Ornithology researchers watched 22 species of insectivorous birds in a deciduous forest in New Hampshire – warblers, thrushes, vireos, chickadees, sapsuckers, wrens, etc.- and classified the birds’ feeding maneuvers into 17 different categories. Observers noted behaviors like hawking and probing and sallying, the height at which each of these maneuvers occurred, and in which of eight species of trees they took place in. The researchers recognized that the 22 birds can be grouped into guilds – assemblages of birds that feed in similar ways. (“Guild”, from Medieval times, means a group of craftsmen, workers, or merchants who shared the same interests.) The bird guilds are: the ground foragers, tree trunk and branch foragers, canopy feeders, and those that feed in other parts of the vegetation. Within each guild the birds subdivided themselves by their differential use of foraging substrates (e.g. bottom or top of leaf,), the use of different tree species (e.g. oak or maple) and foraging maneuvers (e.g. hovering or probing). This research demonstrates that avian communities can be defined and studied by using categories based on feeding habits. There are numerous similar schemes, and almost all of them begin with defining guilds

 Foraging guilds give us sort of a flowchart of an avian community, so let’s look at the guilds found in typical ecosystems.  We begin with decomposers - the  vultures, obligate scavengers that rarely feed on living prey. They serve not only to recycle dead bodies, but limit the spread of diseases. They fly slowly and soar over wide areas, often in groups to increase their chances of discovering food. They are big because the unpredictability of their food sources necessitates survival on their body reserves between feeding bouts. They compete with microbes, not other birds, as microbes quickly make carcasses unpalatable or toxic. In dense habitats like forests, omnivores like crows or ravens dispose of carcasses.

Granivory (seed and grain eating) evolved in tandem with birds’ becoming lighter as their ability to fly evolved. About 100-140 million years ago some birds had crops to store and predigest seeds. Teeth were replaced by a horny beak, and the job of chewing was relegated to the gizzard. In a deciduous forest about 15 percent of the avifauna, mainly finches and sparrows, are primarily granivorous.  In a coniferous forest about 35 percent of birds are seed eaters, in grasslands 60 percent, and in grain fields up to 90 percent. Seeds have to survive on their own until they germinate, so they are packed with nutrients, up to 65 percent carbohydrates, some fiber, some protein, and a bit of fat. Granivores need to supplement their diet with insects during the breeding season and pre-migration for more protein.

Beaks allowed herbivorous birds to eat plant parts such as roots, shoots, or leaves. Approximately ten percent of the plants in any ecosystem are eaten by herbivores, but only about three percent of bird species use vegetation as a major food source as plant parts are fibrous and difficult to digest; they contain only about 20 percent digestible and less than 20 percent protein. Herbivorous birds strategically select parts of plants that are high in protein and low in fiber. The Vegetarian Finch of the Galapagos Islands feeds primarily on buds, leaves, flowers fruit, and soft bark under twigs; the bird has a parrot-like bill, a large gizzard, and an exceptionally long intestine to digest plant materials. Many herbivorous birds have to supplement their diet with insects for protein throughout the year.

Frugivores, about 12 percent of all bird species, eat fruit. Half are songbirds, but some parrots, pigeons, and woodpecker relatives also consume fruit. High in carbohydrates, low in protein, the fat content of different fruits vary from one to 67 percent and may have undigestible parts such as skin, seeds, a hard seed coat, or distasteful or toxic compounds. Frugivores are mostly neotropical and play a major role in the distribution of plants, a major reason for fruit-eating among birds; many plants evolved fruits with characteristics specifically to attract dispersers. Seed-dispersing frugivores are especially important in revegetating damaged ecosystems or developing habitats on new islands. In providing dispersal services to plants, frugivores ensure their own survival by providing a continual supply of food. There is little interspecific competition or specialization among frugivores because fruits tend to be superabundant when they are ripe and fruit eaters need to be able to handle whatever is available at the time.

Two billion people around the world eat insects.  In Korea boiled silkworm pupae are seasoned and eaten as a snack. Connoisseurs in Japan enjoy aquatic fly larvae sautéed in sugar and soy sauce. In the U.S. you can purchase protein bars made of cricket flour. And why not?  Insects are 50-78 percent protein and 77-98 percent digestible.

Approximately 60 percent of the world’s birds are dedicated insectivores, surviving on arthropods. (The term “arthrovore” is rarely used.) There is such a variety of exploitable arthropods that birds employ a wide array of foraging behaviors like hawking, sallying, gleaning, or probing. In temperate area winters, arthropods are scarce so permanent residents have to be flexible and find dormant insects, larvae or eggs, switch to another food source, or leave. Downy Woodpeckers probe galls or stems of weeds for larvae. Northern Flickers eat ants and beetles from the ground and may take berries and seeds. Great Tits survive the winter on a regimen of berries and the seeds of beech and hazel.

Flycatchers, warblers, swallows, and swifts, dependent on active insects, migrate to the tropics where they have access to arthropods all year except at high elevations. What happens when all these insectivorous birds arrive, sometimes doubling the bird population? Insectivorous birds that are permanent residents in the tropics tend to be specialists, surviving in narrow foraging niches. As examples, eleven percent of insectivorous birds in the upper Amazon basin feed only by acrobatically gleaning insects off aerial leaf litter (dead leaves hanging from understory plants), as some antthrushes and ovenbirds do. Although there are fewer dead leaves than live ones, dead leaves hold more arthropods and thus offer a higher energy yield. Migratory birds arriving in the tropics become opportunists and survive by feeding in the gaps between the residents’ foraging niches. One exception is the Worm-eating Warbler from the Eastern U.S. that winters in Central America. In the spring in the U.S. it spends about 75 percent of its time searching live leaves; on its wintering grounds it forages 75 percent of the time on dead leaves, a lot like tropical ovenbirds and antthrushes.

Nectarivores are nectar-eaters, small but important pollinators such as hummingbirds, sunbirds, honeycreepers, and honeyeaters. Other birds occasionally eat nectar, such as orioles and woodpeckers. As many as ten percent of wild plants in the tropics are bird pollinated by nectarivores and perhaps six percent of agricultural crops such as bananas and papayas. Pollinators are especially important for isolated populations of plants because wind pollination is unreliable. 

Bird numbers are mainly limited by competition, but some are also limited by predation by the carnivore guild. Raptors are the hawks, eagles, owls, falcons, caracaras and relatives. There are nearly 500 raptorial birds, about one third of them nocturnal. Ninety percent of all raptors live either exclusively or mainly in the tropics, a reflection of the productivity of the tropical ecosystem. Some are specialists like the Osprey that feeds exclusively on fish, the Black-chested Snake Eagle of Africa, Pel’s Fishing Owl, and the bird-eating Peregrine Falcon. But most raptors are generalists and opportunists and have few predators so they are mainly limited by competition for food. The nocturnal habit of so many raptors lessens that competition a bit by spreading the carnivore guild over 24 hours.