What causes frounce in birds?

Frounce is primarily caused by a type of yeast known as Trichomonas gallinae. This organism is a protozoan parasite that infects the upper digestive tract of birds, particularly affecting those in the family of raptors and pigeons. When birds ingest infected prey or contaminated food and water, the yeast proliferates, leading to lesions in the throat and crop.

In the context of the other options, while bacteria and parasites can cause issues in birds, they are not responsible for frounce. Nutritional deficiencies can lead to various health problems in birds, but they do not directly cause this particular condition. Understanding the specific pathogen responsible for frounce is crucial for effective treatment and management in falconry and bird care.