Birds have a unique digestive system that allows them to eat and digest food very quickly in order to power flight. The part of a bird’s digestive system where food is primarily stored before being fully digested is the crop.
What is the Crop?
The crop is an enlarged, muscular pouch near the base of a bird’s neck, just above the breast muscle. It is part of the esophagus, which connects the bird’s mouth to its stomach. When a bird swallows food, the food passes down the esophagus and enters the crop for storage.
The crop allows birds to eat larger amounts of food at one time than they could fit in their stomach. The food accumulates in the crop where it is softened by enzymes but does not undergo full digestion. This allows birds to maximize foraging efficiency by eating many small snacks during the day which are stored in the crop until the larger meal can be digested.
Some key facts about the avian crop:
- Storage capacity varies by species. In some small songbirds the crop can only hold a few grams of seeds, while in larger birds like ostriches it can hold several pounds of food.
- Food may be held in the crop from a few minutes up to 10-12 hours before moving to the stomach.
- The crop walls produce enzymes like amylases to start breaking down starches while food is stored.
- The crop lining is very elastic so it can greatly expand and contract as food accumulates and then passes to the stomach.
- Pigeons produce “crop milk” – a high protein secretion fed to young squabs.
Crop Function
The main functions of the avian crop are:
- Food storage – as described above, the crop allows birds to gather and swallow more food than will fit in their stomach for later digestion. This is important for small birds that need to eat frequently.
- Softening of food – the moist environment of the crop starts to soften food like seeds or insect exoskeletons, making them easier to grind up and digest later on.
- Enzyme production – enzymes produced in the crop initiate chemical breakdown of foods like starches before they reach the stomach.
- Moistening of food – the crop adds moisture to dry food like seeds or bread that would otherwise be difficult to swallow.
In summary, the main role of the avian crop is as a food storage chamber where some preliminary digestion processes can take place before the food moves to the stomach and intestines to be fully broken down and absorbed.
Crop Anatomy
The crop is composed of the following anatomical structures:
- Mucosa – this innermost lining contains secretory cells that produce enzymes and mucus to coat and soften food. It has a very wrinkled surface to allow expansion.
- Submucosa – a dense layer of blood vessels and elastic fibers underneath the mucosa. It helps the crop stretch.
- Muscularis – a middle layer of smooth muscle that contracts rhythmically to churn food. It has both circular and longitudinal orientation.
- Serosa – the outermost layer of epithelial cells that secrete fluid to lubricate the crop’s external surface.
The crop is highly elastic to accommodate large volumes of food intake. When empty, it collapses into a small pouch. When filled, it balloons out below the neck and may occupy much of the upper chest cavity below the neck before food passes to the stomach.
Food Movement Through the Crop
Food moves through a bird’s digestive tract in the following order:
- Food is collected by the beak and swallowed down the esophagus.
- It enters the crop for storage.
- Powerful contractions of the crop muscle churn and soften the food.
- With antiperistaltic contractions, the crop periodically empties small amounts of food upward into the lower esophagus.
- From the esophagus, food passes through the proventriculus and then the gizzard for grinding.
- Finally digested material moves from the gizzard into the small intestine for nutrient absorption.
The crop serves as a holding chamber where food accumulates and initial enzymatic digestion occurs. Contractions of the crop keep food churning while in storage and then move it upwards toward the stomach as space becomes available for more food intake. This intermittent upward movement of stored food from the crop allows birds to continuously feed and store more food through the day.
Variation in Crop Design
While all birds have a crop for food storage, crop size and shape varies significantly between bird species depending on diet. For example:
- Pigeons – Have a large bilobed crop to store seeds, grains, and vegetation. One lobe stores newly swallowed food, while the other stores food ready to move to the stomach.
- Owls – Have a long tubular crop optimized for whole rodent swallowing and storage.
- Doves – Have an extra esophageal pouch called a ventricular crop for longer-term storage.
- Pelicans – Have a massive, expandable crop to accommodate whole fish swallowing.
- Penguins – Have no crop since they eat small chunks of food. Food passes directly from esophagus to stomach.
In the pigeon, the bilobed crop allows selective passage of food from storage to digestion. One lobe acts as short-term storage, while the other long-term storage. This improves the efficiency of food movement through the crop for these frequent eaters.
Diseases of the Avian Crop
Some common diseases affecting the crop in birds include:
- Crop impaction – compacted mass of food is stuck and unable to empty. Causes bulging neck.
- Crop infection – bacterial or fungal infection causes thickening and necrosis.
- Crop puncture – hole in crop wall allows food contents to leak into body cavity.
- Crop stasis – decreased crop motility and food transit.
- Sour crop – spoiled food accumulates due to slowed crop emptying.
These conditions can be caused by decreased muscle tone, foreign bodies, parasites, toxicity, and infections. Treatment depends on the specific cause but may involve surgery, anti-microbials, probiotics, prokinetics, or flushing of the crop.
Prevention involves avoiding foreign object ingestion, proper food hygiene, and monitoring bird health. Infected birds may show decreased appetite, difficulty swallowing, neck swelling, or discharge from the beak.
The Avian Digestive System
Here is a summary overview of the key parts of the avian digestive system:
Structure | Location | Function |
---|---|---|
Beak | Head | Seizes, manipulates, and swallows food |
Esophagus | Neck | Tube for food transport to crop |
Crop | Lower neck | Food storage and initial enzymatic digestion |
Proventriculus | Upper chest | Secretes gastric juices to further digest food from crop |
Gizzard | Lower chest | Muscular grinding organ to mechanically reduce food |
Small intestine | Abdomen | Absorbs processed nutrients from food |
Large intestine | Cloaca | Compacts and excretes indigestible waste |
Cloaca | Base of torso | Collects waste from digestive tract for elimination |
This system allows birds to maximize nutrient absorption from food in order to power flight. The crop provides temporary food storage and preprocessing before the stomach and intestines complete digestion.
Conclusion
In summary, a bird’s crop is the enlarged pouch near the base of the neck serving as the primary storage site for food before full digestion. Located along the esophagus, the elastic crop allows birds to swallow larger amounts of food than the stomach can hold. Food is detained in the crop for varying lengths of time from minutes to hours, depending on the species. While stored in the crop, food is softened by crop secretions and enzymes which start chemically breaking it down. Contractions of the crop muscle also churn the food and move it upwards into the esophagus in small amounts as space becomes available. After preprocessing in the crop, food passes to the stomach (proventriculus) and then the gizzard where digestion is completed before final nutrient absorption in the small intestine. The crop allows birds to maximize foraging efficiency during the day by enabling intermittent storage of multiple small meals which can be later digested. The large storage capacity and ability to start digesting food makes the avian crop a unique and specialized organ that is critical for flight.