Evaluating Models of Crop Emptying in Hummingbirds

Hummingbirds have received considerable attention in studies of feeding and energy regulation because of their small size, frequent feeding, and their use of energy-rich sugar solutions (e.g. Diamond et al. 1986, Ewald and Carpenter 1978, Hainsworth 1981). They store nectar in a crop, and this internal reservoir supplies energy between foraging bouts (Hainsworth and Wolf 1972, Karasov et al. 1986). The rate of crop emptying, therefore, should influence foraging frequency (Karasov et al. 1986, Wolf and Hainsworth 1977). Because the crop is a major interface between feeding input of energy and outflow for energy use, there has been interest in the determinants of cropemptying rates (DeBenedictis et al. 1978, Hainsworth 1981, Hainsworth et al. 1981, Diamond et al. 1986, Karasov et al. 1986). Karasov et 02. (1986) report measurements of apparent crop volumes for seven Rufous Hummingbirds (Selasphorus rufus) at various times after they were fed 100/•1 of 0.585 molal sucrose. Approximately 26/•1 remained in the crop following prolonged times after feeding, so subtracting 26/•1 from apparent volumes adjusted volumes relative to the amount fed. The authors used a negative exponential and a linear regression to characterize the pattern of crop emptying. A negative exponential regression gave higher correlation coefficients for 5 of the 7 birds (Karasov et al. 1986). The interpretations for these regressions are inadequately tested, and there are other models that may describe crop function. Six alternative models can be compared based either on studies with other species or physical features of elastic emptying structures (Smith et al. 1984, Stubbs 1977). The models involve (1) a linear, (2) an exponential, (3) a square root, (4) a cube root, (5) a hyperbolic, or (6) an inverse cube root change in volume with time. A linear model describes tomach emptying of glucose loads in monkeys (McHugh and Moran 1979) and laboratory rats (McCann and Stricker 1986). Negative feedback mechanisms have been suggested to produce this pattern from detection of fluid properties in the duodenum with subsequent adjustment of stomach emptying (Hunt 1983, McHugh and Moran 1979). A negative exponential model, where the logarithm of volume decreases linearly with time, was used to describe emptying in a variety of animals where the rate of emptying decreased with time (Sibly 1981). This pattern appears when feedback mechanisms have a minimum impact on emptying (Smith et al. 1984). An obvious appeal for this model is that it produces decreasing rates of emptying with time, but there are other models with this feature.