Taxonomy of the kelp gull Larus dominicanus Lichtenstein inferred from biometrics and wing plumage pattern, including two previously undescribed subspecies

22 Pages·2002·10.4 MB·English

&
Normand David Michel Gosselin 50 Bull. B.O.C. 2002 122(1)
Taxonomy of the Kelp Gull Larus dominicanus
Lichtenstein inferred from biometrics and wing
plumage pattern, including two previously
undescribed subspecies
by FredericJiguet
Received16January2001
The Kelp Gull's Larus dominicanus Lichtenstein extensive range discontinuously
encircles the globe in a broad subantarctic belt, where breeding occurs mainly on
isolated islands. By contrast, Kelp Gulls also breed on continental South America
where their latitudinal range extends over c. 60°, reaching a northern limit at the
equator(Higgins&Davies 1996, Haase 1996). ThespeciesbreedsinSouthAmerica
(includingthe Falklands and South Georgia, andnorthto Ecuadoronthewestcoast
and in Brazil to 26°S on the east coast), Antarctica (and South Shetlands, South
Orkneys, South Sandwich Islands),NewZealand,NewZealandislands, Macquarie
Island, Australia, southern Africa, southern Madagascar and subantarctic Indian
Ocean (Kerguelen, Crozet, Heard, Marion and Prince Edward Islands). There have
been no detailed studies ofgeographical variation in the species, and populations
have not proved to separate clearly into subspecies on the basis ofmeasurements
&
(Dwight 1925),butHiggins Davies(1996)suggestedthatlatitudinalandlongitudinal
analyses of measurements might reveal clines. Kinsky (1963) considered, from
measurements ofNew Zealand birds, that all measurements increase up to three
years old; though often quoted in later literature, this was nevertested statistically.
TheKelpGullhascustomarilybeendividedintotwosubspecies(Brooke&Cooper
1979):L. d. vetulabreedingin SouthAfricaandNamibiaandrecognisedbyitslarge
size and dark iris in breeding adults (Brooke & Cooper 1979), and the nominate
subspecies breeding in all other locations. Birds from Madagascar are ofunknown
status(Brooke&Cooper 1979,Morris&Hawkins 1998, Sinclair&Langrand 1998).
Previousstudiesofgeographicalvariationinthisspecieshaveconsideredbiometrics
andiriscolouration,butnotplumagecharacters.However,Jiguetetal.(2001)recently
illustratedtheexistenceofmarkedgeographicalvariationinprimarypatternandbare
parts colouration. Nominate birds show the classic wing pattern ofthe species: one
whitemirror-onthelongestoutermostprimary-andanaverageoftwowhitetongues
on medianprimaries, isolatingtheblacktip ofthe featheronthe innerweb.
In this study, I first investigate the relationship between biometrics and age in
onepopulation,inordertotestKinsky's(1963)hypothesis. Ialsoinvestigatelatitudinal
clinal variation ofbiometrics for populations inhabiting South America. I finally
segregatebirdsofdifferentgeographical originusingbiometricsalone,orbiometrics
and wing pattern, and identify groups that could correspond to distinct subspecies.
1
Frederic Jiguet 5 Bull. B.O.C. 2002 122(1)
Materialandmethods
Study material
Birds used in the analyses were the 243 specimens held at the Museum national
d'Histoirenaturelle,Paris,France(MNHN)andthe NaturalHistoryMuseum,Tring,
UK(BMNH). Eachbirdwasreferencedaccordingtothelocalityofcollection.Notall
specimenswere sexed, so sample sizesusedinthe analysesvaryaccordingwhether
sex is taken into account. One specimenpreserved atthe BMNH, saidto have been
collected in South Africa, was part ofthe Meinertzhagen collection (registration n°
1965/m/3982). Duetoconcernsoverthemis-labellingofMeinertzhagenspecimens
(Knox 1993),thiswasexcludedfromfurtheranalysesonbiometrics,butwasincluded
inprincipal components anddiscriminantanalysestoverify its geographical origin.
Biometrics and wingpattern
Maximumwingchord,tarsuslength,culmenlength,billdepthatgonysandbilldepth
atthebaseofthenostrilsweremeasured(nearestmm)bytheauthoronallspecimens.
Birdsinactivemoultofthelongestprimarieswereexcludedfromanalysesinvolving
winglength.Twowingplumage characters ofadultbirdswererecorded(Fig. 1): (1)
the number of white mirrors on the two longest primaries (primaries numbered
decendantly; recorded as 1 ifpresent on P10 only; 1.5 ifpresent on P10 and very
restrictedonP9; 2 ifobviouslypresentonbothP9andPI0); (2)thenumberofwhite
tonguesbetweenblacktipandsootyblackbaseonmedianprimariesfromP4outwards
(range 1-3).
Age-related variations in biometrics
Totestwhethersize increaseswithage(Kinsky 1963), onlymalesfrom subantarctic
IndianOceanislandsprovidedanadequatesampleofknown-agebirds.Twenty-four
birds available were 7 first-, 3 second-, 5 third-years and 9 adults. Theirbiometrics
were comparedusingKruskal-Wallistests.
Latitudinal variations in biometrics
Clinal variations ofbiometrics (tarsus andwing lengths) were sought in Kelp Gulls
fromSouthAmerica,involvingbirdscollectedfrom6°S(LobosdeTierra,offPeru)to
56°S (Cape Horn, Patagonia), and includingthe Falklands and SouthGeorgia. Only
adult birds were considered to limit the inclusion ofvagrant birds, as non-breeding
immature birds are more inclined to disperse than adults (Higgins & Davies 1996).
Pearson's correlation coefficients between biometric variables and latitude of
collectionwerecalculatedforeachsex.
Geographical variations
Univariate and descriptive statistics (Analysis of Variance) of biometrics and
multivariatestatistics(PrincipalComponentsAnalysis, MultipleAnalysisofVariance
andDiscriminantAnalysis)ofbiometricsaloneorbothbiometricsandwingplumage
variableswere used to look forgeographical variations.
Frederic Jiguet 52 Bull. B.O.C. 2002 122(1)
Figure 1. Outer primary pattern (from P5 to P10, primaries numbered descendantly) of adult
Kelp Gulls Larus dominicanus from different geographical origins, to show how the mirror and
tongue scores were recorded, a) a nominate dominicanus from eastern South America (mirror
score MS 1, tongue score TS 2); b) an Antarctic bird (MS 1, TS 3); c) a Malagasy bird (MS 1, TS
1); d) a bird from Kerguelen (MS 2, TS 3)
I performedtwo different Principal ComponentAnalyses (PCA). InafirstPCA,
I considered onlythe five biometricvariables, forbirds atleast3 years old(as some
measurements increase until this age, see Kinsky (1963) and below). In a second
PCA, I includedboth biometrics andwingplumage variables foradults only.
)
Frederic Jiguet 53 Bull. B.O.C. 2002. 122(1
I further used parametric Discriminant Analysis and an error rate estimate in
classification - thejack-knife cross-validation technique (Wilkinson 1990) - to test
howbirdsfromgeographicallydistinctoriginsdiscriminate. Thistechniqueprovides
an upper limit of error count estimates. The jack-knife classification matrix uses
functions computed from all data except the case being classified. I performed two
different discriminant analyses involving only adult birds. The first involved only
biometrics,thesecondbiometricsandplumagevariables. Thelocationsofcollection
were grouped into ten different global areas that are well separated geographically:
southernAfrica,easternSouthAmerica(from23°to34°S:northArgentina,Braziland
Paraguay,wherethenominateform/,, d. dominicanusissupposedtooccur),western
SouthAmerica(from6°to41°S: Peru,northandcentralChile),Patagonia(from51°to
56°S: southernArgentinaandsouthernChile,i.e. southernSouthAmerica),Falkland
Islands(52°S),SouthGeorgia(55°S),Antarctica(64-65°S),NewZealand,Madagascar
andsubantarcticIndianOceanislands(Kerguelen,CrozetandHeardIslands). Adult
birds of both sexes and of unknown sex were used together in the discriminant
analyses.Therefore,populationsthatwouldhighlydiscriminateshouldbecontrolled
for sex-ratio, to verifythat they are nottotally biased towards one sex, and that the
statisticalmodelisnotmerelydiscriminatingmalesandfemales.Unfortunately,sample
sizesweretoo smallto separate sexes inthemultivariate analyses.
AllstatisticalanalyseswereperformedusingSYSTAT(Wilkinson 1990). Statistical
testswere consideredsignificantatP<0.05.
Results
Age-relatedandlatitudinal variations in Kelp Gullbiometrics
ThebiometricsofmalesofdifferentagesfromthesubantarcticIndianOceanislands
revealedthatneitherwingchord(df=3;H=3.45,P=0.327),nortarsus(H=0.94,P=
0.816)norculmenlength(H=4.94,P=0.176)variedwithage.Onlybilldepthincreased
withage(H= 12.68,P=0.005 fordepthatgonys,andH= 10.83,P=0.013 fordepthat
nostrils), butno significantdifferencesremainedbetween 3-yearoldandadultbirds
(df= 1;H=3.24,P>0.1 forgonys,andH=0.04,P>0.8fornostrils).Forthesereasons,
furtherunivariate analyses involvingwingchord, tarsusandculmen lengths include
data on birds ofall ages mixed (from fully grownjuveniles to adults), while those
involvingbilldepthsincludeonlydataonbirdsatleastthreeyearsold.Allmultivariate
analyses use only data on adult birds.
Pearson's correlation coefficients between biometric variables and latitude of
collection for each sex ofKelp Gull in South America showed that culmen length
variedsignificantlywithlatitudeinbothmalesandfemales,withthe longestculmen
in the north, the shortest in the south (Table 1). Wing chord was not significantly
relatedto latitude, whiletarsuswasweakly so forfemales.
Biometrics, wingplumagepattern andgeographicallocation oforigin
I found differences in biometrics when comparing birds from different populations
usingunivariatestatistics,withmostdifferencesconcerningwingandculmenlengths
Frederic Jiguet 54 Bull B.O.C. 2002 122(1)
TABLE
1
Pearson's correlation coefficient between latitude and biometric variables in adult male
(n = 33) and female (n = 22) Kelp Gulls of South America, from Peru to Patagonia
(including the Falklands and South Georgia). P values are two-tailed,
and values in bold are still significant after Bonferroni correction.
Male Female
Variable r P r P
Wing -0.085 n.s. -0.119 n.s.
Tarsus -0.145 n.s. -0.540 < 0.01
Exposed culmen -0.619 < 0.001 -0.637 < 0.002
Gonys depth 0.098 n.s. -0.022 n.s.
Nostrils depth 0.088 n.s. 0.102 n.s.
(Table2; seealsoTable6).APrincipalComponentsAnalysis,performedtovisualise
thegeographicalvariationsinthefivebiometricvariablesforadults(Table3),didnot
suggest a clear separation between birds from different origins, orbetween already
recognised subspecies, with large overlaps between all ofthem (Fig. 2a). However
Antarcticbirdsappearedtoseparatewellonagraphwithprincipalcomponents 1 and
3 as axes(Fig. 2b). I furtherperformedaDiscriminantAnalysis, consideringtheten
=
groups ofgeographically different origins as differentapriorigroups (Table 4; n
TABLE 2
Univariate tests comparing biometrics of Kelp Gulls from different populations. See Table 7
for details on biometrics ofall populations considered and their name abbreviations.
Sex Statistic1 P differing populations 2
Male Wing length F =H-3 < 0.001 Ju/Do, Ju/Pa, Ju/Au, Me/Au, Au/Nz, Au/Fa,
9,99 Au/SA
Tarsus length F„02 = 3-55 0.001 Ju/Ve
Exposed culmen F„o = 15.5 < 0.001 Ju/Ve, Ju/SA, Ju/Me, Me/SG, Do/Fa, Do/SG,
2
Do/Au, Au/Ve, Au/SA, Fa/SG, Fa/SA, Fa/
Ve,SG/Ve,SG/Nz,SG/Pa,SG/SA
Gonys depth F = 2 57 0.014 3
,59 -
Nostril depth F,59 = 3-21 0.0303
Female Wing length F = 6.89 < 0.001 Ju/Pa, Ju/Au, Au/Do, Au/Ve, Au/SA, Au/Fa
9g3
Tarsus length F = 2 31 0.023 3
,84 -
Exposed culmen p = 10.8 < 0.001 Ju/Ve, Ju/SA, Ju/Me, Me/Fa, Me/SG, Ve/Fa,
9,3
Ve/SG,SA/Fa,SA/SG
Gonys depth F = 2.35 0.026 3
953
Nostril depth F = 0.85 > 0.5
953
1. Comparisons using One-way ANOVA
2. Using Scheffe a posteriori test (significance level at 0.05)
3. Not significant after Bonferroni correction
Frederic Jiguet 55 Bull B.O.C. 2002 122(1)
2
-i
O Q
Eastern S. America
1 a Western S. America
+ Patagonia
* Antarctica
oCN a Falklands
A
Q_ (g) a South Georgia
••
South Africa
• A+ « A
o Madagascar
• sub. Indian Ocean
- NewZealand
Meinertzhagen spec.
<8>
-4 2 4
PC1
A-
O c A
Eastern S. America
a Western S. America
A
A <A "o + Patagonia
A
• I 00 # / g-A x Antarctica
•« A- A *AA 9 A
OCO • A ®<.^ - O + O^ + Falklands
Q. A +A A A a South Georgia
•• •
1 - South Africa
• • A+ # A • + Madagascar
•
1 • sub. Indian Ocean
• •
- New Zealand
A
Meinertzhagen spec.
I i 1 1
-4 -2 2
PC1
Figure 2. Scatter plots ofthe first three Principal Components by pairs (a: PCI and PC2, b: PCI
and PC3), resulting from a PCA performed on 131 adult Kelp Gulls, using five biometric characters.
The Antarctic birds are individualized on both figures.
Frederic Jiguet 56 Bull. B.O.C. 2002 122(1)
TABLE
3
Results ofa Principal Components Analysis performed on 131 adult Kelp Gulls, using five
biometric characters. PCI is positively correlated to all biometric variables, while PC2 is
negatively correlated to the bill depths measures and positively to the other three measures.
PC 1 PC 2 PC 3
Eigen value 3.169 0.859 0.483
% variance explained 63.39 17.18 9.66
Cumulative % 63.39 80.57 90.23
Correlation with axis
:
- Wing 0.748 0.348 -0.532
- Tarsus 0.799 0.315 0.056
- Culmen 0.741 0.411 0.443
- Depth at gonys 0.836 -0.499 0.014
- Depth at nostril 0.850 -0.470 0.016
130 individuals; five biometric variables only). Multivariate Analysis ofVariance
washighlysignificant(Wilk's Lambda=0.120, F= 7.05, df=45, P<0.001). Using
resubstitution and error count estimates, no individual from Antarctica and South
Georgia, and only 12% ofMalagasy birds, were mis-classified. As resubstitution
gives an optimistic estimate oferror rate, we also used cross-validation error rate
estimate(withajack-knifeclassificationmatrix),whichagaincorrectlyclassified 100%
ofAntarctic birds, 88% ofMalagasybirds, but0% ofSouth Georgianbirds (though
expectedas sample size isverysmall, n=2). Nomis-classifiedbirdfromPatagonia
was attributed to the Antarctic group. In conclusion, segregation is well achieved
betweenAntarctic andto a lesserdegree Malagasybirds and all others on the basis
ofbiometrics.
TABLE 4
Results ofa discriminant analysis performed on 130 adult Kelp Gulls (the Meinertzhagen
specimen is excluded), using five biometric variables only.
Geographical origin No. of % correct in % correct in jack-knife
birds classification matrix classification matrix
eastern South America 8 50 25
western South America 17 35 24
Patagonia 45 27
1 1
Falklands 22 45 45
South Georgia 2 100
Antarctica 7 100 100
New Zealand 17 41 41
Madagascar 8 88 88
South Africa 15 33 27
subantarctic Indian Ocean 23 57 52
Frederic Jiguet 57 Bull. B.O.C. 2002 122(1]
TABLE
5
Results ofa Principal Components Analysis performed on 131 adult Kelp Gulls, using five
biometric and two wing plumage characters. PCI is positively correlated to all five biometric
measures, and negatively to the mirror number. PC2 is positively correlated to the wing
plumage variables and bill depth measures, and negatively to the culmen length.
PC 1 PC 2
Eigen value 3.27 1.51
% variance explained 46.7 21.6
Cumulative % 46.7 68.3
Correlation with axis
:
- Wing 0.771 -0.092
- Tarsus 0.803 -0.043
-Culmen 0.782 -0.381
- Depth at gonys 0.785 0.491
- Depth at nostril 0.805 0.444
- Number ofwhite mirror(s) -0.375 0.660
- Number ofwhite tongue(s) -0.132 0.694
ThesecondPrincipalComponentsAnalysis,thatexaminedgeographicalvariations
usingbiometrics plus thetwowingplumage variables foradults (Table 5), suggests
a separationbetweenbirds fromthe subantarctic IndianOcean from all otherIndian
Ocean populations (Fig. 3). Birds from Madagascar also separate from all other
populations, with no overlap, even with birds from the closest populations
(subantarcticIndianOceanandsouthernAfrica). I furtherperformedaDiscriminant
Analysis, considering again the ten groups from geographically different origins as
TABLE 6
Results ofa discriminant analysis performed on 131 adult Kelp Gulls
(the Meinertzhagen specimen is included with South African birds),
using five biometric and two plumage characters.
% %
Geographical origin No. ofbirds correct in correct in jack-knife
classification matrix classification matrix
eastern South America 8 63 38
western South America 17 41 29
Patagonia 11 55 27
Falklands 22 68 59
South Georgia 2 100
Antarctica 7 100 100
New Zealand 17 29 24
Madagascar 8 100 100
South Africa 16 38 25
subantarctic Indian Ocean 23 100 100
Frederic Jiguet 58 Bull. B.O.C. 2002 122(1)
a South America
+ Patagonia
x Antarctica
a Falklands
a South Georgia
o South Africa
o Madagascar
• sub. Indian Ocean
- NewZealand
©Meinertzhagen sp.
O
Q.
-1
-6 2
PC1
Figure 3. Scatter plot ofthe first two Principal Components, resulting from a PCA performed on
131 adult Kelp Gulls, using five biometric and two plumage characters. The three populations
breeding in the south-western Indian Ocean are individualized (top: subantarctic; middle: southern
Africa; bottom: Madagascar).
different a priori groups (Table 6). Multivariate Analysis ofVariance was highly
significant(Wilk'sLambda=0.027,F=9.27,df=63,P<0.001).Usingresubstitution
anderrorcountestimates,noindividualfromAntarctica, SouthGeorgia,subantarctic
Indian Ocean and Madagascar was mis-classified. The cross-validation error rate
estimateagaincorrectlyclassified 100%ofAntarctic, subantarcticIndianOceanand
Malagasybirds.Nomis-classifiedbirdfromPatagoniawasattributedtotheAntarctic
group. Inconclusion, segregationiswell achievedbetweenatleastthreegroupsand
all otherpopulations, on the basis ofbiometrics and wingpattern combined. All of
these groups include males and females with no highly skewed sex ratio. In this
discriminantanalysis, the Meinertzhagenspecimenwasclassifiedwithsubantarctic
Indian Ocean birds; this origin is also supported by its position on Fig. 3 and its
observedphenotype(veryshortbill:44.2mm,twowhitemirrors,threeobviouswhite
tongues; seeTables 7 and 8). This bird could be eithermis-labelled (Knox 1993), a
vagrantfrome.g. MarionIslandcollectedonmainlandAfrica(recordsofnonvetula
birdsareveryscarcebuthaveoccurred; Brookeetal. (1982)), orabirdcollectedon
e.g. Marion Island (a South African possession) and labelled 'South Africa'. Itwas
excluded from calculations inTables 7 and 8.
Frederic Jiguet 59 Bull. B.O.C. 2002 122(1)
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Figure 4. Comparisons of some biometrics (in mm) of females (0 and males (m) of the five
different subspecies of Kelp Gull Larus dominicanus; a) maximum wing chord (black dots and
unbroken lines) and exposed culmen length (white dots and dotted lines); b) bill depths at gonys
(black dots and unbroken lines) and at base ofnostrils (white dots and dotted lines).