Mexican Jays

The Auk 116(4):924-936, 1999

INFLUENCE OF CLIMATE ON REPRODUCTIVE SUCCESS IN MEXICAN JAYS

SHou-HsIEN LI AND JERRAML. BROWN• Departmentof BiologicalScience, State University of NewYork, Albany, New York •2222, USA

ABSTRACT.--Inthe arid southwesternUnited States,many birds initiate breedingin the driestmonths of the year,March to May, longbefore the monsoonrains arrive in Julyand August.Although breeding success in thesespecies is thoughtto be sensitiveto precipitation, the relationshipshave not been rigorouslydescribed based on long-termstudy of a singlespecies. We studiedthe relationshipsamong reproductive success, precipitation, and temperaturein MexicanJays (Aphelocoma ultramarina) in the ChiricahuaMountains, Arizona. We identifiedthree orthogonalfactors that accountedfor 85.8%of the variancein the 11 reproductive-successvariables. Factor 1, the mostimportant, was associatedwith production of youngand with the fractionof femalesthat werebreeding. Because the MexicanJay is a plural breeder(more than onebreeding female per group),reproduction in the population canbe influencednot only by the successof individual females,but alsoby the proportionof femalesin eachflock that breedin a givenyear. This factor was positively related to the amountof precipitationboth at the onsetof breedingin Marchand April and during the previouseight months.Brood size at banding(14 to 15 days),which was stronglyas- sociatedwith Factor2, wasnegatively related to the numberof adult femalesper flockand relativelyinsensitive to yearlyvariation in climate.Success of the youngestfemales was associatedwith Factor3 and dependedon a differentset of variablesthan that of olderfemales. Althoughproduction of youngwas predictably depressed in droughtyears, the significant relationshipsbetween reproductive success and climatedid not otherwiseenable precise predictionsbased on climatealone. Because predation appears to be highly correlatedwith the numberof nestlingsper unit, the lack of strongpredictability of reproductivesuccess usingclimate variables alone may be causedby the independenceof predationfrom climate variables.Received 3 August 1998, accepted 25 January1999.

BECAUSE OF ALMOST DAILY ATTENTION in the 1998).Such studies are neededto assessthe in- scientificand massmedia to E1Nifio and global fluenceof climatechange on avianpopulations. climate change,especially in 1997-1998,ecol- The most dramatic effects of climate on re- ogistsand eventhe generalpublic are becom- productivesuccess have been reportedfrom ing more attentiveto the effectsof climaticvar- coastalEcuador (Marchant 1959, 1960; Lloyd iationon a varietyof biologicalphenomena, in- 1960)and the GalapagosIslands (Grant 1985, cluding geographicrange (Parmesan1996), 1987; Curry and Grant 1989, 1991; Grant and global photosynthesis(Myneni et al. 1997), Grant 1989).Effects of El Nifio on this region community composition(J. H. Brown et al. have long been known. Recently,however, at- 1997), and timing of reproduction (Beebee mosphericscientists have demonstratedthat 1995, Crick et al. 1997, Forchhammer et al. 1998, variations in climate associated with oceanic McCleeryand Perrins 1998). This paper focuses phenomena,such as El Nifio andthe NorthAt- on relationshipsbetween reproductive success lanticOscillation, actually have more far-reach- and yearly climaticvariation. Many short-term ing statisticaleffects (Taylor et al. 1998).The re- studiesdocument that climateaffects repro- alizationthat globalclimate changes dramati- ductivesuccess, but suchstudies are limited by callyon a decadalscale, and that thesechanges small sample sizes. Fortunately,long-term studies of birds have become a rich source of may affectbird populations(Crick et al. 1997, Forchhammeret al. 1998,McCleery and Perrins data for the studyof climateeffects on reproduction (e.g. Brown and Li 1996, Skinneret al. 1998),provides a newstimulus for thestudy of effectsof climateon breedingbiology of North American birds. However, before we can ap- 1Address correspondence to this author.E-mail: preciatethe possibleeffects of El Nifio and [email protected] globalclimate change on a particularspecies, 924 October1999] ClimateVariation and Reproduction 925 we needdetailed information on how that spe- awkward because it would entail fractions of flocks. ciesreacts to yearlyvariation in climate.There- Further details on this populationare availablein fore,in responseto the needto evaluateeffects Brown and Brown (1985, 1990) and Brown (1994). of globalclimate processes on North American Sexand age.--Most birds were banded as nestlings birds, we undertooka comprehensiveexami- or when age couldbe determinedreliably (i.e. 0 to 2 nation of the statistical effects of climate on re- years).Jays in their first two yearsof life were as- signedan age using the methodsof Pitelka (1945), productionin the MexicanJay (Aphelocoma ul- whichwe haveconfirmed using birds banded as nes- tramarina). tlings and trapped at later ages(Brown 1994).Rem- In warm, arid climates,reproductive success iges,rectrices, the alula,and somegreater and mid- of birds is relatedto rainfall becausemost spe- dle secondarycoverts are retainedbeyond the first cies do not breed until rains arrive and success prebasicmolt. Thesefeathers are distinctlyduller is diminished when rain is insufficient. Breed- and moreworn than the newerplumage. Reduced ing successtends to be sensitiveto the duration areasof lightcolor on the bill persistto agetwo, rare- and amount of precipitation in Galapagos ly to agethree. A few birdswere agethree or older finches (Grant 1986, Grant and Grant 1989), wheninitially banded (5 to16%per year). These were assignedthe ageof threeat the timeof bandingand resident passerinesof East African (Sinclair usedin this analysis.Females were birds known to 1978,Dittami and Gwinner1985), White-front- have incubated. Males were birds that built nests ed Bee-eaters(Merops bullockoides; Wrege and with a female.We determined sex for all birdsof age Emlen 1991), wrens of the Venezuelanllanos 8 or older, 96.4% of the birds at the most common (Piper 1994), and desert birds of Australia breedingages (4+), and 93%of birds age3 or older; (Keast and Marshall 1954, Nix 1976). In Ari- the rest were not sexedand were assignedhalf to zonapopulations of the MexicanJay, however, each sex for calculation of certain variables. Further breedingoccurs not in the rainy seasonof July details of determinationof age and sex are given and August (monsoon),but in the driest elsewhere (Brown 1994, J. L. Brown et al. 1997, monthsof theyear, namely March through June Brownand Bhagabati1998). Reproductivesuccess.--We observed reproductive (Brownand Li 1996).The purpose of ourstudy successin thepopulation by monitoringnests of each was to determinemore preciselyhow repro- femaleeach year Fieldwork began in mid-Marchor ductivesuccess in thesejays is correlatedwith earlierand lastedinto Juneexcept in the years1980 climaticvariables in the monthsand years pre- to 1991and 1993to 1995,when field work beganin cedinga breedingseason. January.Observers were presentthrough the end of Juneor closeto it. In addition, many scientistswere M•THODS at SWRSall throughthe summereach year If a nest had beendiscovered by observersin Julyor August, Studypopulation.--The study area is on the South- it is likely that the nestwould have been brought to western Research Station of the American Museum our attention; however, none were. From March to of Natural History,adjoining private lands, and the July,we locatednearly every nest in our studypop- CoronadoNational Forestin Cave Creek Canyonon ulation and determined the individual who incubat- the east side of the Chiricahua Mountains, Arizona ed the eggsand the identityof her mate.We do not (31ø53'N,109ø12'W). The habitat is pine-oak-juniper know of any neststhat we missed,but becausefind- woodland (seeBrown and Brown 1990,Brown 1994). ing nestsis difficult,it is possiblethat we misseda The numberof flocks(also termed "groups" or few,especially ones that failed early in incubation. "units") under study increasedfrom six in 1972 to However,we foundno fledglingsfrom undiscovered nine in 1994. This led to an increase in the area under nests.Mexican Jays tend to be sociallymonogamous studyby about20%. Individuals that were known by in that typically a singlepair builds a nest, unless directobservation to be livingin the studyflocks on usurpationoccurs, and only the femalebuilder in- 1 May of a given year were definedas the census cubates.Studies using allozymes (Bowen et al. 1995) populationfor that year Birdshatched in the same and DNA microsatellites(Li and Brown unpubl. year as the censuswere excludedas flockmembers data) have not revealedany casesof intraspecific that year Thepopulation was color banded and ob- multiplematernity. We monitoredthe reproduction servedevery year from 1969through 1996. During of everynest until the youngwere bandingage or the periodanalyzed here (1972to 1996),the popu- until they left the nest (i.e. fledged).The seasonal lationvaried in sizebetween 63 and 141jays. Each patternof breedinghas been described in detail by flockoccupied a stablegroup territory that varied lit- Brown and Li 1996. tle in locationand size from year to year Therefore, We employed11 measuresof reproductivesuccess we indexeddensity on a per-flockbasis rather than for eachyear in a factor analysis(Appendix). We in relationto a fixed study area, which would be chosethese variables because they are components of 926 LI AND BROWN [Auk, Vol. 116

TABLE1. Summarystatistics for variablesexpressing reproductive success in MexicanJays in southeastern Arizona, 1972 to 1996.

Variables œ SD Min. Max. \ CV 1. Fractionof females3+ yearswith eggs 0.52 0.13 0.23 0.68 23.9 2. No. young/bandedfemale 2 and 3 years 0.49 0.52 0.00 1.60 106.0 3. No. young/bandedfemale 4+ years 1.28 0.63 0.18 2.56 49.2 4. Brood size at banding 3.18 0.50 2.00 4.38 15.7 5. No. young/successfulnest 3.18 0.51 2.00 4.37 16.0 6. No. nestswith eggsper unit 3.05 0.91 1.33 4.78 29.8 7. No. nestlings/unit 4.91 1.81 1.11 8.50 36.9 8. No. successful nests / unit 1.36 0.54 0.22 2.38 39.7 9. No. fledglings/ unit 4.37 1.85 0.56 8.50 42.3 10. No. fledglings/nestwith eggs 1.46 0.64 0.29 2.92 43.8 11. Fraction of nests successful 0.45 0.17 0.12 0.71 37.8

populationgrowth. We did not knowa prioriwhich, ing seasonto two yearsearlier. We suspected that the if any,variables would be sensitiveto climaticfactors varioustypes of food usedby jays,including acorns, or how they would be relatedto eachother, so we arthropods, and small vertebrates,would be influ- included a variety. Somevariables are expressedas encedby various environmentalfactors at different the valuesfor the populationdivided by the number timesof the year of social units, or flocks. Thus, some of the variables Populationvariables.--Because reproduction might are the averagesper unit for eachyear (variables 6 to be densitydependent, we tried to controlfor various 9 in Table 1). densityvariables when necessary.Because the area In the MexicanJay, which is a pluralbreeding spe- understudy was not constantbut wasdefined by the cies,the numberof breedingfemales per group is territoriesof the groups,we usedmeasures that were variable and typically more than one, unlike the independentof the size of the study area, suchas morecommon singular breeders. Therefore, climate numbersof yearlings,adults, malesor femalesper couldaffect the numberof birdsbreeding per unit group, as definedbelow. Population increases were (variables 1 to 3, 6 to 9, and 11) and the successof reflectedin increasesin groupsize. The origin of new thosethat do breed (variables4, 5, and 10). groupsand the lossof existinggroups were rare and Reproductivetiming variables.--Because timing var- apparentlywere not directly related to population iables are related to climate (Brown and Li 1996), we density or climate. We also tried to control for age examinedonly their relationshipsto reproductive structure, because reproductive successincreases success.For eachyear, we calculatedthe meandate with agein this species(J. L. Brownet al. 1997),and of first clutchfor all femalesthat bred,accounting for we investigatedpossible effects of sex ratio on re- leap years.Subsequent clutches were not included. production. We usedas the datefor eachclutch the Julianday of Statisticalanalysis.--All variables were normally the year for the third eggof the clutch,which is the distributedexcept precipitation in May and Juneand date of initiation of incubation (Brown 1994). Most the numberof youngbanded per femaleof age2 to clutchescontain four or five eggs.We did not inspect 3 years.We applieda square-roottransformation to mostnests at the time of layingbut insteadcalculat- the former variable,but the latter variable,which had ed laying datesfrom the agesof eggsor nestlings, many zeros,could not be transformedto a normal making supplementaryuse of observationsof eggs distribution. Because the effect of deviation from and incubation.By trackingindividually color-band- normalityis mainlyto weakenthe powerof the test, ed females, we were able to determine the order of we left it in the analysis;however, this variablewas their clutches. Clutches of unbanded females were not critical for our results. Sample sizes were 25 not usedin this analysis.We alsoexamined the frac- years(1972 to 1996)for all analysesexcept where in- tion of femaleswith a secondclutch each year. dicated. Climatologicaldata.--The climatological data used Becausethe 11 measuresof reproductivesuccess in our analyseswere recordeddaily by employeesof that we used were often correlated with each other, the SouthwesternResearch Station within our study we usedthe principalcomponents method of factor area.We obtainedthese data from the monthlysta- analysisin SPSSto reducethe numberof variables, tion summariesfor Portal,Arizona, published by the an approachused in other studiesof avian repro- Environmental Data Service of the National Oceanic ductive success(Skinner et al. 1998).We rotated the and AtmosphericAdministration, United States De- factormatrix orthogonally by the Varimaxmethod. partmentof Commerce.Climate variables were cho- We then used the scores of the rotated factors instead sento covera range of periodsfrom the samebreed- of the original breeding variablesas dependentvar- October1999] ClimateVariation and Reproduction 927

Highest monthlyprecipitation in this period was al- • 3o waysbelow 30 mm. Thelargest fractions of annual precipitationwere monsoon(41% of total) • '1o and winter rains (Decemberto February;51%). Lowest Precipitationfrom Marchto Junemade up only 8% of annual precipitation.Annual precipita- -lOo tion varied greatly (from 252 to 781 mm; œ= 560 + SD of 129.2mm), as did precipitationin 180 the criticalmonths of early springand Julyto February(Fig. 2). Snowoccurred regularly in • 140 winter and couldreach 15 to 25 cm in depth. • 120 The coldesttemperatures of theyear typical- 80 ly occurredin January(mean lowestdaily min- ima for January= -12.6øC; Fig. 1). Although

20 ' the desertwas only about10 km away,maxi- •. 400 * mum temperatureson the study area in most yearswere below 38øC. June and July were the Month hottestmonths, when the averagedaily maxi- FTC.1. •on•hly •at•e•n • •em•e•a•u•ean• •e- ma typicallyranged from 29 to 34øC.The hot- cJ•itationon the s•y a•ea•om •972 to test singleday of the month typicallywas be- est • hi•hes• •em•e•a•u•eo• month,an• tween 35 and 37øC. lowes••em•e•a•e • month. Box = median, Annualvariation in breeding-success.--Annual an• 75th•e•cenfiles, whJske•s = 5•h an• 95th production of nestlings varied greatly from centi]es,an• •o•s = •an•e. year to year (Fig. 2), but measuresrelevant to reproductivesuccess did not always vary together.Altogether, we examined11 measures iablesto studythe relationshipbetween reproduc- of reproductivesuccess. Descriptive informa- tive successand climate.We usedmultiple linearre- tion for each variable is shown in Table 1. Most gressionmodels to examine the statisticaleffects of variables had a coefficient of variation between climaticvariation on the threeprincipal factors rep- resentingannual reproductive success. We usedthe 20 and 50, but brood size (of those neststhat stepwiseselection procedure to reducethe number reachedthe nestlingstage) was relatively con- of independentclimate variables in the regression stant(15.7), and the highestcoefficient of var- models. iation belongedto the per capita successof Becausereproduction may be influencedby factors youngfemales (106). suchas population density, sex ratio, and agestruc- The breeding-successvariables can be sum- ture, we used a multiple linear regressionmodel to marizedby threeindependent factors that to- detectpossible relationships of thesevariables before we examined the climatic variables. Variables getherexplained 85.8% of variationin annual that had linearrelationships with the reproduction reproductivesuccess (Table 2). Thesethree fac- of Mexican Jayswere controlledusing partial re- torswere the onlyones with eigenvalues>1.0. gressionwhen examiningstatistical effects of cli- The first factor accounted for 55.7% of the to- mate variables. tal variance.The most important variablesfor Factor1 werethe numberof nestswith eggsper RESULTS unit, the numberof successfulnests per unit, the fraction of females with a nest, and the Seasonalpatterns of precipitationand tempera- numberof nestlingsper unit. Eachof thesevar- ture.--In the Chiricahua Mountains, the heavi- iableswas highly correlated with Factor1 (r -> estrains of theyear occur during the monsoon 0.82,P < 0.0001).High scoresof this factorin- monthsof July(œ = 112.4mm) andAugust (œ dicatethat (1) a higherfraction of breeding-age = 97.7 mm; Fig. 1). In September,mean precip- femaleswas engagedin reproduction,(2) more itationdrops to 59.9 mm, and monthlyprecip- femalesbuilt nests,(3) more had successful itation gradually declinesto 32.5 mm by Feb- nests,(4) morenestlings were raisedper flock, ruary. The driestseason is betweenMarch and and (5) more fledglings were produced in a June,when Mexican Jaysbreed. The mean givenyear Thus,the first factorrepresents the 928 LI ANDBROWN [Auk,Vol. 116

] /• ß July-February[•] l/ -- •-100•

,•4• 8o o, .•$00 [•] 40

I 3•

A • • Nestings

Z 3

82 •S .....fulnes•

1972 1974 1978 1978 1980 1982 1984 1986 1988 1990 1992 1994 1998

Year

F•c, 2, Annual variationJn two measureso• p•ecJpJtafionand •our measureso• reproductivesuccess •ex•can •ays.

TABLE2. Rotatedfactor matrix of breedingvariables with factorloadings. Highest values (above 0.8) are in bold.Variable numbers correspond to Figure3 and Appendix.

Variable Factor 1 Factor 2 Factor 3 1. Fractionof females3+ yearswith eggs 0.810 O.O36 0.122 2. No. young/bandedfemale 2 and 3 years 0.033 0.102 0.807 3. No. young/bandedfemale 4+ years 0.591 0.550 0.158 4. Brood size at banding 0.077 0.944 0.210 5. No. young/successfulnest 0.035 0.936 0.257 6. No. nestswith eggsper unit 0.846 -0.194 -0.348 7. No. nestlings/unit 0.835 0.393 0.282 8. No. successful nests/unit 0.840 0.083 0.484 9. No. fledglings/unit 0.748 0.368 0.505 10. No. fledglings/nestwith eggs 0.214 0.580 0.771 11. Fraction of nests successful 0.270 0.336 0.843 October1999] ClimateVariation and Reproduction 929

variablesthat contributedmost heavily to total 1.2 productionin the population.The main con- 1.0

tributing variablesto Factor1, as listed above, 0.8 are high when morethan onefemale per unit breeds. Their maximum values in Table 1 reflect 0.8

largernumbers than canbe producedby only 0.4 onebreeding female per flock.Factor 1 wasnot, however, correlated with the number of fe- 0.2

malesof breedingage (3+ years)per unit (r = 0.0 0.21, P = 0.313).As expected,the numberof -0.2 nestlings per unit was positively associated with the othervariables having high loadings on Factor1, namely,the fractionof females breeding,the numberof nestswith eggsper 0.8

unit, and the numberof successfulnests per 0.6 unit (r >- 0.539, P -< 0.005). The secondfactor explained 20.4% of the to- 0.4 tal varianceof the 11 reproductive-successvar- 0.2

iables.Variables having a high loadingon Fac- 0.0 tor 2 were broodsize and numberof youngper successful nest. This factor seems to reflect con- -0.2 ditions for rearing young but not conditions -0.4 conduciveto initiatingbreeding. The third factorexplained 9.7% of the vari- -0.2 0.0 0.2 0.4 0.8 0.8 ancein the reproductive-successvariables. Fac- Factor 1 tor 3 had a positiverelationship with the frac- F•c. 3. Clusteringof variablesaccording to load- tion of nests that were successful and the num- ings on the three factorsfrom Table2. The ellipses ber of youngper bandedfemale for the youn- facilitatecomparison of the upper and lower figures gestbreeding ages (2 to 3 years). and are purely arbitrary. Whenthe factor loadings in Table2 areplot- ted (Fig. 3), the 11 variablesmay be somewhat ductionof MexicanJays we usedoriginal scores arbitrarilyassigned to threeclusters that reveal of Factors1, 2, and 3 as dependentvariables in closeaffinities among the variablesbetter than the multiple linear regressionmodel. None of in Table2. Thosevariables that loadedheavily thepopulation variables was selected for the re- on Factor I and low to medium on Factors 2 and gressionmodel of Factor 1 ("fraction breed- 3 (1, 3, 6, 7, 8, 9) formed a clusterrelated to nes- ing"). Therefore,we usedthe originalscore of tlings/unit and fraction of mature females Factor1 directly for climate analysis. breeding.The variablesthat loadedheavily on Factor2 (brood size at banding) was nega- Factor2 but low on Factor1 (4, 5) formed a clus- tively related to the numberof mature (3+ ter related to brood size. A third cluster was years)adults per unit (b = -0.4738 + 0.0999;t formedby variablesthat loadedheavily on Fac- = 4.74, P = 0.0001).Factor 3 was negativelyas- tor 3 but low on Factor1 (2, 10, 11). Thesere- sociatedwith averageunit size(b = -0.2063 + flectedthe breedingsuccess of youngfemales 0.0900; t = 2.29, P = 0.0231). BecauseFactors 2 and the overall fraction of nests that were suc- and 3 were significantlyrelated to the popula- cessful. tion variables, we used partial regressionto Statisticaleffects of density,sex ratio, and age controlfor the confoundingeffect of population structure.--Toinvestigate the statisticaleffects variableson Factors2 and 3 in our analysisof of climaticvariation on reproduction,we need- climate variables. ed to controlthe statisticaleffects, if any, of Statisticalassociation of climatevariables with populationvariables such as density,sex ratio, reproductivesuccess.--To investigate possible re- andvarious aspects of agestructure (see Meth- lationshipsbetween reproductive success and ods). To investigatethe statisticaleffects of climate,we employedstepwise regression us- theseseven population variables on the repro- ing eachof the threefactors as dependentvar- 930 LI ANDBROWN [Auk, Vol. 116

0.02).For example, the numberof nestlingsper unit was positivelyassociated with precipita- 1 tion in March and April (b = 0.0313;t = 2.87, P = 0.009) and July to February (b = 0.0066;t o 0 = 2.54, P = 0.0186). After controllingfor the number of adults per unit, Factor2 ("brood size") was not significantlyassociated with any climatevariable -2 thatwe examined.Factor 3, aftercontrolling for the averagenumber of birdsin the group,was -3 negativelyassociated in step1 with precipita- 10 20 30 40 50 60 70 80 90 100110120 tion from Septemberto February two winters earlier (adjustedr 2 = 0.178;t = 2.73, P = 0.012) Early spring precipitation (mm) and in step2 with minimumtemperature from Octoberto November(adjusted r 2 = 0.291;t = 2.16, P = 0.042; full model F = 5.92, df = 2 and 22, P = 0.009).

1 Timingof laying.--Becausethe climatevari- ablesassociated with reproductivesuccess in this study differed partially from thosethat wereassociated with the timingof breedingin our previousanalysis (Brown and Li 1996),we examinedthe relationshipbetween two timing variablesand reproductivesuccess using the four reproductive-successvariables that were most associated with Factor 1. Mean date of -3 first clutchwas positivelyassociated with the 20 30 40 50 80 70 80 numberof nestswith eggsper unit (r = 0.617, July-February precipitation (mm) P = 0.001)and with the numberof nestlings F•c. 4. Relationshipof Factorl scoresto precip- per unit (r = 0.511,P = 0.011)but not with the itationin Marchand April (upperpanel) and the pre- fractionof femalesbreeding or with the num- cedingJuly through February(lower panel). Each ber of successfulnests per unit (n = 24; 1996 point is one year. data excluded).Minimum temperaturein winter wasnegatively associated with meanJulian iablesand controllingfor populationdensity as date of first clutch (Brown and Li 1996)but was stated above.Factor 1 was associatedin step 1 not associatedwith any of the threereproduc- with precipitationin Marchand April (adjust- tive-successfactors considered in thispaper. ed r2 = 0.305,t = 3.15, P = 0.005;Fig. 4) and in The secondtiming variablefrom our previ- step2 with precipitationthe preceding July to ouspaper that we examinedin relationto re- February(adjusted r 2 = 0.431, t = 2.46, P = productivesuccess was the fractionof females 0.022; full model F = 10.08, df = 1 and 23, P = that had a second clutch. This fraction reflected 0.001;Fig. 4). Althoughthese two precipitation the extentof breedinglater in the season(May variablesoften vary together (Fig. 2), theywere to June).Because this variablewas alreadyre- not significantlycorrelated with eachother (rs ported to be positively related to precipitation = 0.329, P = 0.108). Factor 1 was not associated in the previous monsoon and winter, we with precipitationin the previousmonsoon or wished to know how the fraction of females any earlierperiod that we tested.As a checkon with secondclutches was related to the repro- theseresults, we ran separatestepwise regres- ductive-success variables that we use here. We sionson eachof the four reproductive-successfound significant, positive Pearson.correlations variableswith thehighest loadings on Factor1. (n = 26years) with fivevariables, including the As expected,each was significantlyassociated four that were moststrongly associated with with oneor moreof the precipitationperiods Factor1 (fractionof femaleswith eggs, r = includedin the periodDecember to April (P -< 0.388, P = 0.05; numberof nestswith eggs/ October1999] ClimateVariation and Reproduction 931 unit, r = 0.661,P • 0.0001;number of nestlings mustbe involvedto explainmuch of the ob- of banding age per unit per year, r = 0.457,P served variation in Factor 1 and the number of = 0.019;number of banding-agenests per unit nestlingsper unit. Theremay also be statistical per year,r = 0.646,P < 0.0001;and the fraction effectsof climatethat do not showup in our of bandedmales ages two yearsor olderwith analysisbecause of scale,nonlinearity, or the a nestrecord, r = 0.493,P = 0.011)but not with factthat climateis mainly a collectionof vari- otherreproductive-success variables. These re- ableswhose statistical effects are likely to de- suitssuggest that one of theways in whichpre- pendon otherintervening variables that we did cipitationin Julythrough February could influ- not measure,such as various kinds of jay food. enceFactor 1 is by extendingthe breedingsea- Consequently,although we may concludethat son,thus allowing more second clutches by in- precipitationhas significantassociations with dividuals whose first nest had failed. reproduction,we cannot predict reproductive successprecisely on the basisof precipitation DISCUSSION aloneexcept in severedroughts, like that in 1996, whose disastrous effects we did in fact Precipitation.•Ourfinding that yearlyvaria- predict accurately.The negativerelationship tion in the amountof precipitationis relatedto betweenFactor 3 and the amountof precipita- annual reproductivesuccess of Mexican Jays tion two winters earlier is difficult to under- was not surprisingconsidering the arid envi- stand,and we canoffer no obvioushypothesis ronmentin whichthey live, but it wasnot clear that might explainit. a prioriwhich months would be critical.Precip- Predation.--Predationis a likely explanation itation in March and April, at the beginning for someof the variancein reproductivesuc- and peak of the laying season,had the stron- cessbecause indirect evidencefor it is frequent- gesteffect on Factor1, whichbest reflected rely seen,but predationis virtually impossibleto productionin the population.March and April estimatedirectly because the predatorsare so are amongthe driestmonths of the year (Fig. diverse and difficult to observe.Major avian 1). Therefore,even a little rain in this period predatorsof MexicanJays include Common Ra- couldhave a largeeffect on the foodsupply of vens (Corvuscorax), three speciesof Accipiter, jaysby promotinggrowth of herbaceousplants various owls, and other hawks. The most im- and their animaldependents. Thus, precipita- portantmammal for the MexicanJay is proba- tion at this time mightinfluence the decisionof bly thecoati (Nasua narica), which hunts in day- a femalejay to breedthat year,thereby influ- light,can probably hear begging nestlings, and encing the fraction of mature femalesthat can climb tall trees (Brown and Li unpubl. breed,which in turn affectsthe total produc- data).We alsosuspect that the ringtail (Bassa- tion of nestlingsper unit. riscusastutus), which is arboreal and occurs An additional statistical effect on Factor 1 regularlyon the studyarea, molestsjay nests. camefrom the accumulatedprecipitation from Thesuite of predatorson our studyarea is var- the previousJuly through February, which we ied, and a simple connectionwith climate showed earlier to be related to the fraction of seemsunlikely. females with a secondbrood (Brown and Li Therelative importance of predationand cli- 1996).Precipitation in thisperiod should large- mate may, however,be examinedusing the ly determinethe levelof the watertablein May. fractionof neststhat were successful(fledged Therefore,through its effectson trees and other at leastone young)as a proxy variablefor pre- plants, accumulatedprecipitation should also dation,as doneby Woolfendenand Fitzpatrick influencethe growthof biomassduring the dry (1984).Use of thisapproach depends on ourob- season,including food for jays. servationsthat loss of entire broodsthrough Althoughin certainyears (e.g. 1974,1976, starvationwas rare and that predation often re- 1990,and 1996) droughtappeared to depress sulted in loss of entire broods. The fraction of reproductivesuccess (Fig. 2), the percentageof neststhat were successfulloaded heavily on varianceof Factor1 thatis explainedby precip- Factor 3, which collectivelyexplained only itation variablesis not impressive(adjusted r e 9.7% of variancein reproductive-successvari- = 0.413).Thus, althoughprecipitation is im- ables.To examinethe importanceof predation portant,especially in dry years,other variables moredirectly, however, we ran a stepwisere- 932 Li ANDBROWN [Auk, Vol. 116

gressionanalysis using the number of nestlings soon-to-winter was only 55.2% and 86.4%, re- per unit as the dependentvariable (more rep- spectively,of averageprecipitation in the same resentativein our study than fledglingsbe- periods in all years. Therefore, prolonged causeof larger samplesizes). Independent var- droughtlasting into early springmight still iables were the fraction of nests successful and have some influence on brood size in some the two climatevariables previously shown to years. be significant.The first variableto enter the Other acorn-eatingspecies.--Birds that rely equationwas the fractionof neststhat were heavilyon storedacorns for winter foodshould successful(adjusted r2 = 0.303),the secondwas be influencedby the size of acorncrops in the precipitationin March and April (combined precedingyear, which in turn may be influ- adjustedr 2 = 0.441),and the third wasprecip- encedby precipitationat the time acornsare itation from July to February (combined ad- produced.Woolfenden and Fitzpatrick (1984) justedr 2 = 0.541).The final equation was high- examinedannual variation in the "average ly significant(F = 10.44, P = 0.0002): numberof fledglingsproduced per pair" in the acorn-eatingFlorida Scrub-Jay(Aphelocoma coe- y = -0.711 + 4.352 x• + 0.00553x= + 0.0245x3, rulescens).They estimatedthe importanceof (1) four factorsthat might influencereproduction where y = numberof nestlingsper unit, x• = by a femaleterritory owner, namely clutch size, fractionof nestssuccessful, x2 = precipitation hatchingfailure, nestlingstarvation, and nest in March andApril, andx3 = precipitationfrom predation.Although mean clutchsize in first Julyto February.These results suggest a strong nestswas strongly correlatedwith rainfall in associationof predationwith productionof the previoussummer, a significantassociation nestlings,but onethat is relativelyindependent betweenfledglings per pair and mean size of of our climate variables. As a check on the in- first clutchwas absent.However, using "the dependenceof the predationproxy variable we proportionof neststhat surviveto fledging"as did an additional analysis. In a stepwise re- an index of predation,they showedan "over- gression,the fractionof nestssuccessful was whelming" associationwith the number of not correlatedwith any of our 15 climatevari- fledglingsper pair. Therefore,they concluded ablesexcept minimum temperaturein autumn that "the annuallevel of nestpredation deter- (adjustedr 2 = 0.392, F = 5.85, P = 0.0006), a minesthe meannumber of youngthat fledge" relationshipthat is not easyto explain. (Woolfendenand Fitzpatrick 1984:181).The Broodsize.--Although we expectedbrood proportionof neststhat producedfledglings sizeto groupwith the variablesin Factor1 be- was also correlatedwith rainfall during the causeof its directrelationship with reproduc- preceding10 months,but the mechanismby tive success,it groupedwith the variablesin whichpredation is influencedby climateis still Factor2. Nevertheless,brood size was signifi- obscure.Because the effectof densityof breed- cantly correlatedwith two of the variablesin ing pairson total productionin the population Factor 1, number of nestlings per unit (r = was not considered,direct comparisonwith 0.583,P = 0.001,n = 27) and numberof fledg- our study is not possible;but becausethe lings per unit (r = 0.576, P = 0.002, n = 27). scrub-jayhas only onebreeding female per ter- Broodsize was the leastvariable of our repro- ritory, densityeffects should be smallerthan in ductive-successvariables (Table 1) and was not the MexicanJay. The variableof oursthat most associated with climate variables in the manner resemblesthat used with the scrub-jaysis of Factor 1. fledglingsper nestwith eggs(no. 10); this var- Althoughwithout strong statistical support, iable does not reflect the fraction or number of we found a positiverelationship between pre- femalesbreeding. In our study,fledglings per cipitationand brood size in certain years,such nestwith eggswas highly correlatedwith the as1976. Mexican Jays had the lowestbrood size proxy predationvariable (no. 11; r = 0.696,P < (œ= 2) in 1976.In that year,jays alsohad the 0.0001,n = 25). Thus, by this measurealso, lowest number of successfulnests per group yearly variation in predation is strongly asso- (0.5)and the secondlowest nesting success rate ciatedwith reproductivesuccess. (0.21) of our study period. In 1976, precipita- Relationshipsbetween reproduction and cli- tion in early springand in the previousmon- mate have also been examined in the Acorn October1999] ClimateVariation and Reproduction 933

Woodpecker(Melanerpes formicivorus), a species plain the negativerelationship between repro- which, like the MexicanJay, relies on stored ductivesuccess and springprecipitation. Even acornsfor muchof its food (Koenigand Mum- summerprecipitation was negatively related to me 1988).In contrastto the two open-nesting fledging success.Thus, the relationshipbe- Aphelocoma,predation in Acorn Woodpeckers tween reproductionand precipitationjust prior was not an important causeof nestling loss, to and during breedingin this closelyrelated whereasstarvation was an importantfactor A species(Espinosa de los Monterosand Cracraft strong correlation occurred between three 1997)is oppositethat of the MexicanJay. measuresof reproduction(clutch size, young/ TheGalapagos Mockingbird (Nesomimus par- group, young/breeding female) and winter vulus)lives in theregion most strongly affected rainfalltwo yearsearlier The authorsfelt, how- by El Nifio and consequentlyis subjectedto ex- ever,that this result was not explicablein terms treme variationin precipitation.Like someGa- of either acorncrop or insectabundance, and lapagosfinches (Grant 1985,1987; Grant and they concludedthat "reproductivesuccess... Grant 1989),this mockingbirdproduces many appearsto be independentof any directly in- youngin wet yearsand relativelyfew in dry terpretableeffect of the weather"(Koenig and ones(Curry and Grant 1989,1991). It doesso, Mumme 1988:124). however,by a differentmechanism than in the Their data do show, however, that the two MexicanJay. The breedingseason in the Gala- best yearsfor reproduction(in a 10-yeardata pagosis extendedduring wet years,but suc- set) were precededby the two years of good cessper nestis relativelyconstant. Unlike the acorn crops, lowest insect damage to acorns, situationin the MexicanJay, "Breeding density andhighest levels of autumnbreeding. Depen- is amongthe leastvariable ... of the popula- denceof reproductivesuccess on acorn quan- tion'sdemographic characteristics" (Curry and tity and quality would explainthe lag time of Grant 1989:457)and "the densityof breeding two years betweenrainfall and breeding suc- birds remained relatively constant." Thus, cessif an abundanceof high-qualityacorns en- with the exceptionof somevery dry years,the abledwoodpeckers to breed in autumn,over- fractionof femalesbreeding was insensitive to winter in good condition,and enter the next precipitationin a given year but increasedas breeding seasonwith a surplus of stored densitydecreased. Plural breedingwas mainly acorns.In fact, the authorsreported a strong a functionof theproportion of yearlingfemales correlationbetween the numberof fledglings in the population,again in contrastto the Mex- per breeding female and "total maximum kJ icanJay, in whichplural breeding is regularin per individualstored in granariesduring the all flocksand yearsand involvesfemales of all prior winter" Sucha scenariois also feasible matureages. One factorthat maybe partly re- for the MexicanJay, but we have no data on sponsiblefor thesedifferences is that breeding acorn crops. seasonsin Arizona are curtailed by extreme Variable environments.--Long-term studies drynessin May and Juneand by the onsetof have been conductedon two other passerine coldweather in September,whereas breeding speciesthat inhabit unusuallyvariable envi- in the Galapagoscan continuefor 205 daysif ronments.The PinyonJay (Gymnorhinus cyano- the rainspersist. Evidence for a modestexten- cephalus)is well adaptedto useseeds of pinyon sionof the breedingseason of the MexicanJay pine (Pinusedulis), whose yearly cropsare er- resultingfrom precipitationin the previous ratic (Marzluff and Balda 1992). Reproductive Julythrough February is the positiverelation- successin thisjay waspositively related to the ship betweenprecipitation in this periodand sizeof the precedingcrop of pinyonseeds and the fraction of females with a second clutch in negativelyto precipitationduring the breeding the subsequentbreeding season (Brown and Li season(Marzluff and Balda 1992:209).Paradox- 1996). ically,although the PinyonJay lives in a colder Conclusions.--Renewed attention to climate climatethan doesthe MexicanJay, it oftenini- change,human-caused or otherwise,has re- tiatesbreeding much earlier (e.g. February), ap- awakenedinterest in its possibleeffects on avi- parently relying mainly on stored pinyon anpopulations. Comparison of our results with seeds.Very early breedingplaces Pinyon Jays thosefrom otherlong-term studies, however, at risk to snowstormsand thus appearsto ex- hassuggested that generalizations about the ef- 934 Li ANDBROWN [Auk, Vol. 116 fectsof climatevariables on populations will be parentagein the Gray-breastedJay. Condor 97: difficult.Although the matteris far morecom- 503-511. plexthan the simplestatistical associations be- BROWN,J. H., t. J. VALONE, AND C. G. CURTIN. 1997. tween climateand reproductionthat we have Reorganizationof an arid ecosystemin response identified in this and in our previouspaper to recentclimate change. Proceedings of the Na- tional Academyof SciencesUSA 94:9729-9733. (Brown and Li 1996),perhaps knowledge of BROWN,J. L. 1994.Mexican Jay. In Thebirds of North thesephenomena will somedaybe moreuseful America, no. 118 (A. Poole, E Stettenheim,and than presentlyappreciated. Long-term studies E Gill, Eds.).Academy of NaturalSciences, Phil- of avian populationscurrently harbor a large adelphia,and AmericanOrnithologists' Union, amount of relevant data whose value in relation WashingtonD.C. to effectsof variationin climatehas just begun BROWN, J. L., AND N. BHAGABATI. 1998. Variation in to be realized.Studies that attemptto identify mass,wing andculmen with age,sex and season moreprecisely how climatevariation affects re- in the Mexican Jay (Aphelocomaultramarina). productionwill be neededin a variety of spe- Journalof Field Ornithology69:18-29. ciesto evaluatethe potentialeffects of global BROWN,J. L., ANDE.g. BROWN.1985. Ecological cor- warming on North Americanbirds (Brownet relatesof group size in a communallybreeding jay. Condor87:309-315. al. 1999).This paper is part of a plannedseries BROWN,J. L., AND E.g. BROWN.1990. Mexican Jays: of studieson the MexicanJay that may leadto Uncooperativebreeding. Pages 268-288 in Co- a more comprehensiveunderstanding of the operativebreeding in birds:Long-term studies complexrelationships between climate and the of ecologyand behavior(E B. Staceyand W. D. healthof avianpopulations. Koenig, Eds.). Cambridge University Press, Cambridge,United Kingdom. ACKNOWLEDGMENTS BROWN,J. L., E. R. BROWN,J. SEDRANSK,AND S. RIT- TER. 1997. Dominance,age and reproductive Our study would have been impossiblewithout successin a complexsociety: A long-termstudy the cooperationof the SouthwesternResearch Sta- of the MexicanJay. Auk 114:279-286. tion of the AmericanMuseum of Natural History. BROWN,J. L., AND S.-H. LI. 1996. Delayedeffect of The help of J. Rozen,V. Roth,W. and E. Sherbrooke, monsoonrains influenceslaying date of a pas- R. Morse,P. Limberger,C. Swartz,and other em- serinebird living in an arid environment.Con- ployeesof theseinstitutions was invaluable. For per- dor 98:879-884. missionto workon neighboring properties we thank BROWN, J. L., S.-H. LI, AND N. BHAGABATI. 1999. O. Bruhlman,M. Cazier,R. Mendez,E. Bagwell,M. Long-termtrend toward earlier breedingin an Pugsley,and the CoronadoNational Forest.We are Americanbird: A responseto globalwarming? extremelygrateful to the dedicatedfield workers Proceedingsof the NationalAcademy of Scienc- who havemade many of the observationsreported es USA 96:5565-5569. here.These range from undergraduates to post-docs. CRICK, H. P. Q., C. DUDLEY, D. E. GLUE, AND D. L. In chronologicalorder they includeL. G. Herlin, D. THOMSON.1997. UK birds are laying eggsear- Slaymaker,H. Alvarez, E. Wilkins, K. Zemanek, A. lier. Nature 388:526. Lewis,S. Corbett,P. Trail, D. Smith,S. Strahl,G. Lip- CURRY,g. L., ANDP. g. GRANT.1989. Demography of ford, C. and L. Barkan,J. Craig, A. Stewart,R. K. the cooperativelybreeding Galapagos Mocking- Johnson,L. Elliot, D. Siemens, E. Horvath, S. Stole- bird, Nesomimusparvulus, in a climaticallyvari- son,J. Harding,E Kleeman,T. Kennedy,J. Adams, J. ableenvironment. Journal of AnimalEcology 58.' Kaiser,P. Jablonski, L. Herbeck,K. Short,N. Bhaga- 441-463. bati, and E. Brown.We are gratefulto the U.S. Na- CURRy,R. L., AND P. R. GRANT.1991. Impact of cli- tional Institute of Mental Health and the U.S. Na- matic variationon GalapagosMockingbird so- tional Science Foundation for continuous financial cial organization.Pages 1333-1341 in Acta XX supportduring this project.We thankE. Brownand CongressusInternationalis Ornithologici (B. D. the reviewers,R. L. Curry and J.R. Walters,for com- Bell, Ed.). Christchurch, New Zealand, 1990. mentson the manuscript. New Zealand OrnithologicalCongress Trust Board,Wellington. LITERATURE CITED DITTAMI,J.P., AND E. GWINNER.1985. Annual cycles in theAfrican Stonechat Saxicola torquate axillaris BEEBEE,t. J. C. 1995.Amphibian breedingand cli- and their relationshipto environmentalfactors. mate. Nature 374:219-220. Journalof Zoology(London) 207:357-370. BOWEN,B., R. g. KOFORD,AND J. L. BROWN. 1995. 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World jaysinferred from cytochromeb genese- perature and egg-layingtrends. Nature 391:30- quences.Condor 99:490-502. 31. FORCHHAMMER,M. C., E. POST,AND N. C. STENSETH. MYNENI, R. B., C. D. KEELING,C. J. TUCKER,G. ASRAR, 1998. Breedingphenology and climate... Na- ANDR. R. NEMANI.1997. Increased plant growth ture 391:29-30. in the northernhigh latitudes from 1981to 1991. GRANT, g. R., AND P. R. GRANT. 1989. Evolutionary Nature 386:698-702. dynamicsof a natural population:The large Nix, H. A. 1976. Environmental control of breeding, CactusFinch of the Galapagos.University of post-breedingdispersal, and migrationof birds ChicagoPress, Chicago. in the Australianregion. Pages 272-305 in Pro- GRANT, P. R. 1985. Climatic fluctuations on the Ga- ceedingsof the 16th InternationalOrnithologi- cal Congress(H. J. Frith and J.H. Calaby,Eds.). lapagosIslands and their influenceon Darwin's Canberra,1974. AustralianAcademy of Science, finches.Pages 471-483 in Neotropicalornithol- Canberra. ogy (P.A. Buckley,M. S. Foster,E. S. Morton,R. PARMESAN,C. 1996. Climate and species'range. Na- S. Ridgely,and E G. Buckley,Eds.). Ornitholog- ture 382:765-766. ical MonographsNo. 36. PIPER,W. H. 1994.Courtship, copulation, nesting be- GRANT,P. R. 1986.Ecology and evolutionof Darwin's haviorand broodparasitism in the Venezuelan finches.Princeton University Press, Princeton, Stripe-backedWren. Condor 96:654-671. New Jersey. PITELKA,E A. 1945.Pterylography, molt, and agede- GRANT,P. R. 1987.The extraordinaryE1 Nino event terminationof Americanjays of the genusAphel- of 1982-83: Effects on Darwin's finches on Isla ocoma. Condor 47:229-260. Genovesa,Galapagos. Oikos 49:55-66. SINCLAIR,A. R. E. 1978.Factors affecting the food KEAST,J. A., AND A. J. MARSHALL.1954. The influ- supply and breeding seasonof residentbirds enceof droughtand rainfallon reproductionin and movements of Palaearctic migrants in a Australiandesert birds. Proceedings of the Roy- tropical African savannah.Ibis 120:480-497. al Societyof LondonSeries B 124:493-499. SKINNER,W. R., R. L. JEFFRIES,t. J. CARLETON,AND KOENIG,W. D., AND R. L. MUMME.1988. Population R. E ROCKWELL.1998. Prediction of reproductive success and failure in Lesser Snow Geese based ecologyof the cooperativelybreeding Acorn on early season climatic variables. Global Woodpecker.Princeton University Press, Prince- ChangeBiology 4:3-16. ton, New Jersey. TAYLOR, A. H., M. B. JORDAN,AND J. A. STEPHENS. LLOYD,M. 1960. The breeding of some S.W. Ec- 1998.Gulf Streamshifts following ENSO events. uadorian birds. Ibis 102:600-611. Nature 393:638. MARCHANT,S. 1959. The breeding seasonin S.W. WOOLFENDEN,G. E., AND J. W. FITZPATRICK.1984. Ecuadorian birds. Ibis 101:137-152. The FloridaScrub Jay: Demography of a coop- MARCHANT,S. 1960.The breedingof someS.W. Ec- erative-breeding bird. Princeton University uadorian birds. Ibis 102:349-382, 584-589. Press,Princeton, New Jersey. MARZLUFF,J. g., ANDR. P.BALDA. 1992. The Pinyon WREGE,P. H., AND S. t. EMLEN. 1991. Breeding sea- Jay:Behavioral ecology of a colonialand coop- sonality and reproductivesuccess of White- erative corvid. T and A D Poyser,London. frontedBee-eaters in Kenya.Auk 108:673-687. McCLEERY, R. H., AND C. M. PERRINS.1998 .... tern- Associate Editor: E. Greene 936 L• ANDBROWN [Auk, Vol. 116

APPENDIX.List of variablesused to assessthe effectsof weatheron reproductivesuccess in MexicanJays in southeastern Arizona.

Reproductive success 1. Fractionof femalesof age3+ years(breeding or not) recordedwith a nestand eggs. 2. Numberof nestlingsper bandedfemale of ages2 and 3 years,breeding or not. This is the numberof banding-agenestlings produced by femalesof theseages divided by the numberof femalesof theseages. 3. Numberof nestlings per banded female of age4 + years,breeding or not.This is thenumber of banding- agenestlings produced by femalesof theseages divided by thenumber of femalesof theseages. 4. Mean brood size at time of banding(14 to 15 days).Fledging occurs at 24 to 27 days. 5. Number of fledglingsper successfulnest (fledged at leastone young). 6. Numberof nestsper unit (flockor group)that reachedthe eggstage. 7. Numberof banding-agenestlings per unit. 8. Numberof nestsper unit thatreached banding age. 9. Number of fledglings(i.e. leavingnest) per unit. 10. Numberof fledglingsper nestwith eggs. 11. Fractionof neststhat reachthe ageof bandingfor femalesof age3+ years. Precipitation Precipitationin late spring,May and June,of sameyear. Precipitationin early spring,March and April, of sameyear Precipitationin previouswinter, December through February. Precipitationin previousautumn and winter, Septemberthrough February. Precipitationin the previousmonsoon, July and August. Precipitationin previousmonsoon, autumn and winter combined,July through February. Precipitationin previousmonsoon, autumn, winter and early springcombined, July through April. Precipitationin Decemberthrough February, two yearsearlier. Precipitationin Septemberthrough February, two yearsearlier. Precipitationin Septemberthrough November, two yearsearlier. Precipitationin monsoontwo yearsearlier Precipitationin monsoonand winter (July through February),two yearsearlier. Annual precipitationwas calculatedfrom Julythrough June. Temperature Minimum temperaturein early spring,March and April of sameyear. Minimum temperaturein precedingDecember through February. Minimum temperaturein precedingautumn, September, October, November. Maximum temperaturein previousmonsoon, July and August. Maximum temperaturein May and Juneof sameyear. Population characteristics Number of yearlings(second calendar year) per unit. Number of yearlingsand secondyear jays (third calendaryear) per unit. Number of adults (3+ years old; 4th calendaryear) per unit. Numberof adult malesper groupof agethree or older.Number of adult femalesper groupof agethree or older. Averagegroup size during the breedingseason on May 1. Fractionof malesin the populationof agethree or older.