I have three ISA Brown hens. They were about 5 months old when I got them in December 2015, which means they're around 2 years old now. One in particular (her name is Subway) has always seemed a little more "fragile" than the others. She loves attention, loves to gently peck at my clothes or hair for hours if I'd let her, and is okay with being held. Unlike the others, she doesn't run unless she's being chased (or has been left behind), and she sometimes perches 6-12" up, but prefers to stay on the ground. She always sleeps in the nesting box, and the others sometimes do it too. Today, in the late afternoon, she was out in their run. She walked slowly, didn't move around much or seem very interested. After a while she pooped something strange and it took a while before I realised it was an egg with little/no shell [attached]. I shut the others out of the run so she could eat it in peace. She eventually ate most of the yolk, but took at least 30 seconds of blank staring between each peck. Meanwhile the others were desperate to get at it. It's starting to get dark and she's the first one to walk herself up into the coop. Some time in the past few days, someone laid an egg in the nesting box that had a shell, but was so weak that it crumbled when I went to pick it up, and I now suspect that it was her. In the past all of their eggs have had strong shells. Their feed hasn't changed - they get a layer's mix of pellets, sunflower seeds, and assorted seeds. You can see a pile of this feed in the pic. Is this something to worry about? Should I look into calcium supplements? Protein? Could there be some underlying issue?