Proportional Control for Daily Energy Regulation in Hummingbirds

Time budgets and food intake of captive hummingbirds (Eugenes fulgens and Lampornis clemenciae) were monitored for several days to estimate net energy exchange and to assess responses to depletion of energy reserves produced by food deprivation. Meal sizes averaged close to values predicted to maximize short-term rates of net energy gain if hummingbirds behaved optimally. Maximization of rate of net energy gain is analogous to an on-off model of control. However, longer-term, daily rates of energy storage were not maximized but generally were proportional to energy reserve depletion. High daily energy storage rates were associated with (1) high frequency feeding (one species, Eugenes), or (2) meal size (one species, Lampornis), and (3) low rates of energy expenditure between meals (both species). Proportional control may represent a compromise between characteristics of on-off controls, with high sensitivity to changes near set points, and integral controls, with low sensitivity to changes near set points.