RESEARCH Foraging Guilds of North American Birds RICHARD M. DE GRAAF ABSTRACT / We propose a foraging guild classification for USDA Forest Service North American inland, coastal, and pelagic birds. This classi- Northeastern Forest Experiment Station fication uses a three-part identification for each guild--major University of Massachusetts food, feeding substrate, and foraging technique--to classify Amherst, Massachusetts 01003, USA 672 species of birds in both the breeding and nonbreeding seasons. We have attempted to group species that use similar resources in similar ways. Researchers have identified forag- NANCY G. TILGHMAN ing guilds generally by examining species distributions along USDA Forest Service one or more defined environmental axes. Such studies fre- Northeastern Forest Experiment Station quently result in species with several guild designations. While Warren, Pennsylvania 16365, USA the continuance of these studies is important, to accurately describe species' functional roles, managers need methods to STANLEY H. ANDERSON consider many species simultaneously when trying to deter- USDI Fish and Wildlife Service mine the impacts of habitat alteration. Thus, we present an Wyoming Cooperative Wildlife Research Unit avian foraging classification as a starting point for further dis- University of Wyoming cussion to aid those faced with the task of describing commu- Laramie, Wyoming 82071, USA nity effects of habitat change. Many approaches have been taken to describe bird Severinghaus's guilds were not all ba~cd on habitat feeding behavior. Comparisons between different studies, requirements, to question whether the indicator concept however, have been difficult because of differences in would be effective. terminology. We propose to establish a classification Thomas and others (1979) developed lists of species scheme for North American birds by using common by life form for each habitat and successional stage in the terminology based on major food type, substrate, and Blue Mountains of Oregon. This is a use of the guild technique, using foraging guilds. concept in which assemblages of species are defined The guild concept was originally proposed by Root according to common nesting and feeding requirements. (1967) to indicate a group of species that exploit the same Another use of the guild concept was made by Short and class of environmental resources in a similar way. For Burnham (1982), who grouped species into layers of each example, Root defined a foliage-gleaning guild in oak habitat. For example, forest species are associated with woodlands as a group of birds whose diet consisted of subsurface, ground surface, and shrub layer. arthropods obtained from the foliage zone of oaks. Root Verner (1984) defines management guilds as groups pointed out that birds could be a member of several of species that respond in a similar way to a variety of guilds. Thus, seasonal changes that occur in avian feed- changes likely to affect their environment. He states that ing habitats could be accommodated. The guild concept all bird species that depend on tree canopies for their food has become a useful tool in looking at competition, niche supply--whether fruit, buds, leaves, or insects--could be separation, and functional relationships within the com- in one management guild. This would give the manager munity. As a result, biologists can use the guild concept to some idea of the group of species that are affected by show how different taxa interrelate and how impacts of habitat change. Verner's use of management guilds does habitat change influence community dynamics and not not fully consider the functional role originally proposed just individual species. by Root; rather, it considers responses to changes in The guild concept has been used by a number of identifiable zones of forest habitats. authors to compare or classify components of a commu- Standardization of guilds provides a means by which nity. It is possible to establish guilds on the bases of resource managers can compare communities of birds. different criteria. Landres (1983) reviews some efforts to The number of community components are reduced, assign species to guilds. Guilds were used by Severing- thereby facilitating the description. Likewise, biologists haus (1981) to develop the idea of indicator species; he can discuss competition and energy flow using common believed that since all members use the same resource, all terminology (Landres 1984). should respond similarly to changes in their environ- Guilds can be combined in different ways for different ment. There are enough exceptions, in part because purposes. Basically, however, we need to develop bases Environmental Management Vol. 9, No. 6, pp. 493-536 1985 Springer-Vedag New York Inc. 494 R.M. DeGraaf and others for classifying each species into a guild. In this article, we tion seasonally, which may require the use of different have considered food, substrate from which the food was substrates or techniques. We indicate such changes in obtained, and technique used in obtaining the food as the columns labeled as breeding period or nonbreeding peri- common ground to classify 672 species of North Ameri- od. We do not indicate gender- or age-specific differences can birds. These three criteria were selected because they in foraging guilds unless they appear vastly different and can be observed or documented for each species. The would affect the management of the species. components of major food, substrate on which food was The species in this list include all those that can be found, and techniques used to obtain food Provide realis- found in North America during the breeding season or tic groupings of species. The user of this information can nonbreeding season. The geographic range covered by group the species in one of the guilds presently described this list is from Greenland to Florida to California to or utilize different criteria to develop the grouping. With Alaska. Birds of Mexico and the Caribbean are not this classification, based on avian feeding behavior, users included unless they are regularly found north of these could compare the results of different studies for func- areas at the specified season. Accidental species are not tional differences. included. The Check-list of North American Birds, 6th Our description of guilds brings the concept as pro, edn. (American Ornithologists' Union, 1983) was used to posed by Root (1967) into use by resource managers. determine the status of these birds. Despite the different integrations and use of concept, it is now possible to have a ,dictionary" of terms characteris- tic of each species. While debates can continue about the Classification Procedures concept, our classification provides a reference point for application. For food type, we did not necessarily include all foods Birds' names are from the American Ornithologists' taken by each species--only the major food items (20% of Union (1983); scientific names are listed in the Appen- diet during a given period). dix. The major references u.'sed to develop this foraging Carnivore: vertebrates guild classification included A. C. Bent's Life History of Crustaceovore: crustaceans North American Birds series (Smithsonian Institute, US Frugivore: fruits National Museum Bulletins, Washington, DC); Hand- Granivore: nuts book of North American Birds,' edited by Ralph S. Herbivore: plants (leaves, stems, roots) Palmer (Yale University Press, New Haven, CT, 1962); Insectivore: insects Waterfowl of North America, by Paul A. Johnsgard Moltuscovore: mollusks (Indiana University Press, Bloomington, 1975); Ducks, Omnivore: a variety of foods including both animal Geese 6" Swans of North America, by Frank C, Bellrose and plant foods (the less common food group makes (Stackpole Books, Harrisburg, PA, 1976); American up 10% of diet) Wildlife 4r Plants, by Alexander C. Martin and others Piscivore: fish (Dover Publications, New York, 1951); Crows of the Vermivore: sandworms, earthworms, etc. World, by Derek Goodwin (Corneil University Press, Ithaca, NY, 1976); Audubon Water Bird Guide and Substrate refers to the place where a food item is found or Audubon Land Bird Guide by Richard H. Pough (Dou- taken. bleday & Co., Garden City, NY, 1946)i Management Of air: caught in the air Migratory Shore and Upland Game Birds in North bark: on, in, or under bark of trees America, edited by Glen C. Sanderson (International coastal: waters along coast (can include brackish as Association of Fish and Wildlife Agencies, Washington, well as salt water) DC, 1977). coastal beach: beaches and/or tidal flats along coast coastal bottom: floor of continental shelf along coast coastal rock: rocks along coast Foraging Guild Designation coastal surface: surface of coastal waters Often it was difficult to limit this classification to a floral: on or in flowers single guild per species, so in several eases we have listed fresh marsh: freshwater marshes (on mud, in shallow more than one guild. An additional guild was designated water, or on marsh plants) when a second food item represented at least 20% of the freshwater: freshwater habitats (ponds, lakes, rivers, diet of a species or when the specie s spent at least 20% of streams) its time feeding on a different substrate or using a freshwater bottom: bottoms of freshwater ponds and different technique. Some species shift their food Selec- lakes Foraging Guilds of North American Birds 495 freshwater shoreline: shores of freshwater ponds, hover-gleaner: hovers in air while selecting