Sour Crop

TL;DR. Sour crop is an opportunistic yeast infection of a bird's digestive pouch that causes food to ferment, leading to slow digestion, oral plaques, and potentially life-threatening illness if left untreated.

The crop is a specialized muscular pouch at the base of the neck used for temporary food storage.

What is it?

In birds, the crop (scientifically known as the ingluvies) is a muscular pouch located at the base of the neck. It serves as a temporary storage chamber where food is softened by moisture before moving further down into the stomach and gizzard for digestion. Because birds have high metabolic rates, the crop is a vital component of their unique digestive strategy, allowing them to consume food quickly and process it later in a safer environment.

Sour crop, also known as crop mycosis, candidiasis of the crop, or a candidal infection of the crop, occurs when the yeast Candida albicans overgrows within this pouch and the surrounding gastrointestinal tract. Under normal conditions, small amounts of this yeast may live harmlessly in a bird's digestive system. However, if the natural balance of microflora is disrupted, the yeast multiplies rapidly, invading the tissue lining of the crop.

When this overgrowth happens, the crop's muscular walls lose their ability to contract and push food forward—a condition called crop stasis. The trapped food, moisture, and yeast combine in the warm environment of the crop, causing the contents to ferment. This fermentation produces a sour, acidic smell, which gives the condition its common name. Because birds are exotic companion animals, much of our clinical understanding of this disease comes from specialized avian medicine and poultry science.

Causes & risk factors

The primary culprit behind sour crop is Candida albicans, an opportunistic fungal organism. This yeast takes advantage of a compromised immune system or a disrupted digestive tract.

One of the most common triggers for this infection is antibiotic therapy. While antibiotics are necessary to fight bacterial infections, they also eliminate the beneficial bacteria that naturally keep yeast populations in check. Without these friendly bacteria to compete with, Candida yeast can multiply without restriction. This risk is particularly high in neonates and hand-fed baby birds, whose immune systems and digestive microbiomes are still developing.

Other significant risk factors include:

• Poor hand-feeding hygiene: Unsanitary feeding syringes, tubes, or formula preparation utensils can introduce high levels of yeast directly into a chick's crop.
• Incorrect formula temperature: Feeding formula that is too cold slows down crop motility, allowing food to sit and ferment, which encourages yeast growth.
• Immunosuppression: Underlying viral infections, nutritional deficiencies, or chronic stress can weaken a bird's natural defenses.

There are no documented breed or species predispositions for this infection in the primary veterinary record; any bird species with a crop can develop candidiasis.

Signs to watch for

Recognizing the signs of sour crop early is critical, as birds naturally hide illness until they are severely compromised.

• Slow crop emptying (Cardinal): The crop remains swollen, soft, and fluid-filled hours after a meal, indicating that food is not moving into the stomach.
• Sour breath (Cardinal): A distinct, fermenting, or yeast-like odor emanating from the bird's beak.
• Oral lesions (Common): White, raised, plaque-like patches inside the mouth or throat, which can make swallowing painful.
• Mycotic diarrhea (Common): Loose, watery, or abnormal droppings caused by the yeast infection spreading further down the gastrointestinal tract.
• Regurgitation (Common): The bird may bring up sour-smelling, slimy fluid or partially digested food.
• Lethargy and depression (Common): The bird may sit fluffed up on its perch, show little interest in its surroundings, or sleep excessively.
• Weight loss (Common): Due to poor nutrient absorption and a lack of appetite.

White, raised plaques inside the mouth are a common sign of a Candida albicans infection.

How vets diagnose it

The structured veterinary record does not list specific diagnostic tests for sour crop. However, standard-of-care avian veterinary medicine relies on several diagnostic steps to confirm a Candida albicans infection and rule out other causes of crop stasis, such as bacterial infections or foreign body obstructions.

Your vet will begin with a thorough physical examination, gently palpating (feeling) the crop to assess its size, consistency, and whether it contains trapped fluid, gas, or impacted food. They will also look inside the beak for any visible oral lesions.

To confirm the presence of yeast, your vet will typically perform a crop swab or a crop wash. During a crop wash, a small amount of sterile saline is gently infused into the crop through a soft tube and then drawn back out. The collected fluid is placed on a slide and stained for microscopic evaluation (cytology). Under the microscope, the vet will look for the characteristic budding yeast cells and elongated structures called pseudohyphae, which confirm an active, invasive Candida infection.

Treatment options

Treatment for sour crop focuses on eliminating the yeast overgrowth, restoring normal crop motility, and addressing any underlying causes.

Antifungal Medications

Your vet will prescribe specific antifungal medications to target the yeast. These are typically administered orally so they can make direct contact with the lining of the crop and digestive tract.

• Nystatin: This is a highly effective, non-systemic polyene antifungal. Because it is not absorbed into the bloodstream from the gut, it works directly on the yeast within the digestive tract. According to a leading veterinary drug reference, Nystatin is widely used for crop mycosis and enteric yeast infections:

  "For enteric yeast (Candidal) infections:
  a) 200,000–300,000 Units/kg PO q8–12h. Relatively large volume must be administered (2–3 mL). May also be used prophylactic"
  — Plumb's Veterinary Drug Handbook, p. 2617

• Fluconazole or Ketoconazole: If the infection is severe, deep-seated, or resistant to local treatments, your vet may prescribe systemic azole antifungals like fluconazole (a triazole) or ketoconazole. These medications are absorbed into the body to fight more stubborn fungal infections.

Supportive Care

In addition to antifungal drugs, supportive care is vital for a bird's recovery. Your vet may need to gently empty and flush the crop to remove fermented food and toxins. If the bird is dehydrated, warm subcutaneous or intravenous fluids will be administered. If the infection was triggered by antibiotic use, those medications may need to be adjusted or discontinued under veterinary supervision.

Prognosis

Specific long-term prognosis data for companion bird species is limited in the structured record. However, in clinical practice, the prognosis for sour crop is generally excellent if the condition is diagnosed and treated early.

If the infection is caught before the yeast invades the deeper layers of the digestive tract or enters the bloodstream, most birds recover fully within one to two weeks of starting antifungal therapy. If the crop has been severely stretched due to prolonged stasis, it may take longer for the muscle tone to return to normal. In untreated cases, or in very young chicks with weak immune systems, the prognosis is guarded, as severe dehydration, malnutrition, or systemic yeast infections can be fatal.

Prevention

Preventing sour crop relies on excellent husbandry, especially when hand-rearing young birds:

• Strict Hygiene: Thoroughly clean and disinfect all hand-feeding equipment, syringes, and mixing bowls between every feeding. Never reuse formula that has sat out.
• Formula Temperature Control: Use a digital thermometer to ensure hand-feeding formula is served at the temperature recommended for your species (typically between 100°F and 105°F). Formula that is too cold slows digestion, while formula that is too hot can cause severe crop burns.
• Monitor Crop Emptying: Always ensure the crop is completely empty before feeding the bird again, especially before the final feeding of the night.
• Judicious Antibiotic Use: Only use antibiotics under the direct supervision of a veterinarian, and discuss the concurrent use of preventative antifungals or avian-specific probiotics if prolonged antibiotic therapy is required.

When to call your vet

Birds hide their symptoms until they are very ill, meaning that a delay in treatment can be life-threatening. If your bird's crop remains full or swollen hours after feeding, if you smell a sour or yeasty odor from their beak, or if you observe white patches inside their mouth, contact your avian veterinarian immediately.

Other emergency signs that require urgent veterinary attention include severe lethargy, sitting fluffed up at the bottom of the cage, continuous regurgitation, or a complete refusal to eat.

Sources

• Plumb's Veterinary Drug Handbook, pages 2614, 2617.