I saw the photo before I read the question, and I felt that familiar twinge. A tiny chick at a Tractor Supply, its rear end sporting what looks like a fleshy little balloon. The original poster saw it too, and their question is one every chicken keeper has asked at some point, usually at 10:45 PM with a flashlight in one hand and a manual in the other: is it a yolk sac that hasn't been absorbed, or is it a hernia? My honest take is that this is the kind of moment that separates the casual chicken admirer from the full-on, slightly obsessed chicken person. You care enough to *notice* the bumpy butt. You care enough to ask. And frankly, that matters more than the answer itself.
Let's get practical, because I know you want to know what to do. In a chick this young, a protruding yolk sac is the more common culprit. That yolk is supposed to get absorbed into the body during the final days of hatching, providing nutrition for the first few critical hours. When it doesn't, it hangs out like a reluctant carry-on bag. An umbilical hernia, on the other hand, is a hole in the abdominal wall that lets some gut protrude. Both look alarmingly similar to the untrained eye, and neither is something you should poke, squeeze, or panic over. What matters most is the chick's behavior. Is it eating? Drinking? Pooping? Chirping with the general chaos of a typical baby bird? If yes, you have time. If the bump is shiny, dark, or smells bad, that's an infection or a dead piece of tissue, and that chick needs a difficult conversation with a veterinarian who actually sees poultry. Most farm store employees are not that vet.
If a reader asked me about this particular chick, I would tell them to observe first, intervene second. A clean, warm environment is step one. You can try applying a tiny dab of antibiotic ointment (without painkiller) to keep the sac moist and protected, but the real job is watching. The body might pull that sac back in over a week or so. A hernia will not resolve itself and usually becomes a hard, stuck lump. In that case, the kindest option is often a humane cull, not because you failed, but because a chick with a permanent hernia is going to struggle with everything from walking to digesting. Nobody likes to hear that, least of all me. But pretending a serious problem will fix itself just leads to a drawn-out, painful ending. You have the power to make the hard call early.
So what do you do with this information? Next time you're at the Tractor Supply bin, look at the chicks with your new knowledge. And if you see one with a bumpy butt, you know the first step is not a frantic Google search, it's checking whether that bird is thriving or just surviving. That's the difference between being a chicken owner and being a chicken guardian. Be the guardian. And maybe bring home some probiotics while you're at it.