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Foraging Behavior and Switching by the Grasshopper Sparrow Ammodramus Savannarum Searching for Multiple Prey in a Heterogeneous Environment

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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications in the Biological Sciences Papers in the Biological Sciences 4-1988 Foraging Behavior and Switching by the Grasshopper Sparrow Ammodramus savannarum Searching for Multiple Prey in a Heterogeneous Environment Anthony Joern University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/bioscifacpub Part of the Behavior and Ethology Commons, Ornithology Commons, and the Zoology Commons Joern, Anthony, "Foraging Behavior and Switching by the Grasshopper Sparrow Ammodramus savannarum Searching for Multiple Prey in a Heterogeneous Environment" (1988). Faculty Publications in the Biological Sciences. 266. https://digitalcommons.unl.edu/bioscifacpub/266 This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Joern in American Midland Naturalist (April 1988) 119(2). Copyright 1988, University of Notre Dame. Used by permission. The American Midland Naturalist PublishedQuarterly by The Universityof NotreDame, NotreDame, Indiana Vol. 119 April, 1988 No. 2 ForagingBehavior and Switchingby the Grasshopper SparrowAmmodramus savannarum Searching for Multiple Prey in a HeterogeneousEnvironment ANTHONY JOERN Schoolof Biological Sciences, University of Nebraska-Lincoln,Lincoln 68588 ABSTRACT: Switchingamong four alternative grasshopper species by thegrasshop- per sparrow(Ammodramus savannarum) through frequency-dependent predation was ex- amined.Grasshoppers were presented against a heterogeneousnatural background in an outdooraviary where total density was held constantbut relativedensity of each grasshopperspecies varied. Switching among prey, primarily between two of thefour species,and inter-birdvariability in switchingwas observed.Species-specific prefer- ences and interactionsamong the alternativegrasshopper species affectedthe final diet choiceas one grasshopperspecies was alwayspreferentially selected and another disregardedindependent of changesin relativedensity. Attributes of avian foraging behavior,including: (a) proportionof total time spenthandling prey, searching for preyor engagingin nonforagingactivity; (b) capturesuccess; (c) time requiredto catchprey (relative to eithertotal or searchtime), and (d) attackdistance did notvary accordingto the relativeabundance treatments.Results of thisstudy are relatedto quantitativeestimates of predationand relativedensities of grasshopperspecies in a naturalgrassland setting. INTRODUCTION Generalist predatorshave been shown to switchamong alternativeprey as the rela- tive densitiesvary (Hassell, 1978; Murdoch, 1969; Murdoch and Oaten, 1975). Switch- ing involves a frequency-dependentfunctional response where predators change from selectingthe predominantprey type to another as its relativedensity increases. The dynamics of switchingare contained in the functionalresponse of the individ- ual predator.Functional responses relate the predator attack rate to prey density(Has- sell, 1978). Mechanisms proposed to explain switchingbehavior include: foragingfor alternativeprey in differentpatches, the formationof search images by the predator,de- creases in handling time with experience, or increases in searching efficiencywithin patches (reviewed in Murdoch and Oaten, 1975). The firsttwo mechanisms have been studied in most detail. When predators forage for alternativeprey living in different patches and transittime among the patches is not negligible, increased foragingactivity in the patch with higher prey availabilitywill lead to the sigmoidal switchingresponse described by Oaten and Murdoch (1975). Predators may formsearch images where the psychological processes involved with locating and recognizing prey against the back- ground are altered depending on the relative abundances of the various prey types (Dawkins, 1971; Gendron and Staddon, 1983; Pietrewicz and Kamil, 1979, 1981). It is likelythat all of these mechanisms operate in natural foragingsituations. Switching by predators has importantimplications for many other ecological and evolutionaryprocesses. Frequency-dependentpredation within a population is affected by the degree of morphological (or behavioral) polymorphismamong individuals in a population and is expected when foragingon alternativeprey species as well (Clarke, 1962). Visually orientingpredators may select for morphological divergence of coexist- 225 226 THE AMERICAN MIDLAND NATURALIST 119(2) ing prey and thus determinewhich species can coexist (Clarke, 1962) and may explain some patternsof prey assemblies (Jeffriesand Lawton, 1985; Joern and Lawlor, 1981; Rand, 1967; Ricklefsand O'Rourke, 1975; Levin and Segal, 1982). In this manner,the number and taxonomic composition of coexisting prey species may be determined through switching among prey by a visually orienting predator. Other plausible predator-mediatedmechanisms involvingswitching may also affectorganization of prey assemblages (e.g., apparent competition,Holt, 1984; compensatorymortality, Connell, 1978). Most studies which have documented switchinghave employed two alternate prey species presentedto predators in relativelysimple environments(Murdoch and Oaten, 1975). The resultsof these studies must readily extrapolateto more complex and realis- tic situationsif these mechanisms are to be invoked to interpretmultiple-species assem- blies. Whether such extrapolationis justifiedis seldom examined. In the present study, I presented visually orienting grasshopper sparrows (Ammodramussavannarum) with a choice of four grasshopperspecies (Amphitornuscoloradus, Ageneotettix deorum, Cordillacris oc- cipitalisand Melanoplussanguinipes) and varied the density of each prey species. The background was heterogeneousand consisted of naturallygrowing vegetation. By add- ing both multiple prey and a heterogeneousbackground to the experiment,important insightsconcerning the role of switchingin structuringassemblies of insect prey may be gained. I asked the followingquestions: (1) Do components of foragingbehavior such as proportionof time spent searching (or in nonforagingactivity), search time per prey, probability of capturing observed prey and time spent handling prey, change in re- sponse to changes in relative densities of alternate prey? (2) Do grasshopper sparrows switchamong multiple prey in a frequency-dependentmanner when foragingin a het- erogeneous environment?(3) If switchingis observed, do preferencesby the predator exist or are there other synergisticinteractions among available prey which influence the final patternof diet choice by the predator? METHODS Experimentalsubjects. -Experimental subjects were chosen to be representativeof nat- urally occurringassemblages observed at a Nebraska sandhills studysite (Arapaho Prai- rie, Arthur Co.). A detailed descriptionof this site is found elsewhere (Barnes, 1980; Joern, 1982). Although additional grasshopperand bird species are found at these sites, the number of species used in the experimentswas chosen as a compromise between natural complexityand the need fora manageable number of species for experimenta- tion. Four grasshopper species (and mean body lengths) were used in the experiment: Ageneotettixdeorum (Scudder) (19.6 mm), Amphitornuscoloradus (Thomas) (24.1 mm), Cor- dillacrisoccipitalis (Thomas) (23 mm), and Melanoplussanguinipes (Fabr.) (24.7 mm). These species were chosen because of the availabilityof sufficientindividuals of these species to stock experimentsand the general similarityin size. None of these species ex- hibited extensiveintraspecific polymorphism. Only femaleswere used for the firstthree species and mostlyfemales were used forM. sanguinipes.Morphological comparisons be- tween the sexes of M. sanguinipesindicated no statisticallysignificant differences so both sexes were used when there was a shortage of females. Microhabitat use varied some- what among these species. Ageneotettixdeorum and C. occipitalistended to be found in more open areas while A. coloraduswas more likelyto use denser vegetationin addition to open areas; M. sanguinipeswas less restrictivein microhabitatuse than the other spe- cies. Morphologically,Amphitornus coloradus and Cordillacrisoccipitalis are superficiallymore similar,compared with any other species pair, and Ageneotettixdeorum is slightlysmaller than the other species. All species are typically common in Nebraska sandhills and nearby shortgrassprairie assemblages (Joern, 1982). Phenologically,all species overlap 1988 JOERN:FORAGING BEHAVIOR BY GRASSHOPPERSPARROW 227 extensivelyalthough populations of C. occipitalistend to disappear before the other spe- cies. Grasshopper sparrows (Ammodramussavannarum) are small (ca. 16 g), widespread grassland passerines (Smith, 1963; Robbins et al., 1966). Their diet consists largely of insects; grasshoppers(Acrididae) and other Orthoptera comprise the great majority of prey in the diet of grasshoppersparrows at Arapaho Prairie. Grasshopper sparrowsare numericallydominant among the avian insectivoresat Arapaho Prairie (M.E. Kaspari, pers. comm.) and greatlycontribute to the depression of grasshopper densities under natural conditions (Joern, 1986). Feeding trials in the laboratory have demonstrated that these birds readily eat each of the above grasshopperspecies (Kaspari,
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