There's many things that can cause swelling of the abdomen, or just enlargement.Today or tomorrow we can try to start out inspecting a small batch of them.
(What would swollen bellies/abdomens indicate)(And what kind of lumps would I be looking for?)
Reproductive Problems such as Ascites(Water Belly), Feels like a water bed, Internal laying, can feel firm. These are usually seen with the penguin posture though.
Reproductive Tumors can feel firm.
Fat hens have large bellies, are abit in between squishy, & firm.
A hernia would be a sack, with the feel of something inside.
Constipation can cause the abdomen to swell, but constipated birds would be obvious with the birds showing signs of straining, or not being able to poop.