The expenditure of energy involves elaborate chemical reactions, most of which are sensitive to temperature. An endothermic (warm-blooded) animal is relatively independent of environmental temperatures since its body is already warm and ready to go. In birds and mammals the metabolic cost of maintaining endothermy is expensive, requiring at least 90% of total metabolism to be devoted to the regulation of body temperature.
Birds have a somewhat higher metabolic rate than mammals, but not significantly so. Birds tend to be smaller than mammals and thus use more energy; their body temperatures (37.7-43.5 C) are also somewhat higher than mammals’ (36-39 C).
Birds and mammals are similar in their metabolic adaptations, but there are differences:
1. Birds’ feathers are for flight and insulation; mammal fur serves only for insulation.
2. Distribution of fat in birds’ bodies is different for aerodynamics.
3. Birds have no sweat glands and lose heat through their respiratory system and exposed skin.
4. Birds have to incubate eggs outside their body, requiring additional heat.
The energy cost of activities above the basic level seems to be no more in birds than in mammals. The basal metabolic rate is associated with temperature and precipitation. But the behavior of birds tends to make them engage in more energy-costly activities. They are diurnal, while mammals are nocturnal and thus are more exposed to wind, temperature fluctuation, and predators. Most can’t crawl into holes and sleep like mammals; their metabolic rate is almost always at a peak. The smaller the bird, the more the energy used per gram of body weight because the increase in energy need doesn’t increase quite as fast as weight. In the wild a 25 gram bird has enough energy stores to last for perhaps two days, depending on environmental conditions, species of bird, etc. The resting metabolism and regulation of body heat are higher in winter than in summer and reproductive energy higher in the breeding season. Resting metabolism probably requires 35-50% most of the time.
Many researchers do daily time budgets – % of time devoted to feeding, resting, flying, etc. These become useful if you can attach energy costs to each activity. Although there have been a lot of caged bird studies, wild bird studies in the wild are rarer because of the difficulty in obtaining data. Need to consider at least seven major factors:Ambient temperature,Thermal radiation,Wind,Humidity, Behavioral ,Metabolic rate,Water loss, and Body temperature.
Mockingbird Daily Energy Budget might look like this: Foraging 32%, Flying 6%, Singing 40%, Misc. 22%
From caged mockingbird studies we know that the total energy used per day is 30.3 Kcal/day in a cage. Rough estimates tell us that foraging is 3.4x basal metabolic rate, flying is 2.2x and singing is 1.4x.
So to extrapolate: Foraging 32% x 30.3 x 3.4 = 32.9 Kcal/day
Flying 6% x 30.3 x 2.2 = 3.99 Kcal/day
Singing 40% x 30.3 x 1.4 = 16.96 Kcal/day
Misc. 22% x 30.3 x 1 (at least)= 6.7 Kcal/day
These are only estimates, but they tell us that a free-living mockingbird uses at least 60.55 Kcal/day in the wild. And of course, activity budgets vary daily and seasonally.
Several attempts have been made to estimate the total amount of energy required by an avian population. This is not only of interest to ecologists, but may have a practical application. It’s of particular interest to fisherman and fishery biologists because dense colonies of nesting seabirds may have a considerable impact on areas of the coastal ocean. If we know the number of birds and the energy they use we can estimate how much food the birds have to consume. Energy requirements change drastically under certain circumstances – e.g. when a bird incubates eggs, it may need to use considerably more energy.
Well, why do we want to know all of this energy stuff? Because it better explains why birds do what they do and live where they do. Obviously, because of physiological limitations with respect to ambient temperature and consequent use of energy, some birds are forced to live only in certain habitats or migrate between habitats partly because of seasonal temperature changes
For most birds, there is a range of environmental temperatures over which the deep-body temperature remains constant. Temperatures above and below the range of thermoneutrality will both result in increased heat production. Thermoneutrality means that the conditions are such that the bird doesn’t use energy to either lose or maintain body heat. If the air temperature drops below 16.6 C, heat production in the bird increases. If the temperature drops too low, heat production cannot be maintained and the bird dies. If the air temperature goes above 27.5 C (33-380 C for many other species), the bird is stressed to lose heat and actually produces more heat and dies. Birds die faster due to heat stress than cold stress. At very low temperatures, the bird shivers – involuntary muscle contractions generate heat production. Birds with more fat can withstand lower temperatures than birds with less fat.
Birds can behaviorally thermoregulate to some extent to reduce heat loss. The most conspicuous behavior is migration to a warmer climate. In cold environments some birds “hunch down” and/or reduce surface area heat loss by tucking the head or feet or legs (grebes) under the wing or body feathers. Or fluff feathers to trap heat. Birds can reduce 20-50% of their heat loss by sitting. A few species of birds huddle together – e.g. Emperor Penguins- for protection against heat loss. And Brown Creepers by the dozens huddle together for warmth at night. Many mammals make use of burrows but only a few birds do –e.g. Capercaille, which burrows in the snow and the Kagu- parrot of NZ.
Representatives of several orders of birds have the ability to allow their body temperatures to drop – become torpid– under certain circumstances. Some become torpid on a daily basis such as hummingbirds some of whom can drop their body temperature to 12C. The evolutionary reason is that, being unable to feed on insects and nectar in the darkness, the hummingbird would lose too much energy overnight. However, very low outside temperatures (5-8C) are lethal to hummingbirds, even in a state of torpor. The Poorwill of the southwestern US can endure several months without food by dropping its body temperature to 60C.
Since overheating is more stressful than cooling, birds have mechanisms to lose heat as well; some mechanisms are:
1. Non-evaporative cooling (no loss of body water) via radiation, wind, etc.)
2. Cutaneous cooling – heat loss from the skin along with moisture, but moisture loss is low since there are no sweat glands.
3. Respiratory evaporative heat loss – this is the most important form of heat loss in birds and virtually all birds exhibit some form of panting. The respiratory rate of the House Sparrow rises from 57 breaths per minute at 300C to 160/min. at 43C. Many, if not all, birds flutter the throat area during heat exposure, resulting in heat loss from the mucus membranes of the throat. The hyoid bone flexes and the whole area is suffused with blood. This gular flutter may account for 35% of the heat loss of a chicken.
Birds and also avoid the sun by resting in the hottest part of the day, or if sitting in the sun (e.g. while incubating eggs), they orient their back to the sun. Wings can be held away from the body and the feathers elevated. Albatrosses raise their feet off the ground and spread their wings to shade their feet. The Herring Gull orients the white part of the body towards the sun to reflect heat. The Wood Stork, when hot, directs its liquid excrement onto its long legs. The domestic chicken splashes water over its comb and wattles. The Egyptian Plover wets its belly in puddles and cools itself and its eggs and the sand around the nest.
Birds vary enormously in their thermoregulatory abilities immediately after hatching and they have been classified according to these and other capabilities.
Precocial species are those hatched covered with down and are able to respond immediately to changes in the ambient temperature as they are capable of thermoregulation or are able to withstand body temperature fluctuations without harm.Examples are waterfowl, chicken, ptarmigan, etc. Their thermoregulatory abilities increase with age.
Altricial species, those born naked, such as songbirds, woodpeckers, doves, etc. have little or no ability to thermoregulate and are heavily dependent on their parents to warm them. It may be 10 days before they can fully thermoregulate.