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A Classification Scheme for Foraging Behavior of Birds in Terrestrial Habitats

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A Classification Scheme for Foraging Behavior of Birds in Terrestrial Habitats Studies in Avian Biology No. 13:144-160, 1990. A CLASSIFICATION SCHEME FOR FORAGING BEHAVIOR OF BIRDS IN TERRESTRIAL HABITATS J.V. REMSEN,JR. AND SCOTT KROBINSON Abstract. Studiesofavian foragingbehavior in terrestrialhabitats sufferfrom a lack of standardization in the kinds of data gathered and in the terminology used to classify different activities. These incon- sistenciespartially reflect the variety of questionsasked about foraging.If a standardterminology were used, then data on foraging behavior could be included among the standard data (e.g., clutch size, body weight, and mating system) routinely recorded for the biology of a bird species. We propose a system for gathering foraging data for landbirds in which the five basic, sequential componentsofforaging (search,attack, foragingsite, food, and food handling) are quantified separately. Data on searchingbehavior involve measuring continuous variables and are particularly critical for studies of energetics. The “attack” component is most in need of standardized terminology. The system that we propose separatesthe attack perch from the attack maneuver, and further subdivides the maneuvers into near-perch, subsurface,and aerial maneuvers. Each of these general categoriesis further subdivided according to details of attack movements and ways in which substratesare ma- nipulated. Data on attack methods are primarily useful for studies of ecomorphology, but may also be important in bioenergeticand community-level studies. Quantifying the foraging site involves measuring the following variables: general habitat (location in a study area), vertical position, foliage density, and substrate.Although identification and quanti- fication of foods taken in the field is difficult, it can provide valuable information on food size (and taxon for largeritems). Dietary data from stomachsamples are usefulfor studiesof resourcepartitioning when they show dramatic differences,but overlapping diets do not necessarilyindicate that two birds forage in the same way. Food-handling behavior is seldom measured in the field, but is valuable in studiesof optimal foraging behavior and ecomorphology. Intercorrelations between each of these aspectsof foraging can be determined from standard multi- variate analyses.How finely to subdivide categoriesdepends upon the kinds of questionsbeing asked. Key Words: Foraging behavior classification;foraging maneuvers; search;attack; foraging site; diet; food-handling; glean; sally; probe; manipulate. There are almost as many ways of classifying quantify foraging. For example, the “hawk” cat- and quantifying foraging behavior as there are egory of Sherry (1979) and Holmes et al. (1978, papers on the subject. In part, this variety reflects 1979b) includes attacks on prey animals that were the fact that no two species or groups of species flying when first seen, attacks on prey flushed forage in exactly the same way, and that no two from foliage by the foraging activities of the bird, habitats present exactly the same foraging op- and chases after prey that the bird attacked and portunities. It is difficult, for example, to quan- missed. Later papers by Robinson and Holmes tify the foraging methods of bark-foragers in the (1982, 1984) and Sherry (1983, 1984) however, same way that one quantifies the foraging of fo- showed that differences among these aerial ma- liage-foragers (Jackson 1979). Another factor neuvers have important implications for diet. contributing to the lack of standardization is that Remsen (1985) showed that the fine details of different kinds of questions often require differ- substrate use (e.g., dead leaves, moss) that are ent kinds of data. Many studies that focus on often ignored in many community studies were resource partitioning record only the details of particularly important in resource partitioning in foraging site selection and omit data on search a tropical bird community. Rosenberg (this vol- and attack movements (e.g., Hertz et al. 1976). ume) further showed that even within a group as In contrast, studies of ecomorphology emphasize specialized as dead-leaf foragers, species differ in prey-attack methods (e.g., Osterhaus 1962; Fitz- the kinds of suspended dead leaves that they patrick 1980, 1985) whereas bioenergetic and search. The classification of foraging behavior, optimal foraging studies emphasize searching therefore, is more than a semantic problem: one movements as well as prey handling (e.g., Sherry can reach different conclusions simply by clas- and McDade 1982, Robinson 1986). Even stud- sifying foraging methods differently. ies addressing the same questions in ecologically In this paper, we propose a system for mea- similar birds do not always measure the same suring and classifying foraging behavior for non- variables. raptorial landbirds. Our goal is to standardize This lack of standardization, however, reflects data-gathering methods and terminology to per- fundamental inconsistencies in the importance mit among-site and among-species comparisons attached to the individual variables used to that are currently handicapped by the absence of 144 CLASSIFICATION OF BEHAVIOR--Remsen and Robinson 145 a common vocabulary. If some standard system volume). We discuss briefly data on bird diets, and terminology were used by those studying and also propose a standard terminology for food- foraging behavior, then we could ask questions handling techniques. The final section of our pa- concerning the frequencies of various behaviors per deals with some of the ways in which data among communities. Such comparisons may can be analyzed to address questions of resource- provide important insights into community or- partitioning, bioenergetics, and ecomorphology. ganization: in a field such as foraging behavior, in which community-wide experimental manip- SEARCHING BEHAVIOR ulations are difficult or limited, a comparative Searching behavior includes those movements used approach among species and communities may to search for food or substrates that hide food; under be the only method available for testing many our definition of searching behavior, “search” ends hypotheses. once food or food-hiding substrates are sighted and The system presented here separates five se- attacked. Searching methods have usually been quan- quential components of foraging behavior, each tified by recording the lengths and rates of movements of which is quantified separately when data are between perches and the time intervals between move- gathered in the field: (1) search (movements lead- ments (e.g., MacArthur 1958, Cody 1968, Williamson ing up to sighting of food or food-concealing sub- 197 1, Fitzpatrick 198 1, Robinson and Holmes 1982). Other variables are: (1) distance covered per unit time; strates); (2) attack (movements directed at food (2) number of stops and time-spent-stopped per unit item or the substrate that conceals it once sight- time; (3) number of attacks (and % successful) per unit ed); (3) foraging site (including general location time; (4) direction ofmovement between stops, in three and specific substrate); (4) food (including type dimensions if appropriate; (5) sequential distribution and size); and (5) food handling (after food item of locations of stops (to calculate return rates to pre- obtained). We recognize that many of these com- vious stops). Between-foraging-site movements can be ponents are not necessarily independent, but we categorized as: (1) walk, (2) hop, (3) jump (leg-powered prefer to quantify each separately and to allow leaps that cover more space than the typical hop), (4) subsequent analyses to show intercorrelations. run, (5) climb (with notations on whether or not the tail is used as a brace), (6) glide, (7) flutter, and (8) fly. At the outset, we recognize that any classifi- Robinson and Holmes (1984) further distinguished be- cation system inflicts typology on what may be tween hops in which American Redstarts (Setophagu gradients of behavior, but we see no other prac- ruticilla) fanned their tails and lowered their wings, tical solution for organizing foraging informa- and hops in which there were no extra movements. tion. Our goal is to distinguish among what we Some birds also use their wings for support when hop- subjectively perceive to be functionally different ping on thin, weak perches, a movement that could be categories. By using standardized terminology, called a “flutter-hop” (Robinson, unpubl. data). Also data on foraging behavior can be included among of interest are postures during searching that are sel- the standard biological data reported for bird dom qualitatively described (for exceptions, see any E. 0. Willis reference) or quantified but that may have species. At present, reference works on birds, subtle morphological correlates. Postures are particu- which typically include detailed, quantitative data larly difficult to categorize because most species move on variables such as clutch size, body weight, and change postures frequently, and because head, wing, molt and migration schedule, and mating sys- and tail orientations are all simultaneously involved. tem, tend to omit, or describe in superficial, qual- Perhaps the advent of telephoto video-cameras will itative ways, all aspects of foraging behavior ex- permit such analyses; in this paper we deal with pos- cept perhaps diet. Foraging data should be tures only peripherally. More amenable to quantifi- included in such reference works, because for- cation are changes of orientation while searching from aging behavior is an
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