Sour crop is a yeast infection in a chicken’s crop, the expandable pouch at the base of the neck where food is stored before digestion. It’s caused by an overgrowth of Candida albicans, a fungus that normally lives in small amounts in a healthy chicken’s digestive tract. When something disrupts the balance of microorganisms in the crop, Candida multiplies unchecked, fermenting food and producing a characteristic sour smell.
How the Crop Works and What Goes Wrong
A chicken’s crop acts as a holding tank. Food collects there after swallowing, softens, and gradually moves into the gizzard and intestines. A healthy crop maintains a pH of around 5.5, acidic enough to begin breaking down food and keep opportunistic fungi in check. When conditions shift and the crop’s natural microbial balance is disrupted, Candida takes over.
As the yeast multiplies, it thickens the crop lining and can form raised, whitish patches called pseudomembranes. These can appear in the crop, mouth, and esophagus. In more advanced cases, shallow ulcers develop and tissue begins to slough off. The fermenting food produces gas and foul-smelling liquid, which is why a sour, yeasty odor from the bird’s beak is one of the most recognizable signs.
The Most Common Causes
Antibiotics
The single most well-documented trigger for sour crop is prolonged use of oral antibiotics. A course lasting one to two weeks can wipe out beneficial bacteria in the upper digestive tract, leaving Candida free to colonize without competition. This is the same mechanism behind yeast infections in humans after antibiotic use. If you’ve recently treated a bird for a bacterial illness, the risk of sour crop rises significantly in the weeks that follow.
Moldy or Spoiled Feed
Feeding chickens spoiled or moldy grain introduces harmful bacteria and yeast directly into the crop. Mold spores can shift the microbial environment rapidly, giving Candida the upper hand. Feed stored in damp conditions or left in feeders long enough to go stale is a common culprit, especially in humid climates.
Dirty Water
Slimy waterers and unclean drinking systems are a breeding ground for yeast and bacteria. Biofilm builds up in water containers that aren’t scrubbed regularly, and chickens drinking from these contaminated sources are effectively inoculating their crops with the organisms that cause sour crop.
Overeating and Sudden Diet Changes
Chickens that gorge after their feeders have been empty for a while are especially vulnerable. The “feast or famine” instinct causes them to overload the crop, which can slow the normal passage of food and create a stagnant environment where yeast thrives. Sudden dietary changes have a similar effect. Switching from one type of feed to another without a gradual transition can overwhelm the digestive system and destabilize the crop’s microbial balance. Even overindulging in treats, healthy ones included, can tip the scales.
Impacted Crop
Sour crop frequently develops as a secondary problem after a crop impaction. When a physical blockage (from long grass, straw, or other fibrous material) prevents food from moving through, the stagnant contents begin to ferment. The warm, moist, food-rich environment is ideal for Candida overgrowth. A pendulous crop, one that has stretched and lost muscle tone from repeated overfilling, carries the same risk because it doesn’t empty efficiently.
Weakened Immune System
Any condition that suppresses a chicken’s immune system can open the door to sour crop. Parasitic infections, chronic illness, stress from overcrowding, extreme weather, and poor nutrition all reduce the bird’s ability to keep opportunistic fungi in check. Young chicks and older hens are more susceptible for this reason.
How to Tell It’s Sour Crop
The easiest test is checking the crop first thing in the morning, before the bird has eaten. A healthy crop should be empty or nearly flat by dawn. A crop that’s still enlarged, feels squishy or fluid-filled, and produces audible gas or liquid sounds when gently pressed is a strong indicator. The telltale sign is a sour or fermented smell coming from the bird’s beak. Some owners describe it as similar to bread dough or stale beer.
It’s worth knowing the difference between sour crop and a simple impaction. An impacted crop feels firm and solid, like a packed ball of material, and typically doesn’t produce the yeasty smell. Sour crop feels more like a water balloon, with a softer, pliable mass that may slosh when handled. The two conditions can overlap, though, since an impaction that goes untreated often leads to secondary yeast overgrowth.
Other signs include lethargy, reduced appetite, weight loss, and a general decline in condition. The bird may stand hunched or isolated from the flock.
Treatment Options
Mild cases sometimes resolve with conservative care. Withholding food for 12 to 24 hours (while providing clean water) gives the crop time to empty and can reduce the fermentation cycle. Some keepers gently massage the crop several times a day to encourage contents to move through. Adding raw, unfiltered apple cider vinegar to the water at roughly one tablespoon per gallon can help lower the crop’s pH and create a less hospitable environment for yeast.
More established infections typically require antifungal treatment. Nystatin, an antifungal that targets Candida directly, is the standard veterinary option and is given orally over a course of several days. Copper sulfate dissolved in drinking water at a very low concentration (1:2000) is another approach used in poultry flocks, but the margin between an effective dose and a toxic one is narrow. Copper sulfate overdose can poison birds, so this treatment should only be used under veterinary guidance with precise measurements.
If an impaction is contributing to the problem, the blockage needs to be addressed alongside the yeast infection. In severe cases where the crop won’t empty on its own, a veterinarian may need to flush or surgically open the crop to remove the contents.
Preventing Sour Crop
Most cases are preventable with basic management. Store feed in dry, sealed containers and discard anything that smells off or shows visible mold. Clean waterers at least every few days, scrubbing away any slime or biofilm. Nipple-style drinkers tend to stay cleaner than open troughs, though they still need maintenance.
Keep feeders consistently stocked so birds don’t gorge after periods without food. When changing feed types or introducing new foods, do it gradually over a week or more. Limit access to long, tough grass, especially in spring when chickens are eager to forage on new growth. Cutting pasture short or offering chop instead of letting birds tear off long strands reduces the risk of impaction that can lead to sour crop.
If you’re treating a bird with antibiotics for any reason, consider adding apple cider vinegar to the water once the antibiotic course is finished (not during, since the acidity can interfere with some medications). Providing a probiotic supplement designed for poultry after antibiotic treatment also helps repopulate beneficial bacteria in the gut and crop, reducing the window of vulnerability for Candida overgrowth.
Keeping the flock’s overall health strong through proper nutrition, parasite management, and low-stress housing goes a long way. A chicken with a robust immune system can keep Candida in check on its own, even when exposed to the same conditions that would trigger sour crop in a weaker bird.