Potential impact of nutritional strategy on noninvasive measurements of hormones in birds.

The dietary preferences, gastrointestinal anatomy, digestive physiology, biochemical capabilities, and commensal microflora of a bird are collectively known as its nutritional strategy. Measurement of hormones in droppings requires an appreciation of an animal's nutritional strategy in order to optimize collection protocols, validate techniques, interpret results, and minimize variability and artifacts. Foods of animal origin, nectar, and seeds are highly digestible by relatively simple digestive tracts and result in low rates of feces production. Most frugivorous species also have simple digestive tracts, and they digest the fruit's simple sugars and proteins, but not the fiber in its pulp. Consequently, retention time of food in the digestive tract is short, and their droppings are voluminous. Herbivorous species possess enlarged ceca that house microorganisms that aid in the digestion of fibrous components of their food. Part of the digesta enters the ceca and is subjected to lengthy microbial fermentation. The rest is excluded and quickly passes through the rectum, and is quickly defecated. For measurement of hormones in droppings it appears prudent to collect only rectal feces and to avoid cecal feces. One-third of the avian families are omnivorous and consume a wide variety of foods. Their digestive strategies are highly variable and change with diet, as does the amount and composition of feces and the rate of passage.