Anatomical adaptation of the gut to diet in red-winged blackbirds (Agelaius phoeniceus)

-I examined gross and microscopic anatomical responses of the gastrointestinal tract of Red-winged Blackbirds (Agelaius phoeniceus) to experimental variation in diet quality. Birds fed an energy-dense diet had the smallest measures of gizzard, small intestine, and villi. Birds fed an energy-dilute diet had the largest villus length, lamina muscularis depth, and small-intestine outer diameter. These responses are consistent with observations in other birds and are related to increased intake. Received 21 May 1990, accepted 22 December 1990. THE VERTEBRATE gastrointestinal tract is a dynamic and energetically expensive organ system. The small intestine is characterized by high rates of epithelial cell loss and cell production. For example, cell turnover occurs approximately every 2-5 days in chicks (Gallus gallus, Imondi and Bird 1966) and every 2-3 days in healthy laboratory rodents and humans (Johnson 1987). The energetic cost of maintaining a gut is unknown in birds, but may contribute up to 40% of basal metabolic rate in ruminant mammals (Webster 1981). Because birds have high metabolic rates and poor energy-storage capacities, conserving energy allocated to gut tissue may provide a means to temporarily reduce the cost of existence (Sibly 1981). Changes in both gross and microscopic gut anatomy may be important to a bird's ability to maintain energy balance. Seasonal changes in the sizes of digestive organs occur in wild birds, but the proximate factors for such changes are poorly understood. The size of digestive organs can be greatest during times of year when intake is high, possibly due to low available energy of the food, rapid processing rates, or high metabolic demand. Such changes were documented for the gizzard, small intestine, or cecum in the Anseriformes (Miller 1975), Galliformes (Moss 1989; Savory and Gentle 1976a, b; Remington 1989), Columbiformes (Kenward and Sibly 1977), and Passeriformes (Davis 1961, Al-Joborae 1979, Al' Present addresses: U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Denver Wildlife Research Center, 2820 East University Avenue, Gainesville, Florida 32601 USA, and Department of Zoology, University of Florida, Gainesville, Florida 32611 USA. Dabbagh et al. 1987, Ankney and Scott 1988). In contrast, captivity, and the associated increase in diet quality or decrease in activity, can produce reductions in gross measures of the gizzard, small intestine, or ceca (Moss 1972) as well as in the microanatomy of the small intestine and ceca (Hanssen 1979, Goldstein 1989). I used Red-winged Blackbirds (Agelaius phoeniceus) to examine avian digestive responses to dietary manipulation because they have (1) a simple digestive anatomy and (2) seasonal fluctuations in factors that could influence cellular growth, such as food type, physiological status, and metabolic activity. Red-winged Blackbirds have an expandable esophagus but not a crop (McLelland 1979), a small proventriculus, enlarged gizzard, short intestines, and small ceca (Brugger 1989). Simplicity of the digestive tract limits the options for response of gut tissues to external or internal influences. However, seasonal variations in food quality (Beal 1900), coupled with variation in reproductive or migratory status (Payne 1969) and variation in energy requirements (Brenner 1966), could create asynchronous fluctuations in energy supply and demands. Thus, energy conservation by varying tissue allocated to the gut may be important to Red-winged Blackbirds. My objective was to determine how the gross and microscopic structures of the gastrointestinal tract varied in Redwinged Blackbirds in response to experimental manipulations in diet quality.