I have had a thin hen recently who is now gaining weight but without special dietary help I'm pretty sure she wouldn't have made it. I tried lots of foods but these are the ones I found most effective/popular with this chicken: Suet pellets with insects - designed for wild birds these are very energy dense and apparently delicious even to a sick hen. I softened them by adding them to warm moist layers mash. Sunflower seeds - I used black whole seeds to begin with but switched to sunflower hearts as I felt that as the husk would not be digested they occupied precious space in her crop. Scrambled egg - Oddly seemed very appealing when she was off her other foods but started to reject it as soon as she felt a little better. Meal worms - I used the dry ones. She didn't want these when she was feeling very unwell but as soon as her appetite started to return she couldn't get enough of them. The suet pellets really were the big hit, at one point it was all she would eat. Maybe others would like to list some other foods they have had success with.