F. Reed Hainsworth

Time and Energy Budgets of Territorial Hummingbirds

From laboratory data of flying and resting metabolism and field data on time budgets of territorial male Eulampis jugularis hummingbirds we were able to estimate caloric energy budgets associated with territorial behavior. The relative time and energy expenditures for territorial defense, foraging, and sitting varied in territories with different species of flowers. For example, territories centered around banana flowers had lower foraging costs than those centered around Myrtaceae or Inga—Hibiscus. The costs for flycatching, hovering,and perch changes accounted for relatively little of the time and energy budgets at any of the territorial defense and foraging were expended in sitting on perches within the territory. Territorial defense was of relatively low cost in time and energy because of the inactive defense utilized by Eulampis. Selection will optimize time and/or energy budgets, but the type of budget optimized will depend in large part on the exploitation technique of the species. For birds that do not fly to forage, but exploit food items that require a long search time per calorie, time that can be released for other activities may be more important than major shifts in energy budgeting. For hummingbirds, however, optimizing energy may be more important to insure a positive energy budget.

Temporal patterning of feeding by hummingbirds

Abstract For five species of hummingbirds in the laboratory, time between meals was related to energy intake on the first meal and rate of energy expenditure between meals. Field observations gave similar results. Average meal sizes were similar at one intake rate independent of food caloric density; females averaged longer bouts than males. When rate of intake was approximately halved, meal duration approximately doubled and volume intake remained similar. We postulate that feeding is initiated when crop contents reach a lower threshold and that feeding is terminated after ingestion of an optimal volume determined by the added weight of the meal.

Optimal Meal Size in Hummingbirds

Optimization theory is applied to hummingbird foraging to explain the observation that birds in the laboratory with access to functionally unlimited food supplies usually do not take as large a meal as they could. Foraging bouts are modeled in terms of time and energy; birds are constrained by the increased energetic costs associated with the added weight of their meal. Predictions of optimal meal size which maximize the rate of net energy gain or which maximize efficiency agree closely with laboratory and limited field data; we presently are unable to discriminate between these two optimality criteria. Predictions which assume optimal use of time or which maximize net energy gain or which neglect weight of the meal all proved unsatisfactory, generally by predicting optimal meal sizes in excess of capacity. Sensitivity analysis revealed that model predictions may be much influenced by parameter values potentially under physiological and/or behavioral control by a hummingbird. While we presently do not understand what governs the values of these parameters, we suggest that this control will eventually prove explicable by an optimality model of bird behavior.

Crop volume, nectar concentration and hummingbird energetics

Abstract 1. 1. X-ray measurements of hummingbirds indicated that initially food passed only to the crop. 2. 2. Food intake experiments with nine tropical species of hummingbirds (2·7–10·1 g) indicated that crop volume is a linear function of body weight. 3. 3. Nectar concentrations of flowers visited by hummingbirds in the lowlands were equivalent to 0·61–1·23 M sucrose, while nectar from highland flowers was equivalent to 0·29–0·68 M sucrose. 4. 4. At a given ambient temperature, small hummingbirds have less storage capacity in their crops relative to their metabolic rates per g than larger hummingbirds. 5. 5. A major factor for the energetics of hummingbirds is the lower nectar concentration from flowers in the highlands where the hummingbirds are exposed to lower ambient temperatures.

Regulation of Oxygen Consumption and Body Temperature during Torpor in a Hummingbird, Eulampis jugularls

The West Indian hummingbird, Eulampis jugularis, maintained its body temperature in torpor at 18� to 20�C over an ambient temperature range of 2.5� to 18�C. At ambient below 18�C oxygen consumption during torpor increased linearly with decreasing temperature. Thermal conductances were the same for resting and torpid Eulampis regulating their body temperatures at 40� and 18�C, respectively.

Energetics of Foraging: Rate and Efficiency of Nectar Extraction by Hummingbirds

The efficiency with which several species of hummingbirds extract nectar was estimated by converting the rate of extraction of nectar volumes to the energy expended and the energy obtained from the nectar. The extraction rates depend on corolla and bill morphologies, and the extraction efficiencies depend, in addition, on the bird weight and nectar concentration. Differential extraction efficiencies can be exploited by plants to increase pollinator specificity.

TEMPERATURE REGULATION IN NESTLING CACTUS WRENS: THE NEST ENVIRONMENT

Thermogenesis and thermolysis require resources (energy and water, respectively) that may often be present in the environment in limiting quantities. Birds are usually thought of as being limited by food during the breeding season (Lack 1954) and we have indicated elsewhere (Ricklefs and Hainsworth 1968b) that the activities of the Cactus Wren may be restricted by lack of free water during the hot parts of the breeding season. Under these conditions, evolution of the reproductive strategy should proceed toward the most efficient utilization of food and water. The insulative properties of the nest, huddling behavior of the nestlings, and brooding behavior of the adults all act to conserve heat. Conversely, these strategies would impede the dissipation of heat in a hot climate. Huddling and brooding are responsive to changes in ambient temperature and they create few problems when conditions requiring heat conservation and heat loss arise within short periods of time. Nest construction, however, is fixed and must represent a compromise strategy in the southwestern deserts where diurnal changes in ambient temperature may exceed 200C and the hottest temperatures may be several degrees higher than normal nestling body temperatures (Ricklefs and Hainsworth 1968a). Under these conditions the location and construction of the nest may be critical to the efficiency of temperature regulation of the young. The role of the nest of the Cactus Wren in maintenance of homeothermy of nestlings is evaluated indirectly in this report through a study of the relationships between temperatures within occupied nest cavities and the surrounding air.

Spatial learning as an adaptation in hummingbirds.

An ecological approach based on food distribution suggests that humming birds should more easily learn to visit a flower in a new location than to learn to return to a flower in a position just visited, for a food reward. Experimental results support this hypothesis as well as the general view that differences in learning within and among species represent adaptations.

On the tongue of a hummingbird: Its role in the rate and energetics of feeding

Abstract 1. 1. Rate of intake of hummingbirds (3·1 and 7·9 g) visiting a feeder was greater for larger hummingbirds but was not dependent on sugar concentration up to 1 ·0 molar sucrose. 2. 2. Most flowers produce nectarwith concentrations (0·24-1·48 molar sucrose) that should not influence rate of nectar intake. 3. 3. Rate of tongue licking was independent of body size (2·6-3·8 licks/sec), but the larger hununingbird obtained more nectar per tongue lick. 4. 4. Morphological studies indicated that the grooves on the tongues of hummingbirds may play a minor role in determining rate of nectar intake at a feeder with large nectar volumes, but they could be important in emptying small nectar volumes from flowers. 5. 5. Adding a “corolla” to the feeder resulted in a linear decrease in rate of intake with increasing “corolla” length. However, corolla curvature, position and the volume of nectar in flowers are also important for determining rate of nectar extraction from flowers.

Pollination and pre-dispersal seed predation: net effects on reproduction and inflorescence characteristics in Ipomopsis aggregata

SummaryWe examined net seed production for the self-incompatible, monocarp, Ipomopsis aggregata, by monitoring pre-pollination seed parasite (Hylemya sp.) oviposition and hummingbird mediated fruit set on 21 plants of variable height. Both pollination and seed predation increased with inflorescence height. Net seed production (incorporating seed predator mortality) also was positively related to height, and this would have been the case if pollination or seed predation were doubled. Although results suggest pomopsis aggregata should be under selective pressure to maximize inflorescence height, generation time and resource limits could result in advantages for inflorescences of intermediate height.