Transport of water and food particles through the avian ceca and colon

This paper presents the historical steps in the study of transport through the large intestine of birds. Since the early 1920s, it was known that urine may reach the ceca. In the mid-1960s, the retrograde transport of urine was regarded as a natural and normal process in many birds. About 10 years later, it was shown that urine rinses water-soluble substances and small particles out of the colonic contents and brings them into the ceca. The most easily fermentable particles of the digesta are thereby brought into contact with the cecal microbiota, and poorly fermentable material can be rapidly excreted. In addition, water, electrolytes, and nutrients may be absorbed from the liquid having entered the ceca. This paper also gives results from a preliminary study showing that the retrograde flow is affected by the nitrogen content of the food. The share of produced urine reaching the ceca was 6.1% on high-nitrogen food, 12.1% on medium-nitrogen food, and 22.4% on low-nitrogen food. All mean values were significantly different.