|The lack of information available on bird nutrition on a species by species basis makes determining what wild birds need in their diet a challenge. In the early 1970′s ornithologists explained, “the number of species whose nutritional requirements are known with any precision is relatively few. Of the mammals, only about a dozen species have been studied out of a total of over 5,000; the situation with birds is worse.” Though even thirty years ago it was evident that “there (was) a need for determining the energy contents of different diets and the effect of these diets on the heat production of the birds that consume them,” (Sturkie 182), information is still scarce. Even today, most avian nutrition studies have been done on chickens, not wild birds (Fisher 431-432). According to a study done in 1997, “limited information is available on nutrient requirements for avian species not under intensive commercial production” (Angel 41).The variations in bird species, age, and size, as well as other factors such as weather and seasonal behavior make knowing the exact amount of fat, protein, fiber, and energy specific birds need and derive from food difficult. Even in experiments that attempt to understand specific bird species’ energy needs, the time of day, level of activity of the bird, intensity and duration of daylight, environmental conditions to which the bird is adapted before the tests, and the level of food intake will all affect the level of heat, or energy, produced by the bird (Sturkie 153). Also, the size and age of birds will affect nutritional needs. Small birds eat more per unit of body weight than large birds (Sturkie 188), and young birds need protein rich foods, such as insects, even if they are not insect eaters as adults (Brooke 123). These and other variations in dietary needs make specifying birds’ nutritional requirements very difficult. However, studies done on certain wild birds, pet birds, and chickens offer general information that can guide the wild bird enthusiast in providing proper nutrition for his or her backyard birds.
Energy expended is lowest when the bird is inactive, unfed, and the surrounding temperature is high (in the “thermoneutral zone”). The amount of energy expended in this state is called the Basal Metabolic Rate, (BMR) and is measured by the quantity of oxygen consumed or carbon dioxide produced in exhaling (Brooke 133). However, it is rare for a bird to be in these ideal conditions.
When the ambient temperature is lower than the thermoneutral zone, heat must be produced to maintain the bird’s body temperature. The metabolic rate of an unfed, resting bird that is also producing heat is known as the standard metabolic rate (SMR) . . . (However), the daily energy expenditure of an active bird in the wild is often at a level of three or four times the basal level. (Brooke 134)
Even if scientists could identify a bird’s BMR, their findings still would not be accurate because BMR varies with the time of day, geographical location and season. BMR varies with the weight of the bird, but passerines (songbirds) have a higher BMR than non-passerines of the same weight (Brooke 133). BMR also varies on an annual basis due to bird activity levels and weather (Brooke 134).
Birds’ energy requirements above the BMR vary, and are difficult to determine in a laboratory. Measuring oxygen consumption in a lab requires the bird to be in an enclosure, which inhibits the bird from exhibiting his normal activities, which does not allow for an accurate reading of the bird’s energy needs. There are other methods of measurement, but even “under normal conditions, it is clearly impossible to compile a comprehensive energy budget of a bird, but the energy expenditure may be estimated over a given period of time by several different techniques.” These include a “time and energy budget” in which a bird is observed in its daily activities, such as sleep, flight, and non-flying activity. Taking information already gained on the oxygen intake required for that activity, compared to the time the bird spends doing that activity, it is possible to analyze the heat production of the bird without interfering with the bird’s normal activity (Sturkie 175).
On average, “free-living birds allocate about half of their daily energy expenditure to maintenance requirements; the rest is available for various activities of which foraging for food itself is particularly demanding of energy” (Brooke 134). To survive, the amount of energy expended in finding food must be less than the amount of energy the food gives, so the bird will have enough leftover energy for sleeping and looking for mates (Brooke 128).
Seasonal behavior and bird size also affect daily energy expenditure. When birds are laying eggs and feeding their young, their daily energy demands are high, but molt and incubation periods require little energy. Energy is also required to protect territory. Hawaiian honeycreepers use 17% more energy defending their territory than non-territorial birds. (Brooke 134) The size of the bird is also important when determining their energy needs, as “large birds use more energy than small ones in absolute terms but they use relatively less (energy) per unit weight” (Brooke 133).
One of the greatest expenditures of energy is flying. The energy costs of flight average about twelve times the BMR but this depends on the size of the bird, its aerodynamics, and the method and speed of flight. Swallows, martins and swifts have very low flight costs for their size (only 4-5 times the BMR), due to their streamlined shape, long, large wings, and their gliding flight. On the other hand, the short flights of European Robins cost twenty-three times the BMR because landing, taking off, and flapping are energy expensive. However, since robins don’t fly very often, the energy requirements of flight are not great when added up over a given day (Brooke 136).
Due to these variations of energy use, birds have different nutritional needs. Small birds may need to eat their weight in food every day because of their high BMR. For example, a Blue Tit weighs 11g, and it needs 1 kcal per gram of body weight each day in winter, which is about 300 small insects, about 10g in weight. Large birds, however, need relatively less food. A Kestrel that weighs 220g needs only 1/3 of a kcal per gram of body weight each day in winter, or about 120g of food (Brooke 134).
Fat as a Source of Energy
Some scientists believe that energy requirements and fat tissue stores drive birds’ appetite and food intake. When there is a certain amount of fat in the tissues, this causes a decrease in food intake, and vice versa (Sturkie 188). For example, House Sparrows regulate their food intake by the amount of energy they will need at night and during the day (Fisher 434).
Fat is very important to wild birds’ diet, especially during the winter, for energy, heat insulation, and migration. Birds need extra energy and fat reserves in winter to keep warm and sustain life during food-scarce conditions. However, birds do not store an excess of fat for winter. For example, the Blue Tit may only store enough fat on his body for one night, or a day or two of poor feeding conditions.
Of course, some birds migrate to warmer climates for winter where the food is more abundant and not as much energy is needed to keep warm. However, these migrating birds also need fat in their diets to have enough energy for the flight. Fat is important because, “as a migration aid, fat yields two times more energy and water per gram metabolized than does either carbohydrate or protein” (Gill 297).
Table 1: Fuels for Migration
Fuel Energy Yield kj)
Chart taken from Gill, p. 297.
Migrating birds use fat as fuel, which provides about twice the energy per unit mass as any other biochemical fuel available (Brooke 183). Birds prepare for the voyage by consuming more fat than usual to store reserves on their bodies. Dunlin birds, “traveling between England and northern Russia, put on some 30g of fat in the spring at a rate of 0.6g a day, enough for a journey of two and half thousand miles (Brooke 134). Some small migrant species that undertake long flights may almost double their body mass in fat and flight muscles by departure. The extra muscle build-up provides additional power to carry the heavy bird, and the muscle can also be a protein reserve if there is not enough food at the destination (Brooke 183).
In preparation for migration, birds vary in their intake of fat for reserves. White-throated Sparrows store about 17% of their body weight in fat just before spring migration, and after the migration the fat only counts for 7% of their body weight. Three species of Wood Warblers, which have a longer migration flight than sparrows, have about 30% of their body weight in fat before migration, and only 6% after they had reached their destination (Fisher 449).
Birds carry a “safety margin of fuel” within their fat reserves. The Red Knot, a shorebird, was studied. These birds had used during flight only about half the fat stored before departure. The remainder was available as an energy reserve for use on the Arctic breeding grounds to aid survival in bad weather or possibly to accelerate egg production. Indeed, certain geese species that nest in the Canadian Arctic lay down enough fat before migration to provide the constituents for egg formation and a reserve of energy to last them through incubation. (Brooke 183)
However, the amount of fat stored as this “safety margin” depends on the distance the bird will be traveling and the size of the bird.
Table 2: Percentage of Body Weight of Stored Fat, Before Migration
Non-Migrating 3 – 5%
Short Distance 13 – 25%
Long Distance 30 – 47%
Information taken from Gill pg. 298.
If possible, songbirds will only fly several hundred miles and then spend one to three days refueling. However, some songbirds fly for several days until their fat reserves have been completely used up. Songbirds seem to plan their migrations to “coincide with the appearance of abundant food at these sites, where they build up fat reserves required for the next leg of their journey. Conservation programs for shorebirds currently are directed toward the protection of these critical staging areas” (Gill 298). Therefore, it is important to offer birds fat-rich food during migration season, so the migrating birds have the energy they need to refuel and reach their destinations. To obtain the fat needed for winter and migration, birds need fat-rich foods. Seed merchant Kaytee claims, “as a single food item, there is probably no better food for the bird than the sunflower seed” (Kaytee 6). According to Cornell Laboratory of Ornithology, “six of eight bird species in some Canadian provinces and northern states preferred fatty sunflower seeds over less oil-rich seeds more often than did individuals of the same species farther south.” For example, Black-capped chickadees “chose sunflower seeds in almost 100 percent of observations in eastern Canada but in only 80 percent of observation at the southern extreme of the range. Mourning doves in northern areas were twice as likely to pick sunflowers compared to mourning doves in Florida.” Higher calorie fatty seeds are probably preferred because they offer extra energy for heat production during cold weather (Butler 10).
Amino acids, the building blocks of protein, are also important to birds’ diet because they build strong tissues and feathers (www.kaytee.com 2). For example, both the chicken and the Tree Sparrow need about 7% dietary protein. More than half of that, about 4% of dietary protein, may be the absolute minimum for maintaining life at a reduced body weight. However, a different study showed that White Crowned Sparrows could not maintain body weight on a bird seed diet (about 12% protein), but if given a 20% protein starter diet, they could retain their weight. The difference in protein requirements may be due to the age of the birds in the studies, as younger birds need more protein than older birds (Fisher 435).
Baby growth, feather quality, and resistance to disease all hinge on amino acids. To be more specific, “ten essential amino acids are required by birds: lysine, methionine, arginine, histidine, tryptophan, threonine, leucine, isoleucine, valine, and phenylalanine. Unsupplemented seed diets do not provide these essential amino acids in a balanced form, although the percent protein may be high.” This is because seed-only diets for pet birds are low in lysine and methionine (Kaytee 3).
Protein is also a good source of nitrogen, which is required in higher doses than normal maintenance for reproduction, growth, and molting. It has been observed that Red-winged blackbirds increase the amount of protein rich animal food they eat during breeding season.
Although the diet of adult House Sparrows consists of 97% vegetable matter, the nestlings receive foods that are 68% animal matter and only 31% vegetable. Young Mallards fed on low protein diets grow more slowly than do ducklings on high protein diets. (Brooke 247)
Nestling birds are fed a protein rich diet even when adults may not need as much protein. Therefore, it is important to offer high-protein foods during breeding season.
Carbohydrates and Fiber
Carbohydrates are important to bird nutrition as well. They are classified as either soluble or insoluble. Soluble carbohydrates include starches, disaccharides, and monosaccharides. However, fiber protein, or the insoluble part of carbohydrates, is indigestible. The insoluble carbohydrates, or fibers, are indigestible because birds lack cellulase, the enzyme that breaks down cellulose. So, it is not efficient for birds to eat much fiber (Kaytee 3).
The mineral calcium is needed in large quantities for egg production. Vultures, who only eat the soft, low in calcium, parts of dead animals may be restricting their reproductive output. So, some African vultures “supplement their diets with small vertebrate prey that can be swallowed whole.” (www.stanfordalumni.org 1)
The egg-laying female Great Tit needs 33 milligrams of calcium per day, and her young need about the same amount in their first 2 weeks out of the shell. However, the normal source of calcium, insects, does not provide enough for the egg-laying season’s needs, and adult tits do not store extra calcium in their skeletons for use during this season. (Females’ body calcium content before and after egg laying is about the same.) So, Tits take in calcium in raw form, such as snail shells and tiny calcinaceous pebbles, and mortar pecked from walls. They also bring these sources to their young (Economist 9).
According to an observational study performed by Cornell, “during the breeding season birds need increased amounts of dietary calcium during egg laying and nestling rearing. Passerines acquire calcium shortly before and during laying and do not store calcium for egg formation” (Dhondt 1). Snail shells and calcareous grit are sources of extra calcium the birds may eat to obtain this needed nutrient. Cornell’s study consisted of volunteers who provided calcium at feeder sites over North America.
We documented that (1) the use of extraneous calcium varied during the breeding season; (2) the location in which species preferentially took the calcium varied with typical feeding location, aside from swallows which, somewhat surprisingly, favored calcium offered on the ground . . . (4) calcium use varied among species with thrushes, wrens, and woodpeckers taking calcium only rarely, and swallows, pigeons, and corvids taking calcium frequently. The most surprising result perhaps was the very high frequency of calcium use in jays; Blue Jays may hoard calcium in the fall. (Dhondt 1)
This study shows that calcium is important to wild birds during egg-laying season, but the intake of calcium varied from species to species.
Wild bird nutrition is a field in which little is known in comparison to the amount of information available for human nutrition. It is known that fat, protein, calcium, and carbohydrates are important to birds’ health, but it is not known exactly how much of each category of nutrients birds need. It would be difficult to pinpoint these because of the variations between bird species, size, age, weather, and annual behavior that require different levels of nutrition. However, the bird enthusiast can build upon general information to design a balanced diet for his backyard birds.
Angel, Roselina and Rochelle D. Plasse. “Developing A Zoological Avian Nutrition Program.”
Proceedings American Association of Zoo Veterinarians. P. 39-43. 1997. “Body Building.”
Economist. Vol. 336, Issue 7922, 7/8/95, p 79. Brooke, Michael and Tim Birkhead. The Cambridge Encyclopedia of Ornithology. Cambridge
University Press. Cambridge, England, 1991. p. 121-136. Butler, Ralph. “High-Fat diets are for the Birds.” National Wildlife. Feb/Mar 97, Vol 35, Issue 2, p. 10.
Dhondt, Andre A. and Wesley M. Hochachka. “Variations in Calcium use by Birds During the Breeding Season.” The Condor. Laboratory of Ornithology, Cornell University, Ithaca,
NY 14850. (Abstract) Fisher, Hans. “The Nutrition of Birds.” Avian Biology Volume II. Academic Press, Inc., NY, 1972.
Gill, Frank B. Ornithology, Second Ed. W.H. Freeman & Co. NY, 1995.
Sturkie, Paul D. Avian Physiology. Springer-Verlag New York, Inc. NY, 1976.
“What Every Bird Owner Should Know about Avian Nutrition.” “Technical Focus”. Kaytee
Products, Inc. 2000.
www.standfordalumni.org/birdsite/text/essays/Diet_and_Nutrition.html — Link Not Active 2005