Q: My chicken has a lump or tumor on her chest--what should I do?
A: You are probably seeing your chicken's crop. A chicken's crop is a part of her digestive system, and is located on her breast. You see it in the illustration below, closest to the front. When it fills up with food, it may stick out a little bit.
It will be smaller in the morning before she has eaten, and larger after she is full. The crop or craw holds food before it goes further down their digestive tract. Chickens basically store food in their crops; it is like a chicken lunchbox. Then it moves into the stomach or proventriculus and gets soaked with digestive juices, before finally getting "chewed" in the gizzard.
The gizzard chews food by using small stones, or grit, to grind the food up. Chickens need grit because they don't have teeth. Chick starter is typically milled finely enough so that baby chicks don't usually need grit unless they're eating something other than starter. Chickens will need grit once they begin eating treats and other foods, for example if they're foraging in the yard. If your chickens free range, they will probably pick up grit, gravel or small stones on their own, but we usually recommend providing extra just in case.
So the message here is: you probably shouldn't worry at all. Chicks fill up their crops because they grow fast and eat a lot relative to their size.
If the lump doesn't change in size or is hard all the time (don't press too much, because you can force the food back up into your chicken's mouth and cause her to aspirate her food--very dangerous!), your chicken might have an impacted crop, and you will need to get her to a vet. An impacted crop can happen if she doesn't get enough grit or gravel to digest her food, or if you feed her long strands of grass or hay that are too fibrous for her system to handle. Grass is a nice treat when chickens eat it on their own, but keep in mind that a foraging chicken will eat only small pieces, not long strands that can get wound up into a ball in her crop! If that happens, other food won't pass, and she will need veterinary attention, or she could starve.