Annual variation in foraging ecology of prothonotary warblers during the breeding season

-We studied foraging ecology of Prothonotary Warblers (Protonotaria citrea) along the Tennessee River in west-central Tennessee during the breeding seasons of 1984-1987. We analyzed seven foraging variables to determine if this population exhibited annual variation in foraging behavior. Based on nearly 3,000 foraging maneuvers, most variables showed significant interyear variation during the four prenestling and three nestling periods we studied. This interyear variation probably was due to proximate, environmental cues-such as distribution and abundance of arthropods-which, in turn, were influenced by local weather conditions. Researchers should consider the consequences of combining foraging behavior data collected in different years, because resolution of ecological trends may be sacrificed by considering only general patterns of foraging ecology and not the dynamics of those activities. In addition, because of annual variability, foraging data collected in only one year, regardless of the number of observations gathered, may not provide an accurate concept of the foraging ecology in insectivorous birds. Received 29 September 1988, accepted 16 August 1989. DURING the past 20 years, quantitative study of foraging behavior has emerged as one of the major focuses of avian ecologists. This activity has led to the development and testing of theories of optimal foraging, niche partitioning, and community ecology. Typically, foraging data are collected on a study area for several years. These data then are combined across years to produce an overall pattern of species' foraging ecology (e.g. Morse 1968, Holmes et al. 1979, Sabo 1980). Annual variation in foraging behavior, however, generally has been ignored. Recently, there has been concern that, by searching for ecological generalities based upon species' averages across multiple years and areas, information relating environmental factors to behavior is lost (e.g. Wiens 1983, Martin 1987, Wiens et al. 1987). We analyzed the variability in a local population's behavior when exposed to different environmental conditions across consecutive breeding seasons. To date, few indepth analyses (e.g. Landres 1980, Wagner 1981) of annual variation in species' foraging ecology have been reported, and no analyses of this type 'Present address: U.S. Fish and Wildlife Service, North Carolina Cooperative Fish and Wildlife Research Unit, Box 7617, North Carolina State University, Raleigh, North Carolina 27695 USA. have been made on data gathered over >2 yr. Substantial populational (Morse 1971, D. R. Petit et al. 1990), interseasonal (Parnell 1969, Conner 1981), intraseasonal (Williamson 1971, Sherry 1979, L. Petit et al. 1990), and diurnal (Holmes et al. 1978) variability exists in foraging ecology for a number of terrestrial land birds. Consequently it is important to assess the extent of annual behavioral plasticity within a local population. Documentation of the sources of variation in a species' behavior can provide insight into the proximate factors that control those activities. STUDY AREA AND METHODS Our study area along the Tennessee River in Benton and Humphreys counties of west-central Tennessee has been described in detail elsewhere (Petit et al. 1987, Petit 1989). Details of the area, its vegetation, our methods, and the analytical techniques that we used are given in the preceding paper (1990, Auk 107: 133-145). Interyear variation in foraging behavior could have occurred because different environmental conditions existed each year and the warblers simply responded to those changes. This idea was tested indirectly by calculating the magnitude and direction of the deviation of each year from the overall (3 or 4 year) average for all foraging variables. All observations were used to construct these deviations. Because the categories for a given variable are not independent, 146 The Auk 107: 146-152. January 1990 This content downloaded from 157.55. 39.58 on Tue, 11 Oct 2016 04:56:55 UTC All use subject to http://about.jstor. org/terms January 1990] Annual Variation in Foraging Ecology 147 we chose for analysis only one category per variable (except substrate), that one in highest overall frequency. For the variable substrate, we chose the 4 most commonly used substrates (fallen branch, willow, maple, and buttonbush) which totaled ca. 50% of all substrates used in each period. Male and female deviations for each period were calculated separately. Thus, for each sex and for each of the 4 substrate types, there were 4 deviations (=4 yr) for the prenestling period (n = 16 total pairs of deviations) and 3 deviations (=3 yr) for the nestling period (n = 12 pairs). For the other 6 variables, there were 24 pairs of deviations (6 variables, 4 yr) for the prenestling period and 18 pairs of deviations (3 yr) for the nestling period. If proximate environmental cues were important, both sexes should alter their foraging behavior in a similar fashion. In that case, a strong positive correlation (Pearson's r) between male and female yearly deviations would be expected.