maybe prickly things like gooseberries, raspberries etc or things they can't reach like cherries. any experience on this or helpful comments or just plain chat would be very welcome