My runt Dark Cornish had a similar issue last week.
I noticed some yolk stains in the coop but couldn't find the rest of the broken egg. I looked at all the girls and saw Brownie trailing everyone. She also let me walk right up and pick her up w/out fanfare, which is a bit unusual. Her vent was pulsing and I noticed a strip of soft shell hanging out her vent. I pulled what I could see.
I wound up giving her a couple warm baths, not only to clean her up, but hopefully to lubricate and relax her vent. I probed a bit while she was soaking but didn't find any additional soft shell or fragments. I gently manipulated around her belly and vent, pulling toward the vent, hoping if their was a bound egg that it would wiggle loose. Her vent pulsed for two days straight, seemingly non-stop. She was reluctant to eat or drink and isolated herself from the rest of the flock. After her first bath, I put her in an isolation cage in the garage and provided free-choice yogurt, sliced oranges, and water. Atop this, I pushed 4cc of a liquid calcium/Vitamin D supplement (Wellesse brand from
Walmart) down her throat and hit her w/ 0.5cc of tetracycline under the skin. She wasn't happy w/ either.
Day 2 was more of the same. After her morning warm bath, calcium supplement push, and needled meds, I let her free-range w/ the flock. She continued to lag behind and isolate herself. She did make her way to the nest box midday, however I think it was more of a rest stop than urge to lay. She spent most of the day by herself. She made her way to roost on her own in the evening, so I figured I'd let her spend the night in the coop.
Day 3 she was a new chicken, like nothing ever happened. She was eating, drinking, begging for treats, and scratching in stride w/ the rest of the flock. I have yet to confirm she has laid since returning to "normal."
Of note, I have noticed a drop off in egg production for the flock in the past 10 days, which coincides w/ the removal of their heat lamp. I had this on during the night over the winter when temps dropped below 40. I had to remove the heat lamp for my make-shift brooder for a few chicks I recently hatched out. Perhaps the abrupt night-time light change threw the birds' cycle off. Who knows...
