Seasonal abundance of migrant shorebirds in
Baja California Peninsula, Mexico, and California, USA
ROBERTO CARMONA1,3,GORGONIO RUIZ-CAMPOS2 & GEORGINA BRABATA1
•UniversidadAutdnomade Baja CaliforniaSur, Departamentode BiologfaMarina,Apdo.Postal 19-B,La Paz,
Baja CaliforniaSur, C.P. 23080, Mexico. [email protected]
2Facultadde Ciencias;3Facultadde CienciasMarinas,UniversidadAutdnomade Baja California,Apdo,
Postal 1653, Ensenada, Baja California,C.P. 22800, Mexico.
U.S. mailingaddress: PMB 064, PO Box 189003-064, Coronado, California,92178, USA.
Carmona,R., Ruiz-Campos,G. & Brabata,G. 2004. Seasonalabundanceof migrant shorebirdsin Baja
CaliforniaPeninsula,Mexico, and California,USA. WaderStudyGroupBull. 105: 65-70.
Nearctic shorebirdsusethreemain flyways (Pacific, CentralandAtlantic) andthe largestnumbersare found
in the PacificFlyway, which includesthe Baja CaliforniaPeninsula.There, at variousdifferentsites,the
followingpatternsof abundance
havebeenobserved:
verylow numbersin summer;largenumbersin autumn;
low and stablenumbersin winter; and moderatenumbersin spring.In contrast,further north in California,
numbersduringspringmigrationareashighasor evenhigherthanthosein autumn.We contrastthedifferential
utilizationby shorebirds
of BajaCaliforniaduringspringandautumn.In autumn,southbound
migrantsarrive
at sitesin southernCaliforniaandthe upperGulf of California,andtheneithercontinuealongthe mainland
coastof Mexico or alongthe Baja CaliforniaPeninsula.Somecontinuemigratingfurthersouthwhile others
stayfor the winter.Duringthe returnmigrationin spring,modestnumbersoccurthroughoutthe peninsula,
with rathergreaternumbersin the north-west,probablyassociated
with the northwardmigrationthroughthe
extensive wetlands of Sonora and Sinaloa on the mainland coast.
INTRODUCTION
STUDY
AREA
AND METHODS
Of the 49 shorebirdspeciesthat breed in the Nearctic, 40
migrateto temperateandtropicalareasof Mexico, Central
and South America for the northernwinter (Myers et al.
1987).
At typicalshorebirdstopoverandwinteringsitesthroughoutthePacificFlyway,therearedistinctpatternsof seasonal
abundance.
In summer,numbersarevery low becausemost
are breedingin the north. In autumn,most sitesrecordthe
largestnumbersas first adultsand later juveniles migrate
south.Lowernumbersarepresentin winter,buttheyremain
relatively stable becauselittle movement takes place. In
spring,northwardmigrationleadsto increasednumbersas
birdsreturnto their breedingareas(Pageet al. 1979).
Of the three major shorebirdflyways in North America
(Pacific, Atlantic andCentral),the mostimportantin terms
of numbersis the Pacific Flyway (Myers et al. 1987) and
midway along the Pacific Flyway lies the 1,100-kin Baja
Californiapeninsula(Fig. 1). There, duringthe pastdecade,
systematicdataon shorebirdabundancehavebeencollected
for severaldifferent coastallocalities (e.g. Palacioset al.
1991, Gonzfilez1996, Carmona& Danemann,1998). These
studieshaveshownthatthepeninsulais particularlyimportant duringautumnmigrationandof lessimportanceduring
springmigration (Carmona 1997, Fernfindezet al. 1998,
Carmona& Danemann1998). In this paper, we presenta
generalhypothesisto explainthepatternsof seasonalabundanceof migrantshorebirds
alongtheBaja Californiapenin-
We have analysedshorebirdabundancedata for five sites
alongthe south-westcoastof theUnitedStatesandin northwestMexicothathavebeenpreviouslyidentifiedassupporting importantpopulations.
Theseare HumboldtBay (north
California),the CentralValley of Californiaand threesites
in the Baja California peninsula:Punta Banda, Guerrero
Negro andEnsenadade La Paz (Fig. 1).
Although there are many siteswith extensiveintertidal
mudflatsalongthe PacificFlyway in California,we selected
just two (Humboldt Bay and Central Valley) to represent
thosethataretypical- in termsof bothhabitatandshorebird
abundance- of the onesusedby migrantshorebirdsin that
area (Colwell 1994, Shuford et al. 1998).
All five sitesare influencedby a fairly stable,tropical,
high-pressurebelt that bringsdry winds (Coda 1997). Californiaandnorth-westBaja Californiahavea Mediterraneantype climate,while the southernpart of thepeninsula(around
30øN) has an arid climate, except for its southerntip (the
Cape region) which is subtropical(Garcia 1981).
The largestareasof intertidal mudflatsin California are
found in Humboldt, San Francisco and San Diego bays
(Peinadoet al. 1995), while on the west coastof Baja California there are mudflatsin the bays of Todos Santos,San
Quint/n, Ojo de Liebre-GuerreroNegro, San Ignacio and
Magdalena. On the east coastthe largestmudflats are at
Ensenada de La Paz in Bahia de La Paz (Carmona et al.
1994).
sula.
65
Buffetin 105 December
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Wader Study Group Bulletin
I
110 ø
N
1
C•
© United
35 ø
States
PB
--25
ø
Mexico
Pacific
Ocean
Fig, 1. Study area and shorebirdcensussites. (HB = HumboldtBay (northCalifornia);CV = Central Valley
of California;PB = PuntaBanda(north-westBajaCalifornia);GN = GuerreroNegro(mid-westBaja California);
and LP = Ensenada de La Paz (south Baja California.))
Our analysisrelatesto datafrom six studiesduring19911998. Two were carried out at Estero Punta Banda (north-
westBaja California)by Palacioset al. (1991) andGonzfilez
(1996) and coveredsamplingperiodsof sevenmonthsand
oneyear, respectively.
The studysites(Fig. 1) were: (1) HumboldtBay, north
California(38øN, Colwell 1994); (2) CentralValley of California (35øN, Shufordet al. 1998); (3) PuntaBanda,northwestBaja California(31øN, Palacioset al. 1991, Gonzfilez
1996); (4) GuerreroNegro (28øN, Carmona& Danemann
1998); and (5) Ensenadade La Paz (24øN, Carmona 1997).
The frequencyof samplingat eachsitewasrathervariable
anddependedontheparticularobjectivesof eachstudy(see
Appendix).Therefore,to facilitatecomparisons
onbothspatial andtemporalscales(i.e. sitesandseasons)
we calculated
the monthly averageabundanceof each speciesand then
convertedto percentages
on the basisthat the aggregateof
the monthlyaveragesfor eachsitewas 100%.
The datafor HumboldtBay includea numberof separate
countsfor siteswithin the bay (Colwell 1994), so for each
monththesewere aggregated.
For two sites,HumboldtBay
andthe CentralValley of California, therewere no datafor
severalmonths.Therefore,for these,the datawere analysed
on a seasonalratherthan monthlybasisas follows: winter
(December-February), spring (March-April), summer
(May-June) and autumn (July-November). Even on this
basis,therewere no datafor HumboldtBay or CentralValley for summer(seeAppendix).
becauseno data are availablefor Central Valley and HumboldtBay), and(2) seasonal
abundance
of the speciesamong
thethreesiteshavingdatafor all four seasons.
In bothcases,
whenthe independence
hypothesis
wasrejectedin thefirst
test, we determined which site contributed the most to this
rejection,andthenremovedit from the analysis.This process was repeateduntil the hypothesiswas accepted(and
thereforea homogeneous
groupwasformed).In addition,the
excluded sites from the initial analysis were compared
mutually,restartingthe procedure(ct = 0.05 in all the cases;
Zar 1999).
RESULTS
In our monthlyanalysis,HumboldtBay, California,hadthe
greatestabundanceof shorebirds
in autumn(November)and
spring(April), with lowernumbersin winter(February)(Fig.
2a). In the CentralValley of California, abundanceincreased
graduallyfrom autumn(November)to winter (January)to
spring(April) (Fig. 2b). At Punta Banda, Baja California,
therewasgreaterevenness
in seasonalabundances,
with only
smallincreasesin autumn(October& November)andspring
(March) anda notabledecreasein summer(May & June;Fig.
2c). At Guerrero Negro, highest abundanceoccurredin
autumn(August & September)with much lower but more
stablenumbersin winter (December-February)and a small
increasein spring(April; Fig. 2d). At Ensenadade La Paz,
the highestabundanceoccurredin autumn(September&
An independence
Z2test(ct= 0.05;Zar, 1999)wasused October) and winter (January),with no increasein spring
to comparethe abundances
of shorebirdsat PuntaBandaas (March & April; Fig. 2e).
reportedby Palacioset al. (1991) andGonz•lez(1996).LikeIn our analysisby season(Fig. 3), HumboldtBay, Caliwise, two types of comparisonsof shorebirdabundance fornia, had the largestnumbersduring springand autumn
amongsitesweremadeusingZ2 tests:(1) seasonal
abun- migration,with only a smalldecreasein winter (Fig. 3a). In
danceof the speciesamongthefive sites(exceptfor summer contrast,numbersin the CentralValley of Californiawere
Buffetin
105 December
2004
Carmonaet al.:Seasonalabundanceof migrantshorebirdsin Baja CaliforniaPeninsula,Mexico,and California,USA
Fig.2. Meanmonthly
relativeabundance
(%)forthefivestudysites.
30 4
Fig.3. Meanseasonal
relative
abundance
(%)forthefivestudysites.
a. Humboldt Bay
a. Humboldt Bay
15
10
0
SPRING
Aug. Sept, Oct, Nov. Dec, Jan. Feb, Mar. Apr. May June July
SUMMER
AUTUMN
WINTER
40
b. Central Valley
b, CentralValley
35
3O
• 25
lO
5
ND
ND
ND
ND
o
SPRING
Aug. SepL Oct. Nov. Dec. Jan. Feb. Mar, Apr. May June July
SUMMER
AUTUMN
WINTER
40
14 1
c. Punta Bande
Punta Banda
35
30
• 25
!5
10
5
0
SPRING
Aug. Sept. Oct, Nov. Dec. Jan. Feb. Mar. Apr. May June July
SUMMER
AUTUMN
W INTER
60 •
d. Guen•emNegro
d. Guerrero Negro
2oI
to
o
Aug. Sept. Oct. Nov. Dec. Jan, Feb. Mar. Apr. May June July
SPRING
SUMMER
AUTUMN
25
WINTER
e. La Par
e. La Paz
20
5
0
Aug. Sept. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July
67
SPRING
SUMMER
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Table 1. Resultsof •,2testson the relativeseasonalabundanceof shorebirdsat five sitesalongthe Pacific
shorebirdflyway(excludingsummer)(HB = HumboldtBay, CV = CentralValley, PB = Punta Banda,GN = Guerrero
Negro, LP = Ensenadade La Paz).
Groupcompared
Z2
P
d.f.
Site contributing most to the
rejection of the null hypothesis
% of the site more
heterogeneous
HB-CV-PB-GN-LP
61.9
<0.001
8
LP
48
HB-CV-PB-GN
29.7
<0.001
6
GN
57
HB-CV-PB
8.2
0.085
4
-
-
GN-LP
5.0
0.083
2
-
-
Table 2. Resultsof •,2tests on the relativeseasonalrelativeabundanceof shorebirdsat three sites in the
peninsulaof Baja California,Mexico(includingsummer)(PB = PuntaBanda,GN = GuerreroNegro,LP = Ensenada
de La Paz).
Groupcompared
PB-GN-LP
GN-LP
Z2
42.8
P
d.f.
Site contributing most to the
rejection of the null hypothesis
% of the site more
heterogeneous
<0.001
6
PB
58
0.18
3
-
-
4.9
Table 3. Seasonal peak counts of shorebirdsat the five study sites (ND = no data available).
Humbolt Bay Central Valley
Punta Banda Guerrero Negro
Ensenada De La Paz
Winter (Dec.-Feb.)
Spring (March-Apr.)
66,000
83,000
374,000
393,000
1,100
800
33,000
10,000
12,000
1,700
Summer(May-June)
Autumn (July-Nov.)
ND
36,000
ND
177,000
600
1,300
1,900
76,000
470
13,000
greatestin spring, and there were more in winter than in
autumn(Fig. 3b). In Baja California, PuntaPandaheld the
largestnumbersin winter and spring,fewer in autumnand
only smallnumbersin summer(Fig. 3c). At GuerreroNegro,
highestnumbersoccurredin autumn,relatively small numbersin spring,but thewinteringpopulationwasfairly high;
onlytiny numbersoccurredin summer(Fig. 3d). The pattern
was similar at Ensenadade La Paz with highernumbersin
autumnthanin springand only smallnumbersin summer.
However,winteringnumberswere greaterthanat any other
time of year (Fig. 3e).
Comparingthe numbersreportedfor Punta Banda by
Palacioset al. (1991) and Gonzglez (1996), we found that
The sameresultemergeswhenthe datafor thethreesites
in Baja California(includingsummer)areanalysedtogether:
the patternat PuntaBandais shownto be significantlydifferentfrom the othertwo, GuerreroNegro andEnsenadade
La Paz, which are similar (Table 2).
DISCUSSION
Our studyfocuseson patternsof spatialandtemporalabundance,notabsolutenumbers.It is importantto rememberthis
becauseabsolutenumbersusinga sitemay be muchgreater
than those counted at any one time becauseof turnover,
especiallyduringspringandautumnmigration.However,to
therewereno significant
differences
(Z2 = 3.87,d.f. = 5, give an indicationof the scaleof shorebirdmigrationthrough
p = 0.43). However,we decidedto usethedataof Gonzglez the five studysiteswe presentdata on the peakcountsfor
(1996) only becauseit covereda full year ratherthanjust each season(Table 3).
seven months.
As expected,the lowestnumbersat the threePeninsula
Comparingseasonalnumbersacrossall five studysites siteswererecordedin summerwhenmostnearcticmigrants
exceptsummer(Table 1), it is evidentthatEnsenadade La arebreedingin thefar north(Pageet al. 1979,Rappoleet al.
Paz andGuerreroNegrocontributedthemostheterogeneity. 1993). However, relatively more were recordedat Punta
Once thesetwo sitesare removedfrom the analysis,those Bandain the northof the peninsulathanat the two sitesin
remaining(HumboldtBay, CentralValley andPuntaBanda) the south.This might reflect a tendencyfor non-breeding
can be consideredhomogeneous.Similarly, comparison birds (mainly one-year-olds)to over-summercloseto the
betweenGuerreroNegro andEnsenadade La Paz indicates nestingareas(McNeil et al. 1994).
It is noteworthythat at all sites(exceptGuerreroNegro)
thesesitesare similar(Table 1). In summary,the statistical
testsindicatethat there are two homogeneous
groups:the thewinteringpopulationwasvery similarto thepeakpoputhree northern sites (Humboldt Bay, Central Valley and lation in the rest of the year. This may meanthat in winter
PuntaBanda)andthe two peninsularsites(GuerreroNegro eachsiteis usedto carryingcapacity(thoughdetailedfeedand Ensenadade La Paz).
ing studieswouldbe neededto confirmthis).
• Bulletin
105
December
2004
Carmonaet al.:Seasonalabundanceof migrantshorebirdsin Baja CaliforniaPeninsula,Mexico,and California,USA
...
69
i
110 ø
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•
United
•
States
United
States
Mexico
Pacific
Ocean
•25
ø
Mexico
Pacific
Ocean
Autumn
Spring
Fig. 4. Modelof migratory
movements
of shorebirds
in autumnand springalongthe coastsof the south-west
of the United States of America and north-west Mexico.
In spring,relativelyfewer birdsusedthe two sitesin the
south of Baja California than Punta Banda or the sites in
California, but in autumnthe patternwas almostreversed
efforts in the southernparts of the peninsulashouldbe
focussedon the seasonsof autumn and winter; while in the
northmoreeffort shouldbe putinto winterandspring.
with largenumbers
passingthroughGuerreroNegroandLa
The Baja Californiapeninsulais animportantpartof the
Paz (a patternconfirmedby our statisticalanalysis).Our Pacificshorebirdflyway, especiallyin autumn.Moreover
hypothesis
is thatthisindicatesdifferentialuseof theBaja numericallylarge groupsof shorebirdswinter there. HowCaliforniapeninsulain springandautumnwith manypas- ever,it remainsratherisolatedduringthe springmigration.
sagebirdsmovingsouthwards
downthepeninsula
in autumn
(andthenonwardsalongthe southernMexican coasttowards
SouthAmerica)butmostlyfollowingthe mainlandcoastof
Mexico in spring(Fig. 4).
Thuswe suggest
thatin autumnmigrantscomingfromthe
northtravelalongthe Pacificcoastuntil they arriveat the
base- the northernend- of the Baja Californiapeninsula.
There they havetwo options:to follow the mainlandcoast
of Mexico through Sonora and Sinaloa or to follow the
peninsulaof Baja California.Sometake oneroute,somethe
other.Thosethatfollow thepeninsulapassthroughto winteringareasfurthersouthwhile othersstayfor thewinter.In
spring,the northwardmovementof birdsthathavewintered
in theBajaCaliforniapeninsula
leadsto onlya modestpassagethroughLa Paz andGuerreroNegro andfew birds,if
ACKNOWLEDGMENTS
We thankthe personnelof the aquaticbirdsprogramof the
Universidad
Aut6noma
deBajaCalifornia
Sur,fortheirsupportandhelp in thefield. The first authorthanksCONACYT
for thedoctoralgrantthatpermittedthe accomplishment
of
thepresentwork. We alsothankNils Warnockfor comments
on an earlierdraftthat substantially
improvedourpaper.
REFERENCES
American Ornithologists'Union (A.O.U.). 1998. Checklist of North
American birds'.Allen Press,Lawrence, Kansas.
Carmona,R. 1997.Riquezaespecifica,
distribuci6n
y abundancia
deaves
acufiticasen la Ensenadade La Paz, B.C.S., Mdxico. Unpublished
any, passthroughfrom winteringsitesto the south.At this
Final Report CONABIO.
time, mostpassagemigrantsfollow the mainlandcoastof Carmona,R. & G. Danemann.1998.Distribuci6nespaciotemporal
de
Mexico.Thereareprobablytwomainreasons
why theydo
avesen la salinade GuerreroNegro, B.C.S., Mdxico. CienciasMarinas 24: 389-408.
this.First,followingthecoastmaybesaferthanmakingthe
250 km sea-crossing
to the southerntip of the peninsula. Carmona, R., J. Guzmfin, S. Ramirez & G. Fernfindez. 1994. Breeding waterbirdsof La Paz Bay, Baja California Sur, Mdxico. Western
Second,thereare somelargewetlandsalongthe coastsof
Birds 25: 151-157.
Sinaloaandsouthern
Sonora(Engiliset al. 1998)thatprob- Colwell,M.A. 1994.Shorebirds
of HumboldtBay,California:abundance
ablyaffordgoodfeedingopportunities
for shorebirds
using
estimatesand conservation
implications.WesternBirds 25:137-145.
this route.
We hopeourresultswill providea focusfor futurestud-
Coria,R.B. 1997.Climatologia.
pp.27-34.In: L. Arriaga& R. Rodriguez
(eds). Los oasis de la Peninsula de Baja California. Centro de
InvestigacionesBio16gicasdel Noroeste,M6xico.
iesof thephenology
of shorebirds
passingthroughtheBaja
Californiapeninsula
aswell asfor thetargetingof conserva- Cox, G.W. 1985.The evolutionof avianmigrationsystemsbetweentemperateandtropicalregionsof the new world.AmericanNat. 126:451tionaction.Futurestudies
shouldtestourhypothesis
for sev474.
eral of the commonerspecies,usinga varietyof methods Engilis,A., L.W. Oring,J. Cartera, W. Nelson& A. Martlnez-L6pez.
including systematiccounts,banding etc. Conservation
1998. Shorebird surveysin EnsenadaPabellonesand Bahia Santa
Bulletin
105Dec
•
70
Wader Study Group Bulletin
Maria, Sinaloa, Mdxico: critical winter habitats for Pacific flyway
shorebirds. Wilson Bull. lie:
332-341.
Fernfindez, G., R. Carmona & H. de la Cueva. 1998. Abundance and
seasonalvariation of westernsandpipers(Calidris mauri) in B.C.S.,
Mexico.
Southwest.
Nat. 43: 57-61.
Garcia, E. 1981. Modificacidn al sistemade clasificacidnclimdticade
Koppen(para adaptarloalas condiciones
de la ReptiblicaMexicana.
Mdxico
D.F.
Gonzfilez, G.S. 1996. Aves playeras migratorias del Estero de Punta
Banda,Baja California,a travdsde un cicloanual.Bachelor'sThesis.
UniversidadAut6noma de Baja California, Mdxico.
Hayman, P., J. Marchant & T. Prater. 1988. Shorebirds:an identification guide. Houghton Mifflin, USA.
Massey, W.B. & E. Palacios. 1994. Avifauna of the wetlandsof Baja
California, Mdxico: current status.Studiesin Avian Biology 15: 4557.
McNeil, R., M.T. Diaz & A. Villeneuve. 1994. The mysteryof shorebird
over-summering:a new hypothesis.Ardea 82: 143-152.
Myers, J.P., R.I.G. Morrison, P.Z. Antas, B.A. Harrington, T.E.
Lovejoy, M. Sallaberry, S.E. Senner & A. Tarak. 1987. Conservation strategyfor migratory species.American Scientist75: 19-26.
Page, G., L. Stenzel & C. Wolfe. 1979. Aspectsof the occurrenceof
shorebirds
on a centralCaliforniaestuary.pp. 15-32. In: F. Pitelka(ed.).
Shorebirdsin marine environments.
Studiesin Avian Biology. USA.
Palacios,E., A. Escofet & D.H. Loya-Salinas. 1991. E1Esterode Punta
Banda, B.C., M•xico como eslab6n del "Corredor del Pacffico":
abundanciade avesplayeras.CienciasMarinas 17: 109-130.
Peinado,
M., F. Alcarfiz,J.L. Aguirre,J. Delgadillo& J. J•lvarez.
1995. Similarity of zonation within California-Baja California and
mediterranean
salt marshes. Southwest. Nat. 40: 388-405.
Rappole, J.H., E.S. Morton., T.E. Levejoy & J.L. Ruos. 1993. Aves
migratoriasnetirticasen losneotr6picos.ConservationandResearch
Center, National Zoological Park, Virginia.
Shuford, W.D., G.W. Page & J.E. Kjelmyr. 1998. Patternsand dynamics
of shorebirds
useof California'sCentralValley. Condorlee: 227-244.
Zar, H.J. 1999. Biostatisticalanalysis.Prentice Hall, New Jersey.
APPENDIX
Number of monthlycountscarried out in the shorebirdstudiesalong the Pacific Flyway on which our analyses are based.
The yearis givenin parentheses:
(), followedin the caseof Colwell
bbythe numberof sitesvisitedin braces:{ }.
(BC = Baja California,CA -- California,ND = No data available)
PuntaBandaa
HumboldtBayb
PuntaBandac
BC
CA
BC
BC
2(89)
2(89)
2(89)
3(89)
ND
ND
ND
ND
ND
ND
3(88)
2(88)
ND
1(91){37};1(92){32};1(93){30}
ND
1(91){30};1(92){26};1(93){27}
ND
ND
ND
1(91) {22 }; 1(92){29 }
1(90) { 32}
3(90)
3(90)
3(90)
4(90)
3(90)
3(90)
3(90)
3(90)
3(89)
4(89)
4(89)
3(89)
2(97)
2(97)
2(97)
2(97)
2(97)
2(97)
2(97)
2(97)
2(97)
2(97)
2(96)
2(96)
January
February
March
April
May
June
July
August
September
October
November
December
ND
1(90){33};1(91){31 };1(92){27}
ND
Ensenadade La Pazd GuerreroNegroe
BC
1(96)
1(96)
1(96)
1(96)
1(96)
1(96)
1(96)
1(96)
1(96)
ND
1(96)
1(95);1(96)
a Palacios
et al. 1991;bColwell1994;c Gonzglez1996;dCarmona1997;eCarmona& Danemann
1998;f Shufordet al. 1998.
Bulletin
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2004
CentralValleyf
CA
1(93); 1(94); 1(95)
ND
ND
1(92);1(93);1(94)
ND
1(92); 1(93); 1(94)
ND
ND
1(93);1(94)
ND