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The Auk 111(4):962-969, 1994
CLINTON K. MILLER? 3 RICHARD L. KNIGHT, t LOWELL C. MCEWEN, t AND T. LUKE GEORGE2 •Departmentof Fisheryand WildlifeBiology,Colorado StateUniversity,Fort Collins,Colorado 80523,USA;and 2Department of Wildlife,HumboltStateUniversity, Arcata,California95521,USA
ABSTRACT.--Grasshopper populations on the Delta Agricultural Project area in interior Alaska exhibit a strongbiennial periodicity. This phenomenonallowed us to study dietary, reproductive,and numerical responsesof nesting SavannahSparrows(Passerculus sandwichensis)to fluctuationsin prey abundance.Grasshopperdensitiesexceeded25/m 2 in 1990but were lessthan 1/m2 in 1991.In 1990,61%of the identifiablefood itemsbroughtto the nest were grasshoppers.In 1991, Lepidopteralarvae and Diptera were the most common items broughtto the nestsand no grasshopper deliverieswere observed.Fledgingsuccess was97% in 1990and 96%in 1991.At leastone nestlingper nestsuccessfully fledgedeachyear. Mean clutchsize wassignificantlyhigher during the high grasshopper densitiesin 1990.Reduced growth measurements of nestlingswere obtainedin 1991comparedto 1990.Effectson growth
appearedto be greaterearlyin development,ratherthanpriorto fledging.SavannahSparrow densitiesdid not differ between the two years.Although a positive responseto high grasshopper densitieswas observed,the magnitude of the responsewas diminutive compared
with the largedifferencein prey abundancebetweenyears.This suggests that factorsother than food limit reproductive output in this population of SavannahSparrows.Received13 May 1993,accepted 2 July1993.
PREDATORS EXHIBITCHANGES in reproduction, diet, and density in responseto changesin prey density. Numerical responsesoften take the form of a density-dependentrelationship of a predatorcorrespondingto changesin prey density. Predatorsalso respondto changesin prey
In the interior of Alaska, grasshopperpopulations (predominantly Melanoplussanguinipes) fluctuate biennially with populations during
Cherryvale Road, Boulder, Colorado80303, USA.
row density either through immigration into
high yearsreachingdensitiesof morethan 25/ m2; densities in low years are less than 1/m 2
(1990U.S. Dep. Agricultureinternal report by abundanceby switching their diet (Solomon Sluss and Franklin). This natural biennial fluc1949). Modifications in clutch size and changes tuation allowed us to investigatewhether nestsandwichin survival or growth ratesof nestlingsindicate ing SavannahSparrows(Passerculus a reproductiveresponse.Lack (1947, 1954) de- ensis)showed dietary, reproductive, and nuveloped a theoreticalmodel relating food abun- mericalresponsesduring two breedingseasons, dance to reproductive successin bird popula- one with high grasshoppernumbersand one tions. Since then a number of studies have exwith low grasshoppernumbers.We anticipated in the high grassamined whether reproductive successcan be a switch from grasshoppers alteredby manipulatingfoodabundance(Mur- hopper year of 1990 to alternativefoodsin the phy and Haukioja 1986, Martin 1987, Boutin low grasshopperyear of 1991. In addition, we 1990, Simonsand Martin 1990). Many authors expecteda between-yearreduction in feeding became have documentedthe responseof birds (partic- rates to nestlingsas the grasshoppers ularly Passeriformes) to naturalchangesin food unavailable. We predicted SavannahSparrows availability(e.g.Zach and Falls1975,Anderson would show a positive reproductive response 1977, Sealy 1980, Mart and Raitt 1983, Strehl to high grasshoppernumbersin 1990 by inand White 1986). creasingclutchsize,producinglargernestlings and fledging more nestlingsper nest than in 3 Presentaddress:City of Boulder,Departmentof the low grasshopperyear of 1991. Finally, we OpenSpace/RealEstate,OperationsCenter,66 South anticipateda changein adult SavannahSpar962
Sparrow Responses toChanging Food
the 1990 breeding population or recruitment into the 1991 population. METHODS
Our study was conductedduring the summers(25 May-25 July) of 1990 (high grasshopperyear) and
1991(low grasshopper year) on the Delta Agriculture Project,Alaska.The Delta Agriculture Project,covering approximately44,500 ha south of the Tanana River,extendsfor approximately150km eastof Delta Junction,Alaska (64ø00'N,145ø20'W).The entire agricultureprojectis surroundedby spruceforest,making this population of SavannahSparrowsrelatively isolatedfrom other SavannahSparrowpopulations in the region. The Delta Agriculture Projectwas begun in the late 1970sfor farming small grains,and it is a mosaicof different-agedfields cleared from the surrounding forest of black spruce (Piceamariana), white spruce (P. glauca),and aspen (Populustremuloides).Cereals(mainly barley), grassseed,and grass hay are the primary crops grown in the area. Approximately 20% (8,900 ha) of the available land is currently in production.The remainder is idle or in government set-asideprograms. Two sites,each comprisingapproximately150 ha, were selectedfor study.Vegetationof both sitesconsistedof grasses(Calamagrostis spp., Agropyronspp., Festucaspp.) and forbs (Epilobium spp.), interspersed with willow (Salixspp.) and aspenwindrows. SavannahSparrownestswere locatedby dragging a heavy rope over the grassand flushing adultsfrom nests.To facilitaterelocatingnests,a small flag on a 0.5-m-high wire was placed 5 m north of each nest. Nests were visited daily until the first egg hatched (day 0), and then revisited four and sevendayslater. Individual nestlingswere marked with Liquid Paper on their
claws for later identification.
A clutch was
consideredcompleteif the clutchsizewasunchanged for two consecutivevisits. Hatching successwas calculated as the number of unhatched eggsin the nest on day four divided by the completeclutch size. At eachposthatchvisit the number of nestlingsand unhatchedeggswere recordedand growth parameters were measured.Nestling masswas measuredto the nearest 0.1 g using a 30-g Pesola spring scale suspended in a transparent cylindrical plastic tube to minimize wind disturbance.Tarsuslength and length of the ninth primary were measuredfor all nestlings to the nearest 0.01 mm using dial calipers. Mean growth ratesfor broodswere calculatedby subtracting the mean value of each growth variable (ninth primary, tarsus,and mass)on day 4 from the mean value of each variable on day 7, and then dividing by the number of minutesthat elapsedbetweenmeasurements.
This value was then converted
gained per day. Only nestsdiscoveredbefore hatching were used in analyses.A nest was considered
successful if at leastone nestlingfledgedfrom it. The
number of nestlingsremaining in the nest on the secondvisit (day 7) was assumedto be the number of birdsthat successfully fledged.Nestlingstypically fledgedon day 8 or 9. Fledgingsuccess (percentof nestlingssurvivinguntil fledging)was determined by dividing the numberof nestlingson day 7 by the numberof nestlingson day 4. Blowflylarvae(Protocalliphora spp.)were found in nestlingsat 14 nests(4 [15%of all nests]in 1990 and 10 [30%]in 1991). Parasitizednestlingshad reducedgrowth ratesand were not includedin the growth analyses(C. K. Miller unpubl. data). Dietary informationwas collectedat 11 nestsin eachof the two years.Blindswere erected5 to 7 m from eachneston day 3 posthatch,and observations were madeon day 4 posthatch.Eachnestwasviewed continuouslyfor 2.0 to 2.5 h followingthe firstfeeding visit after the observerentered the blind. We monitored21 of the 22 nestsduring the first half of the daylightcycle(0400to 1200AlaskaStandardTime lAST]).Feedingrate did not changewith time of day in a populationof SavannahSparrowsin NovaScotia (Stoboand McLaren 1975). Prey were identified to majortaxawith the aid of binocularsand a spotting scope.This techniqueprobablybiasedsamplestowardslargearthropods. The lengthof foragingbouts wasdeterminedby dividingthe amountof timespent away from the nest by the number of feeding trips made to the nest during the observationperiod. Birdswere censusedusing a line-transectmethod (Emlen 1977).Three line transectsof variablelength (range 538-956 m) were establishedon each site (lengthstotaled 2,079 m at one site and 2,505 m at the other). Flagswere placedparallel to the transect route at 25 and 50 m to aid in estimating the distance of birds from the transect center. The same transects
were used each year and run once when the nests containednestlings(62% in 1990, 50% in 1991) or eggs.Transects wererun duringthe morningactivity
periodof the birds(0400-0800).The perpendicular distance from the line to each bird detection, and the
manner in which the bird was detected (visual, sing-
ing, calling,both visualand auditory),were recorded as the observerslowly walked the transect(ca. 1.5 km/h).
Grasshopperabundancewas indexed in two ways. First, at each pole marking the line transects(poles were approximately90 m apart),contentsof 40 passes of a 0.4-m diameter sweep net swept in a 5-m radius were collected between
1000 and 1200. Collections
were made during: (a) incubationstage(late May to 12June);(b) nestlingstage(13-24June);and(c)fledging stage(>24 June).Grasshoppers were separated from all other arthropods,dried at 100øCin an oven for 48 h, and weighed to the nearest0.0001g. The other arthropodscollectedwere dried and weighed in the samemanner. Mean biomassper nesting stage
wasdeterminedby takingthe total massof 40 sweeps at eachline-transectpole and averagingthoseto ob-
T^BLE1. Biomass'(g; œ + SE) of grasshoppers and other arthropodsduring high (1990) and low (1991) grasshopper yearson the Delta Agriculture Project,Alaska,with resultsof Kruskal-Wallistest.
Grasshoppers Other arthropods
1.37 + 0.20 0.18 + 0.03
0.003 + 0.002 0.075 + 0.013
ßDry-weight biomassper 40 sweepsof a 0.4-m radius insectnet within a 5-m radiuscircle.
tain an estimatefor each transect.Mean biomassper year was estimatedby averaging the means for the three stages. Second,we estimatedgrasshopperdensitiesusing circular aluminum rings (Onsagerand Henry 1977). At eachof two sites,40 0.1-m2rings were positioned in four rows of 10 placed 5 m apart. Eachgrid was placed in the samehabitat. One grid was lessthan 1 km from one study site, while the other was 5 km from the secondstudy site. We countedthe number of grasshopperswithin each ring as they flushed to our approach;vegetationwithin eachring wasbrushed to flush any remaining grasshoppers.Grasshoppers were countedweekly in 1990beginningin late May and continuing into mid-July, and once every three weeksin 1991beginningin early Juneand continuing until late July. After the number of grasshoppers in eachring was summedfor eachrow, the four rows were averagedto obtain a density estimate for the grid. Ring countswere madeonly when temperatures were greater than 19øCand wind velocity was less than 10 km/h.
supply differencesmay existbetweensites(Quinney et al. 1986), we tested for differences between sites
within years. However, we found no differencesin
any of the parameters,including grasshopperand other arthropodbiomass.Therefore,we pooled the information from both siteswithin eachyear. Normality of the data was testedusing a Shapiro-Wilk statistic (Shapiro and Wilk 1965). Clutch size and
Nesting phenologyand grasshopperdevelopment.--Savannah Sparrow nesting phenology was similar betweenyears.The median date for clutchinitiation (first egg day) was 31 May 1990 (n = 23) and 29 May 1991 (n = 42); median hatching dates were 17 June 1990 and 15 June 1991;median fledging dateswere 25 June 1990 and 23 June 1991. More than 52% of clutches
were initiated within a three-day(one day before through one after median)period in 1990, and more than 42% of clutches were within
three-day period in 1991. A unique featureof Alaskangrasshopperdevelopment is a prolonged hatch, which continues over 30 days.This is primarily due to microhabitat temperature differences (1990 U.S. Dep. Agriculture internal report by Slussand Franklin). The first nymphal stagesof grasshopperswere observedasearly as21 May 1990. Secondand third nymphal stageswere present by 30 May. Full adult stageswere seen by 20 June.Grasshoppers were rarely seenin 1991so their phenology was unknown. Grasshopper biomassand densityestimates.Grasshoppers were muchmoreabundantin 1990 than in 1991 as indicated in both sweep-net samples (Table 1) and ring counts. Overall grasshopperdensitiesbetween yearsfrom ring counts averaged 66.4 + SE of 12.1 grasshoppers/m2in 1990and 0.3 _+0.2 grasshoppers/m 2
numberof birdsfledgedper nestwereanalyzedusing a G-test (Sokal and Rohlf 1981:696-721).Hatching success,fledging success,arthropodbiomass,grasshopper biomass,and grasshopperdensitywere analyzed using a Kruskal-Wallis(KW) test (Sokal and Rohlf 1981:429-432).Growth data, feeding rates, in 1991 (H, = 12.20, P < 0.001; KW test). Other length of foraging bouts, and amount of time spent brooding were analyzed using a generalized linear model (GLM; SAS Institute 1988:549-640). Differencesin the amountsof each taxon fed to nestlings between years were analyzed using t-tests. Differencesin the variety of taxa fed to nestlingsbetween yearswere analyzed using a Wilcoxon rank-sumstatistic.Sparrowdensitieswere estimatedusingthe program DISTANCE (Bucklandet al. 1993).Observations were truncated
at 90 m on each side of the line. Tran-
sectdata were fit to a hazard function (Buckland 1985),
and the differencein density estimateswas analyzed using a t-test. Statisticalsignificancewas set at the 0.05 probability level.
arthropods also were more abundant in 1990 than in 1991 (Table 1). There were no between-
site differencesin grasshopperor other arthropod biomasswithin years (1990, F4,3•= 0.21, P = 0.65; 1991, F4,3•= 0.34, P = 0.56). Dietaryresponse.--More grasshoppers were fed to nestlingsin 1990 than in 1991 (t•0 = 5.16, P < 0.001; t-test), while greater numbersof Lepidoptera larvae and Diptera were fed to nestlings in 1991 than in 1990 (t•0= 2.36, P = 0.02; ta0= 2.36, P = 0.02; t-test; Table 2). There were
no differencesbetweenyearsin the number of taxa fed to nestlings (Z = 0.64, P = 0.52; Wil-
Sparrow Responses toChanging Food
TAI•LE2. Food items (number with percent in parentheses)brought to the nestlingsby adult Savannah Sparrows in years with high (1990) and low (1991) grasshopperabundance,on the Delta Agriculture Project,Alaska.
why nestlingsgrew larger in 1990than in 1991. In addition, when brood size was controlled for,
there was no difference in average length of foraging bouts between years (1990, g = 3.2 + 0.4 min/bout,
Arachnida Orthoptera Neuroptera
36 (25.2) 87 (60.8) 0 (0.0)
34 (25.2) 0 (0.0) 1 (0.8)
0 (0.0) 0 (0.0)
8 (5.9) 5 (3.7)
Lepidoptera Adult Larvae
2 (1.4) 8 (5.6)
3 (2.2) 34 (25.2)
Diptera Dipteraa Hymenoptera
3 (2.1) 7 (4.9) 0 (0.0)
16 (11.9) 6 (4.4) 28 (20.7)
Coleoptera Adult Larvae
n = 9; 1991, f = 4.5 + 0.7 min/
bout, n = 11; F2,•7= 0.49, P = 0.50). Reproductive response.--Predationrateson the
study site were unusually low for a groundnestingpassefine(Miller and Knight 1993),with
all nests(28 in 1990and 37 in 1991)being successful.Clutchsizesrangedfrom two to sixeggs per clutch in 1990 and four to five eggs per clutch in 1991 (1990, f = 5.00 + 0.22, n = 22; 1991, œ = 4.60 + 0.09, n = 32). There was a
significant difference in clutch size between
years(G4= 10.07,P = 0.04). Hatching success was similar between 1990 and 1991 (g = 89.9 + 3.6%; f = 87.5 + 3.3%; H• = 0.39, P = 0.53; KW
test). The number of birds fledged per nest
ranged from one to six birds per nest in 1990 and two to five birds per nest in 1991 (1990, f
coxonrank-sum).Althoughadultsfed nestlings = 4.46 + 0.26, n = 28; 1991, f = 3.85 + 0.16, n = 35; G• = 9.86, P = 0.08).Fledgingsuccess was high in both 1990and 1991 (œ= 97.3 + 2.0%;œ
more often per hour in 1990 than in 1991 (f = 9.6 + 0.6, n = 11; f = 7.3 + 0.7, n = 11; F•,2o=
6.57, P = 0.02), when brood size of the observed
no significantdifferencebetweenyears(F2,•9 =
= 96.2 + 1.7%; H• = 0.72, P = 0.40; KW test). The amount of time spent brooding the nestlings was similar between years (F2,•7= 0.02, P
2.12, P = 0.16). Brood size of the observed nests
nests was considered
as a covariate
differed between years(1990, œ= 5.2 + 0.3, n
Nestlingswere larger in 1990 than 1991 (Ta-
= 11; 1991, f = 4.2 + 0.3, n = 11; F•,20= 6.37, P
ble 3). When brood size was considered a co-
= 0.02).If we considerbroodsizesof only four or five nestlings,adultsfed nestlingsmoreper
variate with year, masson day 7 was signifi-
hour in 1990 than 1991 (f = 10.39 + 1.03, n =
0.24 g, n = 23; • = 13.74 + 0.19 g, n = 22; F•,42
cantly lower in 1991 than in 1990 (œ= 13.23 +
5; œ= 7.97 _+0.66, n = 8; F•,• = 4.39, P = 0.06). = 4.32, P = 0.04). If evaluation of brood size is Even though this differenceis not statistically restrictedto four- or five-nestling nests(thus significant,the differencemay shed light on eliminating the brood-sizeeffect),masson day TABI, I• 3. Comparisonof growth parameters(œ+ SE)on days4 (n = 22) and 7 (n = 23) for nestlingSavannah Sparrowson the Delta AgricultureProject,Alaskafor high (1990)and low (1991)grasshopperyears. Growth parameter
Amount gained between days 4 and 7 3.69 _+0.13 3.56 _+0.05 1.71 _+0.06 1.68 _+0.05 1.72 _+0.37 1.68 _+0.35
0.27 0.25 0.38
0.60 0.62 0.54
Primary growth (mm/day) Tarsusgrowth (mm/day) Mass gain (g/day) ßGLM with year and brood in model.
M•r.R •r AL
7 was still greater in 1990 than in 1991 (œ= 13.79 + 0.18 g, n = 12;• = 13.00 + 0.27 g, n = 18; F•,28= 5.04, P = 0.03). Growth parameters differed more on the first visit (day 4) between years,with 1990measurements greaterthan 1991 (Table 3), suggestingthe most vulnerable period for growth may be in the first few days after hatch, rather than from day 4 to day 7. Thisis reinforcedby the similarityin the growth
[Auk, Vol. 111
brood-sizedifferencesrather than changesin food availability. Reproductive response.--Numerousfactorscan
affectclutchsize in birds (Klomp 1970,Hussell 1972)includingfoodavailability(Martin 1987), and Lack (1966) postulatedthat clutch-sizevari-
ation was correlatedwith food availability.Although some authors have indicated no increase in clutch
size due to a natural
(Sealy1980,DaviesandLundratesbetweenyearsfrom day 4 to day 7 (Table in foodresources 3). If this analysisis restrictedto broodsof four berg 1985, Strehl and White 1986), others have or five nestlings, measurementsfor massand
documenteda positive responseof clutch size
ninth primaryon day 4 were still significantly to food availability(MacArthur1958,Zachand lower in 1991 than 1990. Tarsus measurements Fails1975,Anderson1977,Sealy1978,Marr and for day4 do not differ statisticallybetweenyears Raitt 1983, Fleischer et al. 1985, Hussell and (F2,27 = 2.99, P = 0.10). Quinney1986).Savannah Sparrows in ourstudy Numericalresponse.--Densities of adult Savan- had largerclutchsizeswhen grasshoppers were nah Sparrowsincreasedby 42% between the abundant. high and low grasshopperyears (1990, œ= 1.9 _+ 0.3 birds/ha; 1991, •?= 2.7 + 0.5 birds/ha).
However, this increasewas not statisticallysignificant (t9 = 0.94, P = 0.19).
Egg laying was initiated when grasshoppers were hatching (mean initiation date in 1990was
31 May) andat a time when manyof the grasshoppershad already reachedthe secondand third nymphal stages(1990 U.S. Dep. Agricul-
ture internal report by Slussand Franklin). Savannah Sparrowsin our study may have respondedto improvednutrition by laying more SavannahSparrowsnesting on the Delta Ag- eggs (Klomp 1970, Slagsvoid 1988). Marr and riculture Project showed a dietary and repro- Raitt (1983) found that CactusWrens (Campyductive responseto superabundantgrasshop- lorhynchus brunneicapillus) respondedto grassper populations, but demonstrated no statisti- hopper emergenceby increasingclutch size cally discernablenumerical response.The di- when grasshopperswere abundant. Grasshopetary and reproductiveresponseswitnessedin pers are high in nutritional value, exceeding DISCUSSION
ourstudywere in the predicteddirection;however, the magnitude of the responsedid not reflect the overwhelming change in prey abundance between years. Dietary response.--Dietaryresponsesmay re-
50% in crude protein content (Ueckert et al.
1972, DeFoliart 1975). Protein is an important componentin the diet of birds during egg laying (Robbins 1981).
Anderson (1977) reported higher fledging flect a change in diet or a change in feeding successin House Sparrows(Passerdomesticus) in rates,and dietary responsesto changesin food responseto outbreaksof periodiccicadas(Magsupplyarecommon(Bucknerand Turnock1965, icicadaspp.). Strehl and White (1986) also inZach and Falls 1975, Morse 1978, Strehl and dicated greater fledging successand higher White 1986, Steenhof and Kochert 1988, Crawmean numbers of nestling Red-winged Blackford and Jennings 1989, Korpim•iki and Norr- birds(Agelaius phoeniceus) fledgedduring a year dahl 1989, 1991). We observed a dietary re- with abundantfood.SavannahSparrowsin our sponsein that nestlingswere not fed grasshop- study fledged more nestlingsper nest in the pers in the low grasshopperyear. Lepidoptera food-abundantyear; however, the difference larvae and Diptera were usedin greateramounts was not statisticallysignificant. The difference during the low grasshopperyear, presumably waslikely a reflectionof the differences in clutch reflecting the decreasein grasshopperavail- size becausethe percentageof nestlingssurvivability. ing to fledgingwasnearlyequalbetweenyears. Fewer feeding trips were made by adults to Nestling growth can be influencedby food nestsduring the low grasshopperyearthan dur- availability(Anderson1977,Bryant1978,Strehl ing the high grasshopperyear. Differenceswe and White 1986); however, Wiens (1974, 1977) observed in feeding rates probably reflected suggestedfood availability is not a factor lim-
Sparrow Responses toChanging Food
iting nestlinggrowth in shrubsteppeand grass- lished (yet prior to laying), so a within-season land habitat. Growth of nidicolous birds is ilnumerical responsewas probably unlikely. lustrated by a sigmoid curve; growth occurs Although our sampling method for arthroquickly during the first few days of life, then podsother than grasshoppers wasprobablynot slows and may plateau by fledging (Ricklefs adequate for a thorough inventory (Norment 1968, Maher 1973). Nutritious, abundant food
1987), the method we used indicates fewer ar-
may have a more important role in defining growth during the first one to four daysof life for SavannahSparrows.Growth slowsbetween day 4 and 7, and the type and amount of food fed to the nestlingsmaynot be ascritical.Growth of nestlingSavannahSparrowsin our studyappeared to be more affectedat day 4 than day 7. Ninth-primary measurementsand mass were significantlylower in 1991than 1990on day 4.
thropodswere availableto SavannahSparrows
Tarsus measurementson day 4 were lower in
1991,howevernot significantly.In contrast,the only parameter that was significantly lower at day 7 in the year with few grasshopperswas mass,suggestingthat retardationin growth occurred early in development. In addition, growth ratesbetweenday 4 and 7 did not differ betweenyears.If, in fact,the vulnerableperiod of nestlinggrowth occursduring the first four days of SavannahSparrow development,our ability to detect strong differencesin growth was low becauseof the developmentalstageat which we took our measurements.
The first four
daysmay havedictatedthe growthpatternsand final measurements of nestlingsat day 7. This is supportedby the significantly smaller measurementsof ninth primary and massat day 4 in 1991.
during the low grasshopperyear. Savannah Sparrowswere able to exploit successfully this substantiallyreducedfood resourceduring the time when they were completingtheir breeding efforts during the low grasshopperyear. This opportunisticbehavior (Rotenberry 1980) suggeststhat SavannahSparrowsin our study were reproductively limited by factors other than food. Other authorshave suggestedthat vegetativestructure,territoriality, and carrying capacity are important in influencing reproductiveoutput (Fretwell 1972,Wiens 1974,Ross 1980, Holmes et al. 1989, Holmes and Sherry 1992, Richner 1992). ACKNOWLEDGMENTS
We acknowledgethe help of the following people: Patty Miller, Bo Miller, Robert Magill, Tom Sluss, Brett Petersen,
Myers, Paul O'Brien, Ken Burnham, and the landowners on the Delta Agriculture Project.Earlier versionsof thismanuscriptbenefittedby commentsfrom S. Boutin, D. Hussel, P. Marra, B. Van Horne, and two
anonymousreviewers.Support for this projectwas provided by the U.S. Department of Agriculture APHIS-PPQGrasshopper IPM Project,ColoradoState University, Alaska Department of Fish and Game, AlaskaDepartmentof Agriculture,and a grant from the Frank M. ChapmanMemorial Fund of the American Museum of Natural History.
Numericalresponse.--Avertebratepopulation can experiencea numerical responseeither by immigrationor through recruitment(Solomon 1949). A numerical responsedepends on the mobility, reproductive potential and generaLITERATURE CITED tion time of a predator (Korpim•ki and Norrdahl 1991). Numericalresponses by birdsto nat- ANDERSON, T. R. 1977. Reproductiveresponsesof ural fluctuationsin prey densityhavebeendocsparrowsto a superabundantfood supply.Conumented by numerous authors (Pitelka et al. dor 79:205-208. 1955, Morris et al. 1958, Mebs 1964, Buckner and Turnock 1965, Southern 1970, Zach and Falls
1975, Morse 1978, Sealy 1980, Crawford and Jennings 1989, Korpim•ki and Norrdahl 1989, 1991).
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