The purpose of this chapter is to summarize the vast observations on birds’ diets, feeding behavior, and anatomical and morphological adaptations for foraging.
Metabolism and food requirement
A bird’s metabolism largely determines how much food it requires. All birds are homeotherms with a generally high metabolic rate. Homeothermy is energetically costly, necessitating frequent eating. Because of this high cost, most birds either eat high‐energy plant foods such as seeds, fruit, and nectar or they gain high‐energy protein and lipids from eating animals.
Body size and metabolic rate
The metabolic rate typically scales with body size, so that larger birds consume more food in total than smaller birds, but consume less food per unit of body mass. Conversely, small birds have a higher metabolism per unit of mass, which requires a higher feeding rate; this in turn makes small birds more sensitive to feeding interruptions caused by challenges such as inclement weather.
Environment and energy need
Aside from these general patterns, birds vary largely in their energy needs. For example, birds that live in cold environments may need to elevate their metabolic rate just to maintain body temperature. Hot environments also require increased energy expenditure as birds use evaporative cooling by panting and wetting body parts.
For instance, a pelican sitting in the sun on a hot day may visibly flutter the neck membranes of its gular pouch, an effective but energetically expensive form of evaporative cooling.
How birds prepare for migration and save energy
Birds living in seasonal environments undergo annual changes in energy‐demanding activities such as migration. Fats are the most efficient fuel for long‐distance migration. Many birds put on fat reserves to buffer themselves against unpredictable or inhospitable circumstances such as blizzards, monsoons, or droughts that reduce feeding opportunities. Birds that rapidly accumulate fat reserves by increasing their consumption rate are described as going through a period of hyperphagia that is often accompanied by increased metabolism. Some birds may also conserve energy by decreasing their metabolic rate and simultaneously dropping body temperature, entering a state of torpor.
Food intake with nutrients
High-calorie plants
Scientists measure food consumption in energetic units of calories. However, certain nutrients can constitute a disproportionate currency in birds’ diets. Willow Ptarmigans of northern Europe and North America, for example, increase their intake of valuable nitrogen and phosphorus via their choice of particular plant foods. Geese and some ducks also feed selectively on the most digestible and high‐nutrient plants and plant parts.
Calcium source birds seek to produce eggs
Calcium is a chemical element that birds often seek in particular food types. Calcium is often in short supply for female birds that require it to create eggshells for their young. Birds frequently obtain calcium by eating snails, bones, or other calcium‐rich foods that otherwise are not a major dietary component.
A variety of seed‐ and plant-eating birds additionally consume soil at clay licks (a habit known as geophagy).
sodium source in birds for early growth
Studies in South America, where clay licks are known as “colpas,” have suggested that birds like macaws and their relatives visit them primarily as a source of sodium, an element that is rare in plants but important for animal physiology. Evidence supporting this hypothesis includes the low sodium content of plants forming a major part of these birds’ diet, the high-sodium clays selected by the birds, the location of colpas far inland and therefore far from other natural sources of sodium (such as the ocean), the increased use of colpas when feeding chicks by parent birds who need dietary sodium for early growth, and signs of sodium restriction in other animals like ants living in the same regions. The colpa clays consumed by birds may also help them detoxify the alkaline compounds contained in some seeds.
Optimizing when,what, where, and how to forage
All birds must make foraging choices, such as deciding where to feed, what search path to follow, how long to persist in a food patch, what foods to pursue or bypass, whether to join a group and how to balance feeding with other considerations such as avoiding predators. These decisions include the fundamental questions of foraging behavior. Researchers create models as tools to describe how animals make decisions, given assumptions and limitations about the foraging process, and then test these models using observations and experiments.
How birds forage and the strategies used
These models are a significant way to understand animal behaviors such as foraging, habitat use, and communication. The underlying assumption of such models is that natural selection favors those individuals that perform best as foragers.
One of the most essential decisions foraging birds make is whether to spend time and energy attacking a certain prey item they have already encountered or instead to continue searching for more profitable prey.
The constraints include,
the energy in each prey type,
the search time needed to find it,
and the handling time required to catch and consume it,
comparing the costs and benefits of feeding as a generalist (attacking all items encountered) and a specialist (passing over some items).
the overall abundance of the prey (which influences search time) and
the profitability of the prey (which usually is assessed using handling time since more profitable items provide more energy per item and require less handling time).
In a classic study of optimal foraging, accessing food without wasting energy we can see
Birds dropping food items to access food
the crows selected and broke open whelks by flying up and repeatedly dropping these marine snails onto rocks.
A variety of birds drop food items onto hard substrates to access food:
Golden Eagles drop tortoises,
American Crows drop black walnuts,
and Lammergeiers drop bones.
Whelk size and dropping height
In this case, birds selected only the largest whelks and that the drop height was consistently about 5 meters.
But why the crows bypassed smaller whelks containing perfect meat and why the drop height was not lower, which would use less energy?
Larger whelks required fewer drops to break (and thus required less effort) than smaller whelks, regardless of drop height, and drops above 5 meters provided little improvement in breakage, regardless of whelk size. These results support the idea that the crows adopted whelk-foraging tactics that optimize their short-term energy harvest.
How long a forager should stay in a particular patch of food to reduce
This situation is faced by foraging animals that find that finding food becomes harder, with diminishing returns the longer they stay in a patch. These models, formulated with simple algebra, predict, among other things, that foragers need to ignore less profitable items as long as more profitable items are sufficiently abundant.
The bird’s behavioral choice is how long to stay in the patch as determined by experience. The bird should adopt a “give‐up time” rule that causes it to leave the patch when its rate of feeding diminishes to the average rate in the environment overall. The third component involves constraints, which in this model include both the diminishing return as the bird stays in a particular patch and the time cost of leaving a patch to search for a new one.
The real world is not nearly as simple as optimal foraging models assume:
prey often are not encountered sequentially,
foragers often lack the perfect knowledge of all prey types needed to make optimal decisions,
and foragers are selected for traits in addition to optimizing feeding rate over short time intervals. For example, birds might forage, seemingly suboptimally, on low‐quality prey found near safe cover if moving out to forage on better-quality prey would cause the foraging birds to become more vulnerable to predators.
Where to forage and Prey-searching strategies
What is levy flight and how sea birds prey searching strategies work
Optimal Foraging Theory also addresses how birds should optimize their prey-searching strategies. For example, many birds capable of foraging over large areas of open ocean, like penguins and albatrosses, face the problem of detecting sparsely scattered prey patches. A major constraint facing such foragers is a paucity of information to guide them to high‐quality feeding locations. One potential strategy ani- mals employ in such circumstances is Lévy flight named after a French mathematician who introduced the concept in 1937. Lévy flight is characterized by short, randomly oriented searches interspersed by occasional longer flights, and it provides a useful way to understand how some birds forage.
Albastros and Penguins foraging method
Several recent studies have explored how seabirds forage for food over vast distances of open ocean. In one, researchers used GPS satellite data to track Wandering Albatrosses over Antarctic waters. The locational data were coupled with temperature data loggers placed in the birds’ stomachs that allowed the biologists to determine the mass and timing of prey eaten. This integrated approach showed that some albatrosses used Lévy‐like flight patterns, whereas others used random search paths, and some used a mixture of the two.
A different research group examined the search paths of Adelie Penguins foraging on krill in the open ocean near Antarctica using cameras, data loggers, and motion detectors attached to the penguins. They found that these birds also had search patterns characteristic of Lévy foraging ways to optimize the discovery of patchily and sparsely distributed prey.
Foraging in groups
Birds foraging in groups may face additional decisions involving trade‐offs between benefits and costs.
Group‐living individuals face questions including:
(1) whether to join the group, taking into account the number of birds already in the group and food abundance;
(2) how to forage and competing within the group; and
(3) how to take advantage of or collaborate with other group members.
Group living is complicated by the fact that the optimal behavior for an individual bird almost always depends on the choices made by other individuals, also foraging in groups involves risks like,
Thieves in group
Consider, for example, a bird’s choice either to locate its food or to steal food from another individual in the group. If all other individuals in the group are foraging for food, then a thief does well in the absence of competition from other thieves;
Safety during foraging
(Safety) more often when they realize that staying at the feeder exposes them to predation risk and when carrying the food entails relatively little cost. This idea was first tested by simulating the presence of a predator using a model hawk “flown” past a birdfeeder on a wire. For these chickadees, moving to cover had short‐term costs, because the added transport time decreased their feeding efficiency, but it increased their long‐term survival. Predators pose a threat to most birds, which are often particularly vulnerable or conspicuous during foraging.