THE FORAGING MOUNTAIN BEHAVIOR OF BLUEBIRDS With Emphasis on Sexual Foraging Differences BY HARRY W POWER Department of Biology, Livingston College, Rutgers University, New Brunswick, New Jersey 08903 ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1980 NO D.C UNION 28 Frontispiece: Plate I (upper left) Male Mountain Bluebird on top of his nest box carrying orthopteranprey for his young Plate II (upper right) Female Mountain Bluebird with orthopteran prey at entrance to nest box Note dull coloration of folded wings and tail Plate III (lower left) Female Mountain Bluebird with wingsand tail extended.Note the contrastingcolors of the flight surfacesand body Plate IV (lower right) West-facingview of the study area at Calvert, Montana, showingthe savanna-likeecotonebetweenDouglasFir forestand shortgrassprairie Note that trees in the savannaare concentratedon northeasterlyslopes,i.e., those facing toward the right and bottom THE FORAGING MOUNTAIN BEHAVIOR BLUEBIRDS With Emphasis on Sexual Foraging Differences OF ORNITHOLOGICAL MONOGRAPHS This series, publishedby the American Ornithologists'Union has been established for major papers too long for inclusion in the Union's journal, The Auk Publication has been made possible through the generosity of the late Mrs Carll Tucker and the Marcia Brady Tucker Foundation, Inc Correspondenceconcerningmanuscriptsfor publication in the series should be addressed to the Editor, Dr Mercedes S Foster, Department of Zoology, University of California, Berkeley, California 94720 Copies of Ornithological Monographs may be ordered from the Assistant to the Treasurer of the AOU, Glen E Woolfenden, Department of Biology, University of South Florida, Tampa, Florida 33620 (See price list on back and inside back cover.) OrnithologicalMonographs,No 28, x + 72 pp Editor of AOU Monographs, Mercedes S Foster Special Associate Editors of this issue, Jerry Downbower, Department of Zoology, Ohio State University, Columbus, and Douglass H Morse, Departmentof Zoology,Brown University,Providence, Rhode Island Author, Harry W Power, Department of Biology, Livingston College, Rutgers University, New Brunswick, New Jersey 08903 First received, 14 April 1976; resubmitted, 21 July 1978; accepted 24 February 1979;final revision completed, November 1979 Issued July 17, 1980 Price $8.50 prepaid ($7.50 to AOU members) Library of CongressCatalogueCard Number 80-67923 Printed by the Allen Press, Inc., Lawrence, Kansas 66044 Copyright ¸ by American Ornithologists'Union, 1980 iv THE FORAGING MOUNTAIN BEHAVIOR OF BLUEBIRDS With Emphasis on Sexual Foraging Differences BY HARRY W POWER Department of Biology,LivingstonCollege, Rutgers University, New Brunswick, New Jersey 08903 ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1980 NO D.C UNION 28 To Nelson G Hairston who taught me that there is no substitute for an experiment vi TABLE CHAFFER OF CONTENTS EVOLUTIONARY BASES OF SEXUAL FORAGING DIFFERENCES SEXUAL SELECTION INTERSEXUAL FOOD COMPETITION DIVISION OF LABOR AND FORAGING EFFICIENCY INTERSEXUAL EXPLOITATION INTERACTIONS AMONG FACTORS UNDERLYING SEXUAL FORAGING DIFFERENCES FACTORS INHIBITING SEXUAL FORAGING DIFFERENCES CHAFFER STUDY SPECIES,STUDY AREA, AND GENERAL METHODS 1 5 CHOICE OF THE MOUNTAIN BLUEBIRD STUDY AREA GENERAL METHODS CHAFFER NoN-EXPERIMENTAL OBSERVATIONS SEXUAL FORAGING DIFFERENCES OUTSIDE THE NESTLING STAGES SEXUAL FORAGING DIFFERENCES DURING THE NESTLING STAGES PRELIMINARY CONSIDERATIONS OF SEXUAL FORAGING DIFFERENCES POSSIBLE CAUSES OF SEXUAL FORAGING DIFFERENCES YEARLY CHANGES IN FORAGING BEHAVIOR 12 12 14 15 18 22 CHAFFER DESIGN OF THE WORK LOAD EXPERIMENTS EXPERIMENT I EXPERIMENT II 25 25 26 EXPERIMENT LIMITED III SIMPLE SIZE METHOD OF PRESENTATION OF EXPERIMENTAL RESULTS CHAFFER EXPERIMENT I: BROOD SIZE AND WORK LOAD PARENTAL CARE 27 28 28 31 31 PROBLEM BIRDS 31 FORAGING BEHAVIOR HIGH COST SCORES 31 34 CONCLUSION 35 CHAFFER EXPERIMENT II: WORK LOAD FACTORS AND CAUSES OF SEXUAL FORAGING DIFFERENCES PARENTAL CARE FORAGING BEHAVIOR 36 37 38 HIGH COST SCORE CONCLUSIONS 42 43 CHAFFER EX?ERIMENT III: MATE LOSS PARENTAL CARE 47 47 FORAGING 47 MATE LOSS BEHAVIOR 48 CHAFFER RELATIONSHIP OF FORAGING BEHAVIOR TO WORK LOAD FORAGING RATES DISTANCE TO NEST DURING FORAGING vii 53 53 53 TYPE OF VEGETATION CHAPTER ADDITIONAL HYPOTHESES AND EXPERIMENTS ROLE OF TERRITORY CONSORTS AND THE FORAGING BEHAVIOR OF UNPAIRED BIRDS REPRODUCTIVE SUCCESS AND EXPERIMENTAL TREATMENT MALE EXPLOITATION IMPORTANCE OF VEGETATION HEIGHT AND THE HABITAT ALTERATION EXPERIMENT FORAGING LEVELS AND TACTICS CHAPTER 10 CONCLUSIONS AND SUMMARY ACKNOWLEDGMENTS LITERATURE CITED 56 56 59 60 60 61 63 66 69 70 LIST Frontispiece: Plates I-IV 54 OF FIGURES ii Figure Areas near- and away-from-large-perchesin a corner of territory 28 in 1970 Diagrammaticrepresentationof passefinewing shapes Completeblock designof work load ExperimentII High cost scoresin work load Experiment I Wiring diagram of statisticalcomparisonsof the use of aerial stagingpointsby groupsin work load Experiment II Wiring diagram of statisticalcomparisonsof the use of taller vegetationby groupsin work load Experiment II High cost score in relation to several factors in work load Experiment II Effects of mate loss on use of costly behaviors Intensity of foraging effort in relation to distance from the 41 nest site 54 14 19 27 34 42 44 50 10 Foragingtacticsin relationto searching costper foragingpattern, and vegetationheightof foraginglocation 61 11 The effect of scattered trees on use of the area away-fromlarge-perches 64 12 How differences in quality of territories produce variation amongbirdsin use of foraginglevels 65 viii LIST Table Bluebird OF TABLES mensural data Number of foragingpatterns per minute of foraging Proximity of foraging birds to large perches during nonexperimental observations of various stages of the nesting cycle Use of foraging patems during non-experimentalobservations of various stagesof the nestingcycle Use of staging points during non-experimental observations of various stagesof the nestingcycle Visits to the nest box during non-experimental observations Visits to the nest box in Experiment I Fecal sac removals in Experiment I Proximity of foragingbirds to large perchesin Experiment I 10 Use of foragingpatterns in Experiment I 11 Use of stagingpoints in Experiment I 12 Use of vegetationof different heightsin Experiment I 13 Visits to the nest box in Experiment II 14 Fecal sac removals in Experiment II 15 Proximity of foragingbirds to large perchesin Experiment II _ 16 Use of foragingpatterns in Experiment II 17 Use of stagingpoints in Experiment II 18 Use of vegetationof different heightsin Experiment II 19 Visits to the nest box in Experiment III 20 Fecal sac removals in Experiment III 21 Proximity of foraging birds to large perches in Experiment III 22 23 24 25 Use of foragingpatterns in Experiment III Use of stagingpoints in Experiment III Use of vegetationof different heightsin Experiment III Levels and tactics of foragingbehavior ix 13 13 15 16 20 32 32 32 32 33 33 37 38 39 39 40 43 48 48 49 49 51 51 63 CHAPTER EVOLUTIONARY BASES OF SEXUAL FORAGING DIFFERENCES Sexual dimorphismis the set of all differencesbetween males and females in a singlespecies.Sexual foragingdifferencesare an aspectof this dimorphismthat can be definedas differencesin the ways males and females exploit prey In the study of sexualforagingdifferences,evolutionaryevents shouldbe considered alongwith ecologicalonesbecauseevolutionaryand ecologicaleventsare equally dynamic(Ford 1965;Hutchinson1965)and feed back into and conditioneach other continuously.Thus, there can be no profound understandingof present ecologicalinteractionsin the absenceof an understanding of their historicalcontext and the direction in which selectionis currently driving them This report is primarily concernedwith sexualdifferencesin the behavioral pattern of foragingin Mountain Bluebirds(Sialia currucoides).It details observations on foragingbehavior from several breedingseasons,and the results of experimentsdesignedto critically evaluate alternativehypothesesof the proximate and ultimate causesof sexualforagingdifferences.I begin with a review of different possibleevolutionary origins of sexual foraging differences: (1) sexual selection,(2) intersexualcompetition,(3) divisionof labor, (4) foragingefficiency, and (5) intersexualexploitation.I considerhow to separatethese originsin practice and discussfactors inhibiting their productionof sexualforagingdifferences SEXUAL SELECTION Darwin's(1871)theoryof sexualselectionoftencanaccountfor sexualforaging differences whether other factors are involved or not Sexual selection can be defined as non-randomdifferential reproductionof individualsin the contexts of (1) within-sexescompetition,and (2) between-sexeschoice Within-sexescompetition is exemplifiedby male-malecombat over accessto fertile females, and between-sexeschoice by females mating only with males possessingparticular secondarysexual structures,such as bright plumes Sexual selection can result in sexual foraging differencesby producing morphological, physiological,or behavioral differencespreadaptingmales and females for different types of prey exploitation; e.g., the favoring of large males in male-male combat could secondarilyresult in males taking larger prey than females becausemales would be better equipped to capture larger prey The mating systemof a speciescan influencethe expressionof sexualforagingdifferences by affectingthe degreeto which sexual selectionproducessexual dimorphism Polygynousand polyandrousspecieswill usuallybe more sexuallydimorphic than monogamousones becauseof more intense within-sexescompetitionand between-sexeschoice (Verner and Wilson 1969) The greater the dimorphismin structure, function, and behavior, the more divergent will be the foragingtactics that males and females are preadapted to perform Sexualselectionis alwaysat the root of sexualforagingdifferences,irrespective of the presenceof other factors, becauseit is the only form of selectionacting on the sexesper se, andthusthe only form of selectionproducingincipientsexual foraging differences It may seem that other forms of selection could produce sexualforagingdifferences by themselves becausethey favorthe sexesbeing different However, in the absenceof preadaptionstending to make males con- 60 ORNITHOLOGICAL MONOGRAPHS NO 28 byunpaired females relative tOpaired females, i.e.,howcoulda few'consort males have generatedmore intersexual competition than the original male mates? Nevertheless, the arrival of consorts does show that testing for intersexual competitionmust be performed quickly after mate loss Observation of foraging behavior from mate loss to fledging could become increasingly biased by the arrival of more and more consorts until it would no longer be correct to assume an absence of intersexual competition on the territories of mate-deprived birds By taking data on only the first two days after mate collection, I believe I controlled for this potential bias and adequately tested the hypothesis of intersexual competition REPRODUCTIVE SUCCESS AND EXPERIMENTAL TREATMENT In another publication I will detail reproductive successat Calvert for 197077 Here I only mention the relation between experimental treatment and reproductive success.Birds in all groups successfullyfledged all their young except for those that fledgedprematurely after banding Eight nestshad 18 prematurely fledgingyoung, all of which probably died Thirty nests successfullyfledged all young Young from 34 nestsin which fledgingdates were known within two days of occurrence fledged in an average time of 20 days without differences between experimental groups Fledging weights are unknown because behavioral observations precluded weighing nestlings Except for the possibility of differential fledging weights, neither brood size nor mate loss seemedto affect reproductive successin 1972, the year of the experiments If birds adjust clutch size to the probable number of young they can fledge (Lack 1968), brood size would not affect fledging successin these experiments because large broods were of normal size But mate loss might reduce success by halving the work force anticipatedfor rearing young That mate loss apparently did not reduce successin 1972introducesthe possibility that both parents are not normally necessaryto rear full clutches, but this possibility was belied by mass reproductivefailure in 1974-76 when not even both parentscouldprevent nestling starvation Thus, the success of 1972 does not indicate broods are smaller than those that normally can be reared MALE EXPLOITATION The second work load experiment provided data consistentwith an interpretation of male coercion but insufficient to demand acceptance of that interpretation As statedin the Introduction, male exploitationimpliesboth male coercion (with beneficial results to the male) and damage to females The second experiment included no direct measure of damage Therefore, I sought an independent means of testing the hypothesis of male exploitation At the end of the 1976 breeding season, I attached nest boxes to specially constructed observation booths that allowed me to weigh birds as they entered their nests (Power, unpubl data) In 1977 I weighed 14 pair partners a total of 52 times during the nestling stage to determine whether they showed progressive weight changes I hypothesized that male exploitation would be indicated by progressive female weight loss(a measureof damage)coincidentwith either no male weight lossor a slower rate of male weight loss I partitioned the data into early, middle, and late weights POWER: MOUNTAIN BLUEBIRD FORAGING PROXIMITY TO 61 LARGE NEAR PERCHES AWAY 'r TALL z SHORT LOW SEARCHING HIGH COST PERCHHOVER- FORAGE FORAGE FORAGING PATTERN Fig 10 Foragingtacticsin relation to searchingcost.perforagingpattern, and vegetationheight of foraging location When tactic does not provide enoughfood to satisfy their work load, birds increase their searchingcost per pattern (tactic 2) before they increase their use of tall vegetation (tactic 3) corresponding to days 1-4, 5-9, and 10-20 of the nestling period Data were analyzed by the Mann-Whitney U Test I found no evidence of difference in male and female weights or of progressive weight change by either sex (p > 0.15 in all cases) Thus, because the second work load experiment failed to reveal unequivocal evidence of male coercion and becausethe subsequentweighings of birds during the nestling stage did not produce evidence of damage to females, I reject the hypothesisof male exploitation with regard to Mountain Bluebird foraging behavior I admit, however, that weight loss is not the only possible indicator of damage nor, perhaps, even the best IMPORTANCE HABITAT OF VEGETATION ALTERATION HEIGHT AND THE EXPERIMENT In the second experiment, only unpaired birds with large broods intensively soughtprey in tall vegetation, and they did this by more frequently perch-foraging (Tables 17, 19, Fig 10) Birds with lighter work loads used a variety of behaviors including more frequent hover-foragingin short vegetation and bare places awayfrom-large-perches This suggeststhat bluebirds would rather hover-forage in short(or no) vegetation thanperch-forage in tall vegetation eventhoughhover- 62 ORNITHOLOGICAL MONOGRAPHS NO 28 foragingis severaltimes as expensivecalorically as perch-foraging.This, in turn, suggeststhat vegetation height constrainsforagingbehavior more than the caloric cost of foraging.I found this suggestioncuriousbecausemost modelsof foraging tactics (reviewed in Pyke et al 1977)emphasizethe importanceof caloric cost Therefore, in 1976 I performed an experiment to determine whether vegetation height really does constrainforagingbehavior more than caloric cost or whether birds were respondingto different prey types or densitiesin places of different vegetationheight or to some unknown factor(s) Prior to manipulationat each territory in the nestling stage, I noted the geographic distribution of foraging effort for the two hours following sunrise (the periodof mostrapid foraging)and collectedarthropodswith a sweepnet at those placesthat I intendedto use as control and experimentalplots I manipulatedthe habitat by making available a fresh area of short vegetation; I cut a m wide swath through tall vegetation outward from the nest box for a distance of 10 m along one side of either a preexistingfence or one that I built for the experiment This procedure allowed birds to choose between short or tall vegetation within 10 m of their nests while foraging from fences and, thereby, eliminated the need for birds to employ foraging patterns of different caloric cost (hover-foragingvs perch-foraging)while huntingin short and tall vegetation.After manipulation,I again noted the geographicdistribution of foraging effort for the two hours following sunrise and collected arthropods by sweep netting in adjacent control (uncut) and experimental (cut) plots Analysis of the arthropod collection showed that prey types and abundance were greater in tall than short vegetation and decreased when tall vegetation was cut short In particular, the caterpillarsand grasshoppersthat closedcircuit television had shown to be so important in the nestling diet were less than 25% as commonin short as in tall vegetation(N = 29 plots; U = 24; p < 0.001) Despite these results of insect sampling,the behavioral results of the habitat alteration experiment were consistentwith the hypothesisthat vegetationheight is a critical and more important factor in foragingthan the caloric cost of foraging When given a choice between short and tall vegetation on opposite sidesof the samefence and within 10 m of their nests, birds always foraged more frequently (usually exclusively) in short vegetation (N = 7; X = 0; p = 0.008; Sign Test) This showsthat birds prefer shortvegetationdespiteits far lower prey abundance When only tall vegetationwas available adjacent to fences within 10 m of nests (before cutting) and birds had to fly somedistance(up to 700 m in one case) to reach short vegetation, birds still preferred to forage in short vegetation even when it was not adjacentto a fence or other large perch (X" = 66; p < 0.001; McNemar Test) This showsthat birds hunt in short vegetation even when it requires the expenditureof many more caloriesin searchingand transportation;it implies that the distribution of short vegetation has a greater influence on foraging behavior than the caloric cost of foraging Given the reasonableassumptionthat bluebird foraging behavior is adaptive, bluebirds'preferencefor huntingin bare placesand shortvegetationimpliesthat there is an advantageto foragingin those placesthat surmountstheir lower prey abundancesrelative to tall vegetation Bluebirds probably can achieve a higher rate of capturefor themselvesand a higherrate of food deliveryto their nestlings when huntingin placesof short (or no) vegetationbecauseinsectsare probably lessconcealed.Moreover, birds on the groundin shortvegetationand bare places POWER: MOUNTAIN BLUEBIRD FORAGING TABLE 63 25 LEVELS AND TACTICS OF FORAGING BEHAVIOR Level I II III Tacticsper level Total costper leveP Low I + 1+ 2+ High Highest Foraging tactics: I Characteristicforagingpattern of tactica Proximity to large perches Vegetation height PF HF PF + (H• NLP ShoN AFLP ShoN NLP Tall Searching cost per tactic5 Low High Low • Total cost increaseswith the sequenceof levels becauseeach successivelevel addsa new tactic with its own searchingcostto the tactic(s) of the precedinglevel(s) and its (their) attendantsearchingcost(s) • PF = perch-foraging; HF = hover-foraging; Ha = hawking;NLP = near-large-perches; AFLP = away-from-large-perches • All foragingpatternsandtheir attendantstagingpointsare usedby birdson all levels,but only I patternis mostcharacteristic of the tactic initiated with each level Althoughbirdson level hawk afterflyinginsectsmorefrequentlythan birdson otherlevels,hawkingis an unimportantdiagnostic character of tactic compared to perch-foraging • In calories are probablylessimpededby vegetationthat is often wet and sticky, more likely to avoid ambushby concealedpredators,and better able to view their larger surroundings,includingtheir nests,mates,unwantedconspecifics,and approaching predators Bluebirds are not the only bird species foraging in grasslandwhose behavior is constrained by vegetation structure Wiens (1969, 1973) and Cody (1968)foundthat grasslandphysiognomywas moreimportantin the distribution, abundance,and resourceuse of grasslandbirds than either geographiclocation or floristiccomposition.Wiens' (1973)observations on the effect of grazingon speciesdistributionare particularly relevant He found that some species(e.g., Horned Larks, Eremophilaalpestris) were especiallycommonin heavily grazed places,while others(e.g., WesternMeadowlarks,Sturnellaneglecta)were more commonin ungrazedplaces.It would appearthat grazingis importantin bluebird resourceuse becauseit generatesplaces of short vegetation,keeps them short, and speedsthe successionof woody plantsthat provide largeperchesand potential sitesin which woodpeckersmay excavatenestholes.It might, therefore,be true that any impetusfor the cattle industryof the West that promotesgrazing is also an impetusfor Mountain Bluebird populationsprovided that it does not result in grazingso heavy that grasslandis destroyed FORAGING LEVELS AND TACTICS Non-experimentalobservationsrevealeda three step sequenceof foragingbehavior in response to increasing work load I find it convenient to call each of these steps "levels" (definedin Table 25) A level consistsof a foragingtactic initiatedwith it plus the foragingtactic(s)of the precedinglevel Thus, Level II succeedsLevel I by addition of tactic to tactic 1, and Level III succeedsLevel II by addition of tactic to tactics and Tactics are the clustersof foraging behaviorsand locationsmost characteristicof each level; e.g., tactic consists of perch-foragingnear-large-perches into short vegetationor bare places An important feature of the sequenceof foraginglevels is its additive nature 64 ORNITHOLOGICAL MONOGRAPHS NO 28 I00' 80 J 60 4.0 20 ò ò 4'0 ò 80 1:50 TREES Fig 11 The effect of scatteredtrees on use of the area away-from-large-perches(AFLP) Each point representsthe use of the area away-from-large-perches by a female whoseterritorial boundaries were known in 1970 Bluebirdsrespondedto light work loadsby usingthe Level I behaviorof perchforagingnear-large-perches.They respondedto heavier loads by also usingthe Level II behavior of hover-foragingaway-from-large-perches.The work load experimentsalso showed the existenceof Level I and II responsesand tied them to vegetationheight: whether near or away-from-large-perches,birds usually foraged on bare substrateor in short vegetation The work load experiments also revealed a Level III response stimulated by very heavy work loads, i.e., more frequent perch-foraginginto tall vegetation near-large-perches,and to a much lesser extent, more frequent hawking after flying insects The other four foragingpatternsand their attendantstagingpointswere used by birds on all three levels with no particular relation to work load They were related to short-livedopportunities(e.g., flycatchingafter temporarily abundant flying insects) and to habitat peculiarities of territories (e.g., frequent ground foragingon the fallow fields of territory 43, and perch-foraging/hover-foraging by birds with utility lines on their territories) Vegetation height constrains movement between levels more than does the caloric cost of foraging (Fig 10) Thus, birds preferred to hunt in short rather than tall vegetation even though hunting in short vegetationaway-from-largeperchesis most often doneby hover-foraging,a pattern that is far more expensive calorically than the perch-foragingusually employed near-large-perches.However, this rule does not imply that the caloric cost of hunting is in itself unimportant, only that it is less important than vegetation height The intrinsic importance of caloric cost is implied by the relationshipof Levels I and II; birds POWER: MOUNTAIN BLUEBIRD FORAGING 65 J TERRITORY QUAI w ITY w / d •n' Inl 14.2 J w Ill" n Ow w •w Lg w• iLrr 0rr ir TIME ,-) Fig 12 How differences in quality of territories produce variation among birds in use of foraging levels Levels have one to three foragingtactics each (see Table 25) Birds go from one level to the next when the tactics of the preceding level not provide enough food to satisfy their work load A-D are the individualpathsof responseof theoreticalbirds A-D All birds have their own territories where they are confrontedby the same work load T Ois the origin of an interval of responseto work load TR is the time at which A-C reach a goal of reproductionsuchas fledgingof their young Tr is the time at which D fails in its attempt to reach its goal Territory quality is fixed for birds A-D at values a-d, respectively; it is determined by the number of large perches, the area of short (or no) vegetation, and the biomassof catchable, preferred prey per territory All scalingis purely ordinal A's territory is so good it can achieve the rate of food delivery required by its work load (and thus reach its goal) by usingonly the tactic of Level I B and C have progressivelypoorer territories and therefore must use the tactics of Levels II and Ill, respectively, before they can reach their goals D's territory is so poor it cannot reach its goal despite use of the tactics of all levels hunt in shortvegetationby perch-foragingnear-large-perches before they hoverforage away-from-large-perches There is great individual variation in movement between levels If there were not, only three points would appear in Figure 7D relating work load to high cost score(1 point for each level) becausebirds' behavior would be completelydeterminedby their work load However, examinationof the resultsof this study repeatedly showsthat the habitat idiosyncraciesof territories strongly influence foraging behavior For example, in Chapter I showed how habitat affected the distanceat which some birds foragedfrom their nestsduring the work load experiments of 1972 Likewise, Figure 11 shows how the number of scattered treesper territory is reciprocallyrelatedto useof the area away-from-large-perches Realizingthe influenceof habitatfeatureson foragingbehavior,onecanexpect birds to initiate the new tactics of successivelyhigherforaginglevels in accordance with the opportunitiesafforded by the habitat peculiaritiesof their individual territories This principle is illustrated in Figure 12 which shows that birds with the samework load deploythe tacticsof the three foraginglevelsat different times (or not at all) becausethey possessterritoriesof different qualities 66 ORNITHOLOGICAL CHAPTER MONOGRAPHS NO 28 10 CONCLUSIONS AND SUMMARY Sexual foraging differences are differences between the sexes in prey exploitation They may originate from sexual selection, intersexual competition, division of labor, differences in foraging efficiency, or intersexual exploitation Although experimentation is probably necessary in most cases to determine the origins of sexual foraging differences, sexual selection always is involved irrespective of other forces because it is the only form of selection acting on the sexes per se Thus, it is the only form of selection producing incipient sexual foraging differences Other forms of selection can only exaggerate pre-existing sexual differences Despite the importance of sexual selection, I detected in the literature a tendency to overlook its significance even when its presence was recognized In order to bring greater rigor to the study of the evolution and ecologicalexpression of sexual foraging differences than had been previously employed, I observed and experimented on Mountain Bluebirds in the Little Belt Mountains, Cascade County, Montana The pattern of foraging behavior throughout the breeding season was observed in 1970 and 1971 Three complete block design experiments based on the inferences of earlier observations were run in 1972 In these work load experiments, brood size and number of adults per territory were independently varied to provide a mix of potential factors of work load and causes of sexual foraging differences Supplementary observations on foraging behavior and follow-up experiments testing hypotheses derived from the results of the work load experiments were performed from 1974 through 1977 The major conclusionsof this study are: (1) Both sexes are governed in their foraging behavior by certain common factors Mountain Bluebirds stage several distinct foraging patterns from a variety of locations and physical objects in order to exploit a range of insect prey in several grassland-forestecotone microhabitats Their foraging behavior is driven by their need to feed themselves and their dependents but constrained by (a) the distribution and amount of short vegetation and bare places, (b) the distribution and number of large perches, (c) the caloric cost of foraging, and (d) the distanceof foraging sites to the nest (a) The 1972 work load and 1976 habitat alteration experiments showed that birds prefer foraging in short vegetation (less than tarsal length in height) and bare places to foraging in tall vegetation (more than tarsal length in height) They also showedthat the distributionof short vegetationand bare placesmore strongly influencesforaging location and behavior than factors (b-d) Bluebirds foraged as if they sought to exploit all prey in short vegetation and bare places before exploiting any prey in tall vegetation The advantage of foraging in short (or no) vegetation is not fully understoodbecause the habitat alteration experiment also revealed that prey density is generally lower in short than tall vegetation However, elements of that advantageprobably include a higher rate of harvest due to poorer concealment of insect prey, less impeded movement, easier ability to detect predators, and more opportunity to monitor the nest and mate while foraging (b) Observation and experimentation showed that birds prefer to search for prey from large perches, such as scattered trees and fences, as opposedto other POWER: MOUNTAIN BLUEBIRD FORAGING 67 stagingpoints,especiallywhenthatallowsharvesting preyfromshortvegetation and bareplaces.Searchingfrom largeperchespermitsbirdsto use severalforagingpatterns,primarilythe caloricallyinexpensive perch-foraging that in many waystypifiesbluebirdforagingbehavior.Searching fromlargeperchesalsoallows birds to observetheir mates, nests,and approachingconspecificsand predators, all at a distance.Thus, searchingfrom large perchesshouldreducethe risks of predationof adultsandnestcontents,lossor degradation of territory,andcuckoldry or mate loss (c) The patternof useof aerialstagingpointsthroughthe breedingseasonand the work load experimentsimpliedthat bluebirdsuse caloricallyexpensiveforagingbehaviorsonly whencompelledto by hungeror work load Use of aerial stagingpointsin hover-foraging andhawkingallowsprey to be takenfrom short vegetationand bare placesaway-from-large-perches: (d) The firstwork loadexperimentshowedthat birdspreferto forageas close to their nestsas the distributionof short vegetation,bare places, large perches, and probablyprey, allows.Birds almostalwaysforagedwithin 500 m of their nests,and usuallywithin 100 m This preferencefor foragingcloseto the nest seemsto be constantas it was not influencedby work load in the 1972 experiments.Foragingcloseto the nestreducesthe time and costof transporting food to the nest and returning to foraging sites (2) Both sexesuse the sameforaging tacticsin successivelevels of foraging efforts Bluebirdssuccessively usethreelevelsof foragingeffortin responseto greaterhungerandwork loadsby addingforagingtacticsratherthanby substitutingthem Addingtacticsenablesbirdsto use moreand moreof the foraging locations around their nests instead of roaming farther and farther from them Foragingneartheirnestsallowsbirdsto minimizetheirtransportation costswhile maximizingtheirrate of food deliveryto their youngandto quicklyperceiveand interceptconspecifics andpredatorsapproaching theirnests.The threelevelsare: Level I Birds satisfytheir individualfood requirementsand respondto light work loads(e.g., smallbroods)by usingtactic 1: perch-foraginginto shortvegetation and bare places near-large-perches Level II Birds employLevel II tacticswhen Level I doesnot allow them to satisfytheir hungerand/orfood demandsof their dependents This occurswith pairedbirdsrearinglargebroodsand mate-lessbirdsrearingsmallbroods.Level II adds tactic to tactic Tactic consistsof frequent hover-foragingaway- from-large-perches into shortvegetationandbareplaces.Thus,birdsusingLevel II tacticsexploit virtually all the placesof short(or no) vegetationnear their nests Level III Birds use Level III tactics when even the tactics of Level II are insufficientto satisfytheir needs.This occursat very highwork loadsand, thus, was exhibitedonly by mate-lessbirds rearinglarge broodsin the work load experiments of 1972.Level III addstactic3 to tactics1 and2 Tactic3 consists of frequentperch-foraging into tall vegetationnear-large-perches and, to a lesser extent, of hawkingafter flying insects.Thus, birdsusingLevel III tacticsintensively exploit all the foraginglocationsnear their nestsexcepttall vegetation away-from-large-perches Level III producesgreater total energy expenditure than Levels I and II becauseLevel III incorporatesthoseprior levels, but tactic requiresless energythan tactic becausemost searchingis donefrom large 68 ORNITHOLOGICAL MONOGRAPHS NO 28 perchesrather than from hovering positions.The hesitancyof birds to use tactic suggeststhat it producesgreaterrisksthan tactic 2, probablythe risks of ambush by concealedpredatorsand reduced ability to see other predators, conspecifics, and the nest at a distance Use of each level is dependentnot only upon the food demandexperiencedby the individual bird but also upon the features of its territory Thus, birds with a high number of large perchesnear their nests switch to Level II later than birds with the same work load but fewer large perches near their nests Birds with much short vegetation and many ba.re places near their nests switch to Level III later than birds with the same work load but less short vegetation and fewer bare places'near their nests Birds use those foraging patterns, staging points, and foraging locations not listed above at all levels of foraging effort depending upon opportunity (e.g., flycatching on a day of high abundance of flying insects) or the idiosyncratic featuresof territories (e.g., frequent ground-foragingon a territory with few large perches) (3) Number of youngper attendant adult is the work loadfactor to which both sexesrespond in switching between the three levels of foraging effort Number of young per attendant adult is calculatedby dividing the number of young in a nest by the number of adultsattendingthat nest The greaterthe numberof young per attendant adult, the higher the level of foraging effort, other things being equal That number of young per attendant adult is the important factor in work load implies that: (a) Mate loss affects foraging behavior by doubling the number of young per attendant adult (b) Rate of food delivery is crucial in fledgingnestlings.The higher the rate, the higher the probability of fledging (c) The carrying capacity of each territory is less for nestlingsthan for adults (d) Bluebird territories are formed to encompassenoughreadily available prey to ensurea sufficientrate of food delivery to nestlingseven under adversehunting conditions Consequently,food will appear to be superabundantunder good hunting conditions.Other specieswith Type A territories probably also form them to meet the minimum food requirements of their young under adverse conditions Three other possible work load factors (number of birds per territory, number of adults per territory, and number of young per nest) were tested but found not to affect foraging behavior (4) Males have a higher thresholdfor use of at least the second of the three levels of foraging effort This sexualdifferencein responsethresholdgenerates an important quantitativedifference in resourceuse during the nestlingstages At that time females make far greater use of the area away-from-large-perches than males, particularly by hover-foraginginto short vegetation and bare places, thus generatingan incomplete micro-allopatry of the sexes on most territories Thus, the cause(s)of sexual difference in responsethreshold is the cause(s)of sexualdifferencein foragingbehavior Several possiblecauseswere investigated: (a) Males are not lessable to hover-forageor exploit the area awa•y-from-large perches than females Capture success,morphologicalmeasurements,and the POWER: MOUNTAIN BLUEBIRD FORAGING 69 second and third work load experiments showed that males can use the same tacticsas femaleswith the samesuccessin capturingprey and fledgingyoung (b) Males and females not compete to obtain food for themselves when sexualforagingdifferencesoccur Unpaired malesincurredmore costsin foraging for youngthan paired males in the secondwork load experiment, directly contrary to the intersexual competition hypothesis (c) Males probably not exploit females The hypothesisof male exploitation was only equivocally supported in the second work load experiment, i.e., three of four expectedresultsoccurredbut onewascontraryto expectation Moreover, that hypothesis was further weakened in application to bluebirds by a later experiment that showed no evidence of weight loss by females despite their more frequent use of costly behaviors (d) The hypothesisthat males' brighter color exposesthem to a higher risk of predation while using high cost foraging behaviors and, therefore, that males are more reluctant to use high cost behaviors was supported but not fully corroborated The hypothesis was supportedby the occurrence of a sexual difference in response threshold in spite of the proven equal foraging abilities of males and females; it was particularly supportedby the sex role reversal of mate-lessmales in the work load experiments.These males successfullyforaged like females The hypothesis was not corroborated, however, because no predation of males occurred when they foraged like females in the work load experiments But, predators were not directly built into the design of the work load experiments Mountain Bluebird sexual foraging differencesmay reflect a peculiar kind of division of labor The first criterion for inferring that selection for division of labor is the cause of bluebird sexual foraging differences appears to be met, i.e., the differences appear to be based on sexual dimorphism in plumage The third criterion is certainly met, i.e., there is a common effort by males and females to feed nestlings when important sexual foraging differences occur The second criterion is not so obviously met; males and females not appear to differ in efficiencyof use of different foragingtactics in the senseof caloric efficiency,but they may differ in the sense of efficiency of avoiding predation Thus, insofar as bluebird sexual foraging differences are a division of labor, they are of a more indirect kind than such obvious reflections of sexual selection as sexual differ- encesin foragingheight in warblers or sexual differencesin foraging substratein woodpeckers, which are unencumberedby any difficult-to-measureintermediate variableslike differential predation (5) Sexualforaging differencesbesidesthose occurring when adultsfeed young in the nest not produce different uses of resourcesby males and females and are divisions of labor directly reflecting sexual selection This is exemplified by females foraging more rapidly than males during females' brief periods off the nest in the incubationstages,reflectingtheir care of eggs ACKNOWLEDGMENTS This study was made possibleonly by the help of many people, all of whom cannot be thanked here Most of the research reported in this monograph was performed for the preparation of my Ph.D dissertationin Zoology at The Uni- 70 ORNITHOLOGICAL MONOGRAPHS NO 28 versity of Michigan Therefore, I thank the members of my doctoral committee, R D Alexander, G F Estabrook, D R McCullough, and R B Payne R D Alexander chaired that committee for all its years, inculcating the method of evolutionaryreasoningpioneeredby Darwin; I doubt that anythingI_ever will be truly free of RDA's influence, a most fortunate fate! N G Hairston served on my committee for years before leaving Ann Arbor to take an endowed chair at the University of North Carolina; I owe him a special thanks becauseI would never have developeda practical experimentalorientation without his prodding Almost as important as the members of my doctoral committee were my fellow graduate students,particularly E J Brady in Zoology and D K Fox in Statistics L L Wolf, my postdoctoraladviser, provided stimulatingscientificconversation during the long Syracuse, N Y winters of 1974-76 when an earlier draft of this report was written J W Hardy, D H Morse, J Downhower, and M S Foster made many useful suggestionshelpful in the revision of this manuscript Fieldwork in the Little Belt Mountains of Montana was successful because of the help of many people, including R Person, D Pings, my sister Teresa, and my parents Dr and Mrs Harry W Power C Doner, P Hanson, and G Van Vliet made important practical contributions as field assistantsin the later breeding seasons Mr and Mrs W V Thoren and R S Hoffmann influenced my development as a field biologist I especiallythank the Montana ranchingfamilies who allowed me to work on their land, helped me to survive several emergencies, and generally looked out for me and the bluebirds:the J and R Gasvodas,the R Umphres, L Anderson, and R Bond Particularly in the postdoctoral years of this study, Jim Gasvoda proved to be a helpful and informative friend This study was supportedat various times by grantsfrom the Horace H Rackham School of Graduate Studies of The University of Michigan (1971-72), the Syracuse University Research Council (1975), the Rutgers University Research Council (•977), and the National ScienceFoundation(GB-25986 to N G Hairston, 1972; BNS76-08840 to H W Power, 1976; and BNS77-09299 to H W Power, 1977) LITERATURE CITED ALEXANDER,R D 1974 The evolution of social behavior Ann Rev Ecol Syst 5:325-383 ARDREY, R 1966 The Territorial Imperative Atheneurn, New York ASHMOIrE,N P 1963 The regulationof numbersof tropical oceanic birds Ibis 103b:458-476 BATEMAN, A J 1948 Intrasexual selection in Drosophilla Heredity 2:349-368 BocI{, C E 1970 The ecologyand behavior of the Lewis Woodpecker(Asyndesmuslewis) Univ California Publ Zool 92:1-100 BROWN,J L 1969 Territorial behaviorand populationregulationin birds Wilson Bull 81:293-329 CODV, M L 1968 On the methods of resource division in grasslandbird communities Am Nat 102:107-147 DARWIN, C R 1871 The descent of man, and selection in relation to sex John Murray, London DARWIN, C R 1975 Natural selection Edited by R C Stauffer from an 1856-58 manuscript CambridgeUniversity Press, Cambridge, England DAWSON, W R., AND F C EVANS 1960 Relation of growth and development to temperature regulationin nestlingVesper Sparrows Condor 62:329-340 FoRBusi•, E.H 1929.Birds ofMassachusetts andother NewEngland states PartIII Mass Dept Agric., Boston FORD, E B 1965 Ecological genetics, second edition Methuen and Co., London FRETWEI•I•,S 1972 Populationsin a seasonalenvironment.Monogr Popul Biol 5:1-217 POWER: MOUNTAIN BLUEBIRD FORAGING 71 GRIFFIN, D R., AND C D HOPKINS 1974 Soundsaudibleto migratingbirds Anim Behav 22:672678 HAMILTON, T H 1961.The adaptivesignificance of intraspecific trendsof variationin winglength and body size among bird species.Evolution 15:180-195 HAMILTON, W D 1964 The geneticalevolution of social behavior J Theoret Biol 7:1-52 HAMILTON,W D 1970 Selfishand spitefulbehaviourin an evolutionarymodel Nature 228:12181220 HARTSHORNE, J M 1962.Behavior of theEastern Bluebird at thenest.LivingBird1:1•1-149 HOWARD,H E 1935 Territory and food Brit Birds 28:285-287 HUTCHINSON,G E 1965 The ecological theater and the evolutionary play Yale Univ Press, New Haven, CN KILHAM, L 1970 Feedingbehaviorof Downy Woodpeckers,part Preferencefor paper birches and sexual differences Auk 87:544-556 KRUUK, H 1972 The Spotted Hyena: a study of predation and social behavior Univ Chicago Press, Chicago, IL K0CHLER,A W 1964 Potential natural vegetationof the conterminousUnited States Spec Publ No 36 Am Geog Soc N.Y LACK, D 1968 Ecologicaladaptationsfor breedingin birds Chapmanand Hall, London LASIEWSKI,R C 1963 Oxygen consumptionof torpid, resting, active, and flying hummingbirds Physiol Zool 36:122-140 LIGON, J D 1968 Observationson Strickland's Woodpecker, Dendrocoposstricklandi Condor 70:83-84 LIGON, J D 1973 Foragingbehavior of the White-headedWoodpeckerin Idaho Auk 90:862-869 MILLER, W 1970 Factors influencingthe statusof Easternand Mountain Bluebirdsin southwestern Manitoba Blue Jay 29:38-46 MORSE,D H 1968 A quantitativestudy of foragingof male and female spruce-woodswarblers Ecology 49:779-784 PINKOWSKI,B.C 1975 Growth and developmentof Eastern Bluebirds Bird-Banding46:273-289 POWER,H W 1966 Biologyof the Mountain Bluebirdin Montana Condor68:351-371 POWER,H W 1974 The Mountain Bluebird: sex and the evolution of foragingbehavior Unpubl Ph.D dissert Univ of Michigan, Ann Arbor POWER,H W 1975 MountainBluebirds:experimentalevidenceagainstaltruism Science189:142143 POWER,H W 1976a Altruism in Mountain Bluebirds?(Reply) Science 191:809-810 POWER,H W 1976b Altruism: methodologicaland definitionalissues.(Reply) Science 194:562563 POWER,H W 1980 Predationon femalesat the nestcavity and male escortingin MountainBluebirds Wilson Bull., in press POWER,H W., AND C G P DONER 1980 Experimentson cuckoldry in the Mountain Bluebird Am Nat., in press PYKE, G H., H R PULLIAM, AND E L CHARNOV 1977 Optimal foraging: a selectivereview of theory and tests Quart Rev Biol 52:137-154 RAND, A L 1952 Secondarysexualcharacteristicsand ecologicalcompetition.Fieldiana-Zoology 34:65-70 RICKLEFS,R 1968 Patternsof growth in birds Ibis 110:419-451 ROBINS,J D 1971 Differential niche utilization in a grasslandsparrow Ecology 52:1065-1070 ROBINSON,M H 1969 Defensesagainstvisually huntingpredators Evol Biol 3:225-259 ROYAMA,T 1966 Factors governingfeedingrate, food requirementand brood size of nestlingGreat Tits Parus major Ibis 108:313-347 ROYAMA,T 1970 Factors governingthe hunting behavior and selectionof food by the Great Tit (Parus major L.) JoAnim Ecol 39:619-668 SCHALLER, G B 1972 The Serengetilion: a studyof predator-preyrelations.Univ ChicagoPress, Chicago, IL SELANDER,R K 1966 Sexualdimorphismand differentialniche utilization in birds Condor68:113151 SELANDER,R K 1972 Sexual selectionand dimorphismin birds Pp 180-230 in B Campbell(ed.), Sexual selectionand the descentof man, 1871-1971 Aldine, Chicago, IL SIEGEL,S 1956 Nonparametric statisticsfor the behavioral sciences.McGraw-Hill, New York 72 ORNITHOLOGICAL MONOGRAPHS NO 28 So•:^i•,R R., ANDF J RO•L•: 1969 Biometry:the principlesand practiceof statisticsin biological research W H Freeman, San Francisco, CA STOREI•,R W 1966 Sexual dimorphismand food habits in three North American Accipiters Auk 83:423-436 T•NBERGEN,N 1957 The functions of territory Bird Study 4:14-27 TmvEl•S, R L 1971 The evolution of reciprocal altruism Quart Rev Biol 46:35-57 Ta•vEl•S, R L 1972 Parental investment and sexual selection.Pp 136-179 in B Campbell (ed.), Sexual selectionand the descentof man, 1871-1971 Aldine, Chicago, IL TuLI•OC•:, G 1979 On the adaptive significanceof territoriality: comment Am Nat 113:772-775 V^• V^LE•, L 1965 Morphologicalvariation and width of ecologicalniche Am Nat 99:377-390 VAUGHAN, T A 1959 Functional morphology of three bats: Eumops, Myotis, Macrotus Univ Kansas Publ Mus Nat Hist 12:1-153 VEa•EI•, J 1977 On the adaptive significanceof territoriality Am Nat 111:769-775 VERNER,J., AND M F WILLSON 1969 Mating systems,sexual dimorphism,and the role of male North Americanpassefinebirds in the nestingcycle Ornithol Monogr 9:1-76 W^I•^CE, G J 1959 The plight of the bluebird in Michigan Wilson Bull 71:192-193 W^I•^CE, R A 1974 Ecologicaland socialimplicationsof sexual dimorphismin five melanerpine woodpeckers Condor 76:238-248 W•E•S, J A 1969 An approach to the study of ecologicalrelationshipsamong grasslandbirds Ornithol Monogr 8:1-93 W•E•s, J A 1973 Pattern and processin grasslandbird communities.Ecol Monogr 43:237-270 WILLIAMS, G C 1966 Adaptation and natural selection.Princeton Univ Press, Princeton, NJ W•L•^MSO•, P 1971 Feedingecologyof the Red-eyedVireo (Vireo olivaceous)and associated foliage-gleaningbirds Ecol Monogr 41:129-152 W•so•, E O 1975 Sociobiology.Harvard Univ Press,Cambridge, MA WOLF, L L., AND F R HA•NSWORTH 1971 Time and energy budgetsof territorial hummingbirds Ecology 52:980-988 WOLF, L L., AND J S WOLF 1976 Mating systemand reproductivebiologyof Malachite Sunbirds Condor 78:27-39 WắE-EDw^IDS, V C 1962 Animal dispersionin relation to social behavior Oliver and Boyd, London No 21 SocialOrganizationand Behaviorof the Acorn Woodpecker in Central Coastal California, by Michael H MacRoberts and Barbara R MacRoberts 1976 $7.50 ($6.00 to AOU members) No 22 MaintenanceBehaviorand Communication in the Brown Pelican, by Ralph W Schreiber 1977 Price $6.50 ($5.00 to AOU members) No 23 Species Relationships in the Avian GenusAimophila,by Larry L Wolf 1977.Price $12.00 ($10.50 to AOU members) No 24 Land Bird Communities of Grand Bahama Island: The Structure and Dynamicsof an Avifauna, by John T Emlen xi + 129 pp., 38 text figures,appendix.1977.Price $9.00 ($8.00 to AOU members) No 25 Systematics of SmallerAsianNightBirdsBasedon Voice,by JoeT Marshall.viii + 58 pp., frontispiece,15plates,phonodiscsupplement,appendicesI-III 1978 Price $7.00 ($6.00 to AOU members) No 26 Ecologyand Behaviorof the Prairie Warbler Dendroicadiscolor,by Val Nolan, Jr xx + 595pp., colorfrontispiece,42 text figures,8 appendices 1978 Price $29.50 No 27 EcologyandEvolutionof Lek MatingBehaviorin theLong-tailed Hermit Hummingbird, by F Gary Stiles and Larry L Wolf viii + 78 pp., 26 text figures 1979 Price $8.50 ($7.50 to AOU members) No 28 The ForagingBehaviorof MountainBluebirdswith Emphasison Sexual ForagingDifferences,by Harry W Power x + 72 pp., color frontispiece, 12 text figures 1980 Price $8.50 ($7.50 to AOU members) Like all otherAOU publications, Ornithological Monographsareshippedprepaid.Make checkspayableto "The AmericanOrnithologists'Union." 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Tampa, Florida 33620 (See price list on back and inside back cover.) OrnithologicalMonographs,No 28, x + 72 pp Editor of AOU Monographs, Mercedes S Foster Special Associate Editors of this issue,... away-from-large-perches Number (%) No second broods in 1974-75 14 ORNITHOLOGICAL MONOGRAPHS NO 28 Fig Areas near- and away-from-large-perches in a corner of territory 28 in 1970.Circled X is the nest box; dots are... sexesbeing different However, in the absenceof preadaptionstending to make males con- ORNITHOLOGICAL MONOGRAPHS NO 28 sistently different in one way and females in another, differences of a particular