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Citation Details Murphy, Michael T., "The mpI act of Weather on Kingbird Foraging Behavior" (1987). Biology Faculty Publications and Presentations. Paper 82. http://pdxscholar.library.pdx.edu/bio_fac/82
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THE IMPACT OF WEATHER ON KINGBIRD FORAGING BEHAVIOR'
MICHAELT. MURPHY2 Museum of NaturalHistory, The Universityof Kansas, Lawrence,KS 66045
Abstract. Foragingdata on EasternKingbirds (Tyrannus tyrannus) were collected during the early breedingseason in eastern Kansas to test the hypothesis that foragingrate and other aspects of foragingbehavior vary with weather. Foragingcharacteristics of five ad- ditional kingbirdspecies were also examined to assess Fitzpatrick's(1980) generalization that kingbirds(Tyrannus spp.) are aerial hawking specialists. In EasternKingbirds, total foragingrate was independentof air temperature,cloud cover, wind speed, and time of day, but the rate of aerialhawking varied directlywith air temperatureand inverselywith cloud cover (both P < 0.05). Effectsof the two variableswere additive.The percentageof foraging movementsthat were aerialhawks also increasedwith temperatureand declinedwith cloud cover,and, hover-gleaningand perch-to-groundsallying were observed mainly during cloudy weather. Sally (i.e., foragingflight) distance correlateddirectly with perch height and air temperature,and largeinsects were captured almost exclusivelyin long upwardor horizontal flights. I interpretthese data to indicate that foragingbehavior and the captureof large, flyinginsects depends on weatherbecause of how it affectsthe activityof insectprey. Foraging data on kingbirdssupport Fitzpatrick's generalization, but the relativeuse of aerialhawking varies considerablyamong species. Resident TropicalKingbirds (T. melancholicus)are the most specialized foragers,whereas the migrant and widely distributedEastern Kingbird appearsto be the most generalized.Certain habitats also appearto favor the use of particular foragingmethods (e.g., outwardstriking in grasslands,and perch-to-groundsallying in drier, open habitats). Keywords: Aerialhawking;foraging behavior; prey capture; Tyrannus tyrannus; weather.
INTRODUCTION mate weather conditions should be a primary Avian ecologists have quantified tyrant flycatch- determinant of insect flight activity, food avail- er foraging behavior for many purposes ranging ability and therefore foraging behavior of fly- from the study of morphology (Karr and James catchers. However, with only rare exception (Da- 1975, Traylor and Fitzpatrick 1982), resource vies 1977, Foreman 1978), all flycatcher foraging use and community structure (Beaver and Bald- studies to date have explicitly avoided collecting win 1975, Eckhardt 1979, Blancher and Rob- data during poor weather, and as a result, there ertson 1984, Sherry 1984), phylogenetic rela- is a critical shortage of information on how ty- tionships (Fitzpatrick 1980), to analyses of rannid foraging varies with weather. migratory behavior (Verbeek 1975a). Analyses This gap in knowledge is significant for at least of stomach contents have shown that diets are two reasons. From the perspective of under- diverse, but the main flycatcher prey are the Hy- standing the evolution of flycatcher reproductive menoptera, Coleoptera, Orthoptera, Hemiptera, patterns it is essential to determine what factors and Diptera (Bent 1942, Dick and Rising 1965, influence foraging success. In the other large group Hespenheide 1971, Beaver and Baldwin 1975, of aerial foragers, swallows and swifts, foraging Sherry 1984). An unexplored, yet important area and reproductive success vary with weather be- of flycatcher foraging ecology concerns the im- cause of the strong dependence between insect pact of weather on foraging patterns. In view of flight activity and meteorological conditions. Low the dependence between body temperature and air temperatures, precipitation, high cloud cover, capacity for flight in most insects (Heinrich 1981, and wind all reduce the availability of flying in- Kingsolver 1983a), it seems logical that proxi- sects (Bryant 1973; Hespenheide 1975; Davies 1977; Visscher and Seeley 1982; Kingsolver 1983a, 1983b; Jones 1987). The accepted view IReceived 11 1986. Final 13 September acceptance is that life histories of that the May 1987. species exploit 2 Presentaddress: Department of Life Sciences, In- "aerial plankton" have been shaped largely by diana State University, TerreHaute, IN 47809. high, but short-term temporal and spatial vari-
[721] 722 MICHAELT. MURPHY
ation in insect abundance(Lack and Lack 1951; Most observations were of individuals in pas- Bryant1973; O'Connor 1977, 1979;Jones 1987). tures and recentlyabandoned fields where small Likewise,hawkers such as flycatchersdepend on shrubs,fence posts, and fence lines werethe main insect movement for prey detection and capture perch substrates. (e.g., Davies 1977), and Murphy(1983) suggest- Terminologyfollows Fitzpatrick(1980). Sal- ed that weatherhas also been an integralfactor lying refers to the approach flight preceding a in the evolution of temperate-zonebreeding fly- prey capture attempt. All foragingmoves were catcherlife histories.Poor weathernegatively af- initiated by a flight from a perch, and included fects flycatcher reproductive success (Davies aerial hawking(direct flights to capturea single, 1977, O'Connor and Morgan 1982, Murphy flyinginsect), outwardstriking (prey are snatched 1983), yet except for Davies' (1977) study, the from vegetationfollowing a rapid, direct flight), link betweenweather, foraging, and preycapture outward hover-gleaning(prey are located and is not establishedin flycatchers(but see Mahan picked from vegetation following a brief hover- 1964). ing flight),and perch-to-ground sallying(prey are Second, weather's impact on flycatcherfor- picked from the groundafter a direct flight). aging behavioris relevantfor interspecificstud- Foraging birds were observed from 250 m ies of either niche relationships (Beaver and usingbinoculars under all weatherconditions ex- Baldwin 1975, Verbeek 1975b, Holmes et al. cept rain. Observations were recorded vocally 1978), or assessment of foraging specialization with a hand-held tape recorderand later were. (Fitzpatrick1980). Measuringniche overlap us- transcribed.Upon sighting a foragingbird (in- ing only "fair-weather"studies may be mislead- dicated by previous foragingattempts, and alert ing since diet and foragingbehavior are expected posture),I waited for it to initiate a new foraging to diversify when food becomes scarce (Pyke et sequenceand then recordedtime of day, and all al. 1977), which is most likely to occur when furtherperch heights, foraging movements, perch poor weather depresses insect activity. Since changes without a prey capture attempt being competition is most likely when food is least made, sally distances, and flight trajectories abundant,quantification of communityrelation- (above, below or level with the perch).Horizon- ships and foragingplasticity in flycatchersshould tal flightsoccurred within about 150 of the plane not be restrictedto fair weather. parallelwith the groundsurface. Foraging period In this report I quantify foragingpatterns of duration was also timed to the nearest second EasternKingbirds (Tyrannus tyrannus) in east- with a stop watch. Observationswere continued ern Kansas to addressthe issues raised above. I for as long as a birdwas in sightup to a maximum will test predictions that short-termchanges in of 15 min. I ended all observationsat 15 min to weather are the primary determinantsof vari- avoid overrepresentingforaging under any spe- ability in kingbirdforaging behavior and success cific environmentalcondition (e.g., a local insect (Murphy 1983). I will also assess Fitzpatrick's emergence). (1980) classificationof the genus Tyrannusas Within the study area fences and fence posts aerialhawking specialists by evaluatingtheir de- wereabundant, were of uniformheight and spac- gree of behavioral flexibility through (1) intra- ing, and were frequently used by kingbirds. I and interspecificcomparisons of kingbird for- therefore estimated sally distances and perch aging, and (2) by examining the consistency of heights to the nearestfoot and half-foot, respec- EasternKingbird foraging as weatherchanges. tively, by referringto fence lines. I later con- verted distance to meters. Except in the case of METHODS I was unaole to determinewhether lcanc •lISeCLS I collecteddata on EasternKingbirds in May and capture attempts were successful. However, early June of 1981, 1982, and 1983 in Douglas kingbirdshad to kill largeinsects by beatingthem County,eastern Kansas (site descriptionin Mur- againsta perch,which allowedme to recordtheir phy 1986).Foraging behaviors were recorded op- capture. portunisticallywhile kingbirdnests were located At the end of each foragingperiod, I recorded early in the season. I limited my observationsto air temperatureusing a Schulteis fast reading pre-eggstage birds to controlfor seasonalchanges thermometer(shielded from sun and wind) and in food abundance(Murphy 1986) and for vari- wind speed at breastheight (1.4 m) with a Sims ation in energyneeds due to breedingactivities. hand-held cup anemometer. The stall speed of KINGBIRDFORAGING 723 the anemometerwas about 8 km/hr, hence, I was diversity and equitabilityof use of differentfor- able to measurewinds that exceeded 8 km/ and is definedas where only agingcategories, H'/H'max, hr. The wind measurementsno doubt failed to H' = - pilog pi, and pi is the proportionof each precisely reflect the actual convective microen- foragingbehavior. H'max is the maximum diver- vironment of the bird, but I believe they accu- sity possible given n foragingcategories, where rately portrayedgeneral environmental condi- pi = 1/n for all i. J varies between 0 and 1, the tions. Cloud cover was estimated by classifying two extreme values representinga specialistus- sky conditions into five categoriesof percentage ing one behavior (0) and a generalist using all cover (i.e., 1 = 0 to 20%, 2 = 21 to 40%, . . . , foragingmethods equally (1). 5 = 81 to 100%). PREDICTIONSAND ANALYSIS RESULTS GENERALFORAGING CHARACTERISTICS Insect flightactivity increaseswith increasingair temperature,clearing sky conditions, and de- BreedingEastern Kingbirds foraged as "sit-and- creasingwind speeds (see referencesabove). As- wait" predatorssince 85%(n = 477) of searches sumingenergy intake is sensitive to insect activ- endingin a foragingflight. Of these, aerialhawk- ity, I expected the following patternsto emerge. ing accounted for 50.0% of all foragingmoves, First, total foragingand hawkingrate would in- followedby outwardstrikes (39.9%), hover-gleans crease with air temperature,and decline with (8.1%),and perch-to-groundsallies (2.0%). Other increasingcloud cover and wind speed. Second, foragingbehaviors observed infrequently at oth- assumingperches were selected to maximize en- er times (e.g., during rain or the emergence of counters with prey, I predicted an increase in aquaticinsects) included gleaningof lepidopter- perchheight with increasingair temperature,de- an larvae from trees by upwardstriking, surface creasingcloud cover, and wind speed.Third, Leck gleaning from water, and multiple captures of (1971) proposedthat long sally distancesin king- aerialprey by hoveringduring a single flight.The birdsreflect good foragingconditions. Long flights latter behavior, describedalso by Blancherand presumablyindicate greateravailability and se- Robertson (1985), involved many consecutive lectivity of prey. I thereforeexpected sally dis- hover-gleanswhile the birdmoved in a slow flight tance to increase with air temperature,and de- over grass covered fields. Frugivoryon mulber- clinewith increasingcloud cover and windspeeds. ries (Morus rubra; Stapanian 1982) was also Optimal foragingtheory (Pyke et al. 1977) pre- common later in the season when fruit was in- dicts also that diet diversity, and in this case cluded in both adult and nestling diets (M. T. foragingdiversity, will increase as the expected Murphy,pers. observ.). rate of energy intake declines. Hence, kingbirds Perch height averaged 1.3 m (SD = 1.53, n = should switch from being foragingspecialists to 49; range= 0.3 to 9.1 m) and mean sally distance generalistsas weatherdeteriorates. was 2.9 m (SD = 1.58 m, n = 49; range = 0.9 I excluded all foraging bouts in which birds to 7.6 m). The median sally distance of 2.1 m were observed for <3 min. For the remainderI was shorterthan the 3.7 m reportedby Via (1979). calculated foraging rate (foraging moves/min), Kingbirds returnedto the same perch used to hawkingrate (number of aerialhawks/min), mean initiate prey captures 53.5% of the time (n = and median perchheight, sally distance,and the 325), similar to Via's reported value (48.8%; percentageof foragingmovements as aerialhawks t-test for percentages,t = 0.7, P >> 0.05). How- (minimum of three foraging movements re- ever, sally distanceinfluenced the probabilityof quired for inclusion). Percentageswere arcsine perchreuse since mediandistance for individuals transformed.Univariate comparisonsof behav- reusing percheswas 1.5 m, compared to 2.5 m ior to weatherwere made using least-squareslin- for new perches(median test, G = 15.6, df = 1, ear regression and correlation analysis. Multi- P < 0.01; Zar 1974). Sally distance also varied variaterelationships were further examined using with flighttrajectory. Downward, horizontal, and step-wise multiple regression(BMDP2R; Dixon upward flights accounted for about 59, 32, and 1981). I also groupedbirds accordingto weather 9% (n = 376) of all attempted prey captures, conditionsand calculatedforaging diversity with respectively. Median (and mean) sally distance respect to weather using a standardmeasure, J for level (2.3 m [2.9 m]) and upwardflights (2.4 (Eckhardt1979). J takes into account both the m [3.8 m]) did not differ significantly(median 724 MICHAELT. MURPHY
.013 r2= r2 .106 2.8 1.6 PP<.05 w 0 2.1 4 0 o S0.8 0 z 1.4 - *
0.7 * *
0.0 0.0 I 2 3 4 5 CLOUD COVER ABSENT TOTAL ,- FIGURE2. Hawkingrate (number ofhawks/min) vs. cloudcover for Eastern Kingbirds from Douglas Coun- 0.8 ? ty, Kansas. 0-
0.4
0.0 regressionindicated that effects of temperature 12 15 1s 21 242i and cloud cover were additive, and that despite AIR TEMPERATURE(0C) uncontrolledvariation in absolute insect abun- dance among years (Murphy 1986), recent past FIGURE1. Foragingrate (number of moves/min)as and differences birds in a functionof ambientair temperaturefor (a) all for- weather, among hunger agingtactics, and, (b) just hawking behaviors (number level and body condition, the two-variablemod- of aerialhawks/min) for Eastern Kingbirds from Doug- el of air temperatureand cloud cover explained las County,Kansas. a quarterof the variation in hawking rate (r = 0.501, df = 2, 37, P < 0.01). The partialcorre- lation of both and cloud cover were test, G = 0.47, df = 1), but the median of their temperature < when effects of the other combined distributions was longer than for significant(P 0.05) variablewere controlled time downward flights (1.9 m [2.3 m]; G = 5.80, statistically.Adding of to the model little additional df= 1, P < 0.025). Largeinsects were also more day explained variation = df = P < likely to be capturedin upward(6.0% of all for- (r 0.537, 3, 36, 0.01). and level aging moves) (3.4%)flights compared PERCHHEIGHT AND SALLYDISTANCE to downwardflights (0.4%)(x2 = 6.96, df = 2, P < 0.025). Perch height did not vary with air temperature (r = 0.238), wind speed(r = -0.134), cloud cover FORAGINGRATE (r = -0.050), or time of day (r = -0.092). Sally Foragingrates varied widely (X = 1.2 moves/ distancealso variedindependently of wind speed min, SD = 0.76, n = 40), but essentiallynone of (r = -0.049), cloud cover (r = -0.042), and time the variabilitywas relatedto air temperature(r = of day (r = -0.171), but did tend to vary directly 0.110; Fig. la), wind speed (r = -0.033), cloud with temperature(r = 0.250, P = 0.12, n = 40). cover (r = 0.027) or time of day (r = -0.048). As is common in flycatchingbirds (Pinkowski Furtherrestriction of the analysis to timed ob- 1977, Greig-Smith 1983, Moreno 1984), perch = servations >5 min, or min did not affectthe height and sally distance were correlated (r results. _7 0.441, n = 40, P < 0.01), and even higher so if I examined hawking rate separatelybecause birds that perched in trees were excluded (r = kingbirds(Tyrannus spp.) preferentiallycapture 0.623, n = 36, P < 0.001). I thus reexamined insects by aerial hawking (Fitzpatrick 1980). sallydistance in relationto temperatureand perch Hawkingrate increased with air temperature(r = height using multiple regression analysis. The 0.395, P < 0.02; Fig. lb) and decreased with two-variablemodel was significantfor birds that cloud cover (r = -0.325, P < 0.05; Fig. 2). did not use trees as perches (r = 0.683, df = 2, Hawkingrate did not vary with wind speed (r = 33, P < 0.01), but the partialcorrelation of sally 0.072), but tended to decline as the day pro- distanceand temperaturewas not (r = 0.250). A gressed(r = -0.235, P = 0.15). Stepwisemultiple plot of residual sally distance (effects of perch KINGBIRD FORAGING 725