Hunting Behavior, Prey Selection, and Energetics of Snail Kites in Guyana: Consumer Choice by a Specialist Author(s):
نویسنده
چکیده
-The hunting behavior, snail size selection, and time-activity patterns of nonbreeding Snail Kites (Rostrhamus sociabilis sociabilis) were studied in Guyana rice fields. Kites spent 62% of the photoperiod perching, 19% foraging, 13% in maintenance activities, and 6% flying. As the day progressed, the percentage of daylight hours spent perching increased significantly, while foraging decreased significantly. Kites successfully captured and ingested Pomacea snails in 78% of the foraging bouts observed. The mode of hunting was evenly split between coursing (50.7%) and still-hunting (49.3%). Searching and returning times were related to time of day, as significantly more coursing hunts and still-hunts occurred in mornings and late afternoons, respectively. Prey handling time was the most time-consuming component of a foraging bout. Time spent searching for prey, returning, or handling prey was not related to snail size. The size distribution of captured snails differed significantly from that of available snails; kites selected more medium snails and fewer small snails and took large snails in equal frequency to that at which large snails were available. Kites captured the same-sized snails when coursing as when still-hunting. As rice grew, kite utilization of rice fields declined, and the frequency of still-hunting decreased while course-hunting attempts increased. The daily caloric intake of kites was estimated to be 104.2 kcal, the daily energy expenditure 85.7 kcal. Results are discussed in relation to consumer choices of specialists. The most important decision a foraging Snail Kite may make is what patch to search in and how long to search before abandoning patches. Received 14 January 1982, accepted 28 June 1982. LITTLE is known about the hunting behavior and foraging ecology of the Snail Kite (Rostrhamus sociabilis). Long recognized as an extreme diet specialist, the Snail Kite feeds almost exclusively on Pomacea snails (Haverschmidt 1962, Snyder and Snyder 1969, Voous and van Dijk 1973; for exceptions see Sykes and Kale 1974, Beissinger in prep.). A kite captures a snail by flying over a marsh, extending a foot, and grabbing the mollusc with long toes near the water surface. It then perches in order to extract the body from the shell with its thin, sharply hooked bill. Two modes of hunting have been observed (Snyder and Snyder 1969, Haverschmidt 1970): "still-hunting" kites visually search from a perch and capture a snail near that perch after a short flight; "coursehunting" kites fly 3-5 m above a marsh, usually facing into the wind, visually searching until a capture occurs. Snyder and Snyder (1969) and Voous and van Dijk (1973) described in ' Present address: School of Natural Resources, University of Michigan, Ann Arbor, Michigan 48109 USA. detail how kites extract and consume snails, but there is no quantitative information on the behavior and energetics of kite foraging. In this paper, I describe the foraging ecology of the Snail Kite (R. s. sociabilis) on the basis of field observations of hunting behavior, activity patterns, and prey selection in rice fields in Guyana, South America. STUDY AREA AND METHODS Studies were conducted from July-August 1977 at the Rice Research Station and surrounding rice fields on the coastal lowlands (1.4 m below sea level) at the Mahaicony and Arbary Rice Development Scheme (MARDS), Burma, Guyana, South America (6?28'N, 57?45'W). The freshwater marshes or mangroves that once covered this region have been almost totally replaced by rice cultivation (6,721 ha in 1976; Kennard pers. comm.), with few fields remaining fallow. There are two rainy and two dry seasons (Cummings 1965). Rainfall during the study period totaled 14.76 cm in July, the end of the long rainy season, and 3.84 cm in August, the onset of the short dry season. Daily minimum and maximum temperatures ranged from 22-31?C. For a more detailed description of the study area, see Giglioli (1959), Osborne and Boume (1977) and Bourne and Osborne (1978). 84 The Auk 100: 84-92. January 1983 January 1983] Snail Kite Foraging 85 A 26-ha study site of seven fields was selected for behavioral observations of kites. Strip censuses of Snail Kites and Limpkins (Aramus guarauna) were made one to five times weekly from 0700 to 0800. Fieldwork was initiated near the time of sowing. Rice had reached full height by the end of the study 6-8 weeks before harvest. I marked 14 fence posts on the border between two fields and cleared the surrounding area of old snail shells so that I could sample the size of snails selected by kites. As many as 14 kites were observed foraging from these posts during strip censuses but four was the average. Posts were selected because of easy accessibility and use by foraging kites. Fence posts ranged from 7 to 17 cm in diameter and 1 to 1.5 m high. Empty snail shells were collected from beneath each post on 14 occasions with periods of 24-48 h between collections. Aperture length was measured to the nearest millimeter, and after one week three size classes were delineated: small (11-24 mm), medium (24-28 mm), and large (29-36 mm). I attempted to estimate the relative abundance of the three size classes of snails in the two fields from late July through early August. Because water turbidity was great, I could not detect snails visually. I collected snails by walking or crawling a transect through the field, tactilely searching the mud and plant surfaces with both hands. A total of 20 h was spent sampling snails in kite feeding areas. Thirtyfour of these snails (13 large, 9 medium, and 12 small) were collected for caloric analyses. After extraction from the shell, albumen glands were discarded (as kites do not generally ingest them; see Snyder and Snyder 1969), and each snail was oven dried for 28 h at 100?C, when dry weight became constant. Ten snails were individually ground in a Wiley Mill, and caloric determinations were made on 40-mg samples of each through use of a Phillipson Microbomb calorimeter. I monitored Snail Kite activities by observing focal individuals (Altmann 1974) chosen at random for 15min periods from 20 to 75 m away through spotting scope and binoculars. As there was no breeding colony of kites in the vicinity and kites that were observed extracted and ate snails in the study area, none of the focal individuals was assumed to be breeding during the time of the study. Activities were recorded directly into a tape recorder and later timed to the nearest second with a stop watch. Three time periods were chosen a priori to homogenize air temperature trends within periods (morning: from sunrise at 0630 to 1100; midday: from 1100 to 1500; and late afternoon; from 1500 to 1830, just before sunset). A total of 60 focal samples was distributed almost evenly throughout the daylight hours (morning 19, midday 21, late afternoon 20). I tried not to sample the same kite more than once a day by sampling during only one time period a day. Activities were categorized as follows. (1) Perched birds were sitting upon a fence post, vegetation, or mudflat. It was not possible to determine reliably whether perched birds were engaged in searching activities (i.e. still-hunting), and no attempt was made to separate perched birds into those searching or those not searching for snails. Thus, still-hunting kites visually searching for prey while perched were included in this category. (2) Maintenance activities included preening, scratching, feather ruffling, wing stretching, wing drying, and bill cleaning. (3) Flying was defined as movements for the purpose of changing location of perches or hunting areas, as well as flights to or from roosting areas. (4) Aggression was observed mainly in the form of the supplanting of one individual by another or by active chasing of another kite. Finally, (5) foraging included all activities associated with the capture and ingestion of snails. A foraging bout was composed of searching (flight in the pursuit and capture of snails, including both coursing and still-hunting), returning (flight from the point of capture to any perch where the prey was consumed), and handling (the extraction of a snail from its shell and ingestion). The time between foraging bouts ("between") was calculated as the amount of time that elapsed between ingestion of a snail and the initiation of the next searching flight. For each capture clearly observed (79% of the captures), I visually estimated the snail size class. From experience gained while measuring over 1,000 snail shells, I usually was able to estimate the aperture length within 1 mm. A comparison of my estimate of the distribution of snail size classes that kites fed upon during periods of behavioral observation and the distribution of snails collected beneath feeding perches revealed no significant difference (X2 = 0.94, df = 2, 0.50 < P < 0.70). This supported my ability to assign captured snails correctly to the three size classes. Statistical analyses were conducted using parametric procedures on the Statistical Analyses System (S.A.S.) at Miami University and Wayne State University. Assumptions of normality and homoscedastiscity of parametric models were tested by inspecting scatter and residual plots, skewness and kurtosis coefficients, and comparing sample variances. Nonparametric statistics were used when assumptions were violated. Because percentage data were not normally distributed and often outside the 30-70% interval, they were arcsine transforned before analyses. Differences were designated as significant if P < 0.05. RESULTS AND DISCUSSION Daily activity.-Snail Kites usually arrived individually at foraging stations, presumably from a communal roost to the north, between dawn (0630) and 1100, with occasional arrivals or departures during the course of the day. 86 STEVEN R. BEISSINGER [Auk, Vol. 100 w, a. 4a 0 0 30 c ' ?AING wr 2 MA~ }m: 0 2 D MORNING MIDDAY (LATE AFTERNOON
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