Pasty butt sounds like a joke until you understand the anatomy: a chick's vent is the single opening it uses to pass droppings, and when soft droppings dry and cake over that opening, the chick cannot pass anything at all. Waste backs up inside a very small body, and a chick that was merely untidy in the morning can be in real trouble by the following day. It is common in the first week or so of life, especially in chicks that have been shipped, chilled, overheated, or stressed, and it is straightforward to treat if you handle it gently and promptly.
What you are looking at
Check the rear end of every chick daily during the first week — a quick look while you refresh water is enough once you know the picture. A chick with pasty butt has a crusty, dried blob of droppings stuck to the down around and over the vent. The chick may strain, cheep in a distressed way, stand hunched, or go quiet and stop joining the others at the feeder.
Clearing it: warm water, patience, no pulling
Hold the chick securely and calmly in one hand. Soak a cloth or cotton wool in warm — not hot — water and hold it against the caked droppings for a minute or more to soften them, or trickle warm water gently over the area. As the plug softens, work it away in small pieces with the damp cloth, re-wetting as needed, until the vent itself is visibly clear and open. Never pull a dry plug off and never pick at it with fingernails or anything sharp: the skin of a days-old chick tears easily, and a torn vent invites infection and further sticking. If the blockage is stubborn, patience and more warm water are the answer, not force. Once clear, pat the area dry, and if the down is wet, make sure the chick goes straight back under the brooder's warmth to dry out fully, since a damp chick chills fast. A tiny smear of plain petroleum jelly around the vent afterwards can make the next droppings less likely to stick, if you have it to hand.
Finding and fixing the cause
Treating the plug without fixing the cause means doing it again tomorrow. The commonest triggers are temperature problems — a brooder that runs too hot, too cold, or swings between the two produces stressed chicks with loose, sticky droppings. Check your heat gradient and the chicks' behaviour against it. Dehydration pastes chicks up too, so confirm every chick has found the water and is actually drinking; a drinker that is empty, fouled, or too high for the smallest chick explains a lot.
When this is a vet problem
Most pasty butt clears with warm water and a corrected brooder. Escalate to a vet, or an experienced poultry keeper while you arrange one, when a chick's abdomen looks swollen, when it strains with nothing passing after the vent is cleared, when it becomes lethargic, stops eating, or breathes oddly, or when the same chick pastes up repeatedly despite good conditions — repeated blockage can signal something wrong beyond the brooder. A chick found cold, limp, and pasted needs warming as well as clearing, and quickly. When you are holding a pasted chick in one hand and warm water in the other, KinsLe's Kimmy can be asked for step-by-step guidance in that moment, and what you learn can stay noted alongside the chick's profile for the next check.