1. Relatively few birds feed heavily upon green-plant materials, though a number may take substantial numbers of buds. Birds appear not to possess adequate gut biota to break down cellulose efficiently, and even it they did, this system would not be compatible with efficient flight, on account of the long processing times required.
2. Birds have stronger adaptations toward feeding upon seeds and make a particularly heavy impact upon seeds in trees and on those in widely scattered or unpredictable crops.
3. Several families, mostly of tropical distribution, feed primarily upon fruit. Again, they are most successful in exploiting resources reached only with difficulty by other groups. A similar pattern holds for nectar stores. In the case of fruit and nectar, evolutionary complications result because the plants compete for seed dispersers and pollinators.
4. Most primarily herbivorous species feed their young partially or totally upon insects or other animal foods. Only a few species are able to fledge upon an entirely herbivorous diet, and those that do so often have extremely long fledging periods, which subject their young to high rates of predation unless the parents nest in places inaccessible to predators.
5. In general, herbivorous birds make a heavy impact relative to other animals only when they exploit resources that the other groups have difficulty in obtaining (hard to reach, or widely scattered in space and/or time).
6. Insects are exploited by more families of birds than any other food category. Birds are of greatest relative importance in capturing insects on the wing and in arboreal locations. Other terrestrial invertebrates are probably taken by techniques similar to those used for insects.
7. A variety of species take invertebrate prey along the water's edge or in shallow water.
8. Almost without exception, vertebrate prey is of small size, in part a function of the size of birds. Occasionally predation upon small terrestrial vertebrates may be relatively heavy, generally in outbreak situations.
9. Several families feed exclusively upon fish, crustacea, squid, and other aquatic prey. Some indirect evidence suggests that exploitation of these resources may sometimes be heavy, though there is little evidence that these groups often make an appreciable impact upon this food source.
10. Birds appear to be among the most important scavengers of animal remains.
11. Like the herbivorous birds, carnivorous birds appear to exert their greatest impact upon resources that are hard to reach or are widely scattered in space and/or time.
12. One-third of the avian families feed regularly on a variety of resources, as recognized in Table I. Most of these families specialize upon resources located in certain areas (ground, water's edge, trees, etc.). Only 13% of these families regularly use such a wide range of foods that they transcend both of these considerations (food type, segment of the habitat). In very few cases (5 %) does this designation result from some species of a family specializing upon one category of food and others using a different category.
13. There have been few large (> 20 kg), primarily herbivorous birds, though there have been several large omnivorous species. Large carnivorous species have been even rarer. All of these large species have been flightless, and in most cases they have occurred when or where large mammals were scarce or absent.
14. Birds have made extremely limited use of subterranean areas, caves, or deep water, and adaptations facilitating other types of life appear to be virtually incompatible with success in these situations. All these areas are characterized by a rapidly decreasing food gradient.
15. Bats exploit certain, but not all, of the food resources regularly exploited by birds. The resources not used by bats that are exploited by birds are those that are available over a full 24 h period; thus, bats' nocturnal habits do not provide them with an advantage in exploiting them.
16. Insects also use some of the resources exploited by birds. With few exceptions, insects are smaller than birds or bats. Considerable indirect evidence is consistent with the argument that the segregation in time, resource type, and size of flying animals is at least in part caused by competition.
17. While selection for anti-predatory mechanisms can be surmised, the reciprocal density relationships frequently noticed between birds and other organisms suggest that they arise from competition, rather than predation.
18. Most information suggests that interactions between birds and other animals of the same trophic level are usually, though not invariably, over food.
19. The types of relationship existing between birds and other organisms described above are repeated in a variety of other animal groups, though they clearly are not ubiquitous in the animal kingdom.