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228
Commentaries
[Auk, Vol. 100
The Relative Size of Darwin's Finch Eggs
GRANT 1
Scottand Ankney (1983)have pointed out I was in
error in concludingthat Darwin's fincheslay unusually small eggs(Grant 1982). ! discoveredthe error
independently when trying to reconcileNewton's
(1972) statement that European finches lay eggs
weighing approximately10% of adult female body
weight with Rahn et al.'s (1975)finding that North
Americanfincheslay eggsapproximately20% of adult body weight. As detailed by Scottand Ankney
(1983), Rahn et al. (1975) mistakenly equated egg
weight with eggvalue (lengthx breadth2) in Ama-
different slope (0.551 ñ 0.058). The relationship can
be representedas
log•0EV= 0.551 log•0W• - 0.055
or
EV = 0.881V• ø'5'•.
The two regressionlinesfor Darwin'sfinchesand
North American emberizids, respectively,intersect
when log body weight is 1.391, or body weight is
24.6 g. This is the approximate weight of Geospiza
conirostris.
The two speciesof Darwin's finchesabove
don's (1943) treatment of North American emberizid
and Platyspizacrassirosdata. ! repeatedthe error. In fact, for both groups of this weight, G. magnirostris
tris, have relatively small eggs comparedwith the
finches10% is the approximatelycorrectfigure.
Exactlyhow do Darwin's finchesdiffer fron con- North American birds of similar weight (36 g). All
tinental species?To answer this I have followed the remainingDarwin's fincheshave relativelylarge
Amadon's(1943)exampleand used eggvalue (LBS/ eggs,not smalleggsasconcludedearlier(Grant1982).
Do Darwin's finches have relatively large eggs
1,000) as an index of volume (cc)rather than manipulatethe datato calculateeggweight. Amadon(1943) comparedwith fincheson the nearby South Ameriestimated the intercept and slope of the relationship can mainland as well? At the time of my original
betweeneggvalueand bodyweight on a doublelog analysis,I was unableto answerthis question,bescale but did not calculate confidence limits on the
causebody-weightdata were not availablefor the
estimates.Eleven speciesof continentalfincheswere speciesstudiedby Marchant(1960)in Ecuador.I now
involved in his analysis,but two were represented have a set of data from Peru, however, kindly suptwice by two subspecies.To avoid a possiblebias plied by M.D. Williams(pers.comm.;seealsoWilthat sucha repetition might introduce,I repeatedthe liams 1981a,b). The species,with samplesof body
analysisafter first eliminatingthe subspecieswith weights and egg measurementsgiven in that order
the smallestsampleof body weights,namely Passer- in parentheses,are: Sicalisfiaveola(16, 21), Piezorhina
culus sandwichensis nevadensis and Passerella i. iliaca.
cinerea(16, 28), Volatiniajacarina(3, 2), PoospizahisThe recalculatedvalues, with 95% confidence limits,
paniolensis(6, 2), Aimophilastolzmanni(41, 33), and
are -0.382 + 0.057 for the intercept and 0.786 + 0.042 Sporophilaperuviana (21, 9). The relationship befor the slope. I recalculatedthe relationshipwith the tween the means of EV and l,V.•for thesesix species
two subspeciesadded and, separately,with the in- is
clusionof threemore speciesthat Amadon (1943)had
log•0EV= 0.808 log•y•, - 0.414
excludedbecausehe consideredtheir body-weight
or
data inadequate. The resultswere almost the same,
differing in the interceptby 0.02 and in the slopeby
EV = 0.385V• ø'8ø8.
0.01 (maximum).
The relationshipbetweeneggvalue (EV) and adult
body weight in grams(WA)is thus
log•0EV= 0.786 log•0I'E•- 0.382
or
EV = 0.415W• ø'7s•.
The 95% confidence limits on the coefficients are
0.639 and 0.977 for the slope and -0.196 and -0.632
for the intercept. Strictly, analysisof covarianceis
required for a comparisonof regressions.Nevertheless, it is clear that the relationshipfor South American speciesis strikinglysimilar to the relationship
for North American species(above) but is not similar
to the
one
for
Darwin's
finches.
These
results
Ten speciesof Darwin's finchesspanning a nearly strengthenthe conclusionthat Darwin's fincheslay
identicalrangeof adult body weights(Grant 1982) relativelylarge eggs.
The highinterceptfor theDarwin'sfinchregression
give a very different intercept(-0.055 + 0.073) and
means that the smallestspecieslay especiallylarge
eggs.The point is illustratedby convertingegg value
to egg weight (W•,) in grams by means of Schoenwetter's equation (in Amadon 1943)
1 Division of BiologicalSciences,University of Michigan, Ann Arbor,
Michigan 48109 USA.
W•. = 0.5128EV,
January 1983]
Commentaries
then expressingegg weight as a percentageof adult
weight for each species.For the 14 North American
emberizidspercentageegg weight variesfrom 9.5 to
12.8, and for the 6 South American speciesit varies
from 9.8 to 13.2, whereasfor the 10 speciesof Darwin's
finches
it varies
from
8.9 to 17.3.
The
four
smallestspeciesof Darwin's fincheshave higher percentageegg weights than any speciesin the North
American and South American groups: the four
species,and their mean weights, are Certhideaolivacea(8.8 g), Camarhynchus
parvulus(13.0 g), Geospizadifficilis(11.8 g) and G. fuliginosa(13.3 g).
The low slopeof the regressionmeans that Dar-
win's fincheggsincreasein sizeslowlywith increasing body size among species.In this respect they
differ from most other birds. Martin (1981) deduced
that eggweightsamongbirds in generalshouldscale
to body size in the manner
WL.= kW,• ø'75,
where k is a constant.He performed a test with
weights (in milligrams) from 127 speciesand found
goodagreementwith expectation.The empiricalformula was
1og•0WE
= 0.79 log•,W• + 2.34,
with 95% confidence limits on 0.79 of 0.75 and 0.84.
The North
American
and South American
emberi-
zids clearly fit the theoreticalexpectation,because
the slopes(0.786and 0.808, respectively)of log•0EV
regressedon log•,W• are the same as the slopes of
1og•0WE
on log•0WA,
as EV is multipliedby a constant
(0.5128)in eachcaseto give W•,. Darwin's finchesdo
not fit the theoretical expectation, either with calculated egg weights (slope= 0.551) or actual egg
weightsfrom a restrictedsampleof species(slope=
229
Downhower 1976).They are able to manufacturerelatively largeeggsquickly. The advantageof relatively large eggs is that the young hatch at a relatively
advancedstageof developmentas a consequenceof
a relatively long incubation period (Grant 1982), and
this gives them a good chanceof fledgingat a relatively large size (Grant 1981). I suggestfurther that
the risk of nest predation contributesto the selective
forcesacting on egg size. For example, in mainland
environments,where the daily risk of nest predation
is higher, the advantageof largeegg size is counterbalancedby the attendantdisadvantageof a longer
(vulnerable) incubation period, and, in fact, both incubation and nestling periods, which generally covary among speciesfor developmentalreasons(Lack
1968), are shorter on the EcuadorJanmainland than
the Galgpagos(Grant and Grant 1980). On the Galgpagos,nestsof the largestspecies,G. magnirostris,
are especiallyvulnerable to predation by owls (Asio
fiammeus)in a time-dependent manner, the species
does not lay relatively large eggs,unlike its smaller
congeners,and its incubation and nestling periods
are not relatively long (Grant and Grant 1980).
This food-predation hypothesisexplains why the
smallspeciesof Darwin's fincheshaverelativelylarge
eggsandwhytheslopeof therelati,onship
withbody
size is low, but, as the energeticadvantageof small
adult femalesizecouldbe translatedalternativelyinto
a large number of eggs, there is a need to consider
jointly the factorsaffectingegg numberand egg size
(Grant 1982; see also Ricklefs 1968, 1970; Smith and
Fretwell 1974).
I thank B. R. Grant, T. D. Price, D. Schluter, R. A.
Vgisgnen,and J. H. Zar for helpful commentson the
manuscript,M.D. Williams for allowing me to use
unpublisheddata, and C. D. Ankneyand D. M. Scott
for showingme an early versionof their paper.
0.48; note the intercept for this equation should be
-0.24, not log•0-0.24 as reported in Grant 1982).
Why then do Darwin's finchesexhibit the unusuLITERATURE CITED
ally low slope and high intercept?I follow the approach adopted earlier toward a similar question AMADON,D. 1943. Bird weights and egg weights.
Auk 60: 221-234.
(Grant 1982), although not the incorrectdetails. The
surface-areahypothesisof Rahn et al. (1975)and the DOWNHOWER,
J. F. 1976. Darwin's finches and the
metabolichypothesisof Martin (1981)have successevolution of sexual dimorphism in body size.
Nature 263: 558-563.
fully accountedfor variation in relative egg size at
higher taxonomiclevels, perhapsbecauseof the het- GRANT,P.R. 1981. Patternsof growth in Darwin's
erogeneity of the taxa, but neither hypothesis prefinches.Proc. Roy. Soc.LondonB212:403432.
dicts the low slope in Darwin's finches. These two
1982. Variation in the size and shape of
failures suggestthat local ecologicalcircumstances
Darwin's finch eggs.Auk 99: 15-23.
need to be considered.
I suggestthat the most importantecologicalfactor
influencingvariation in relativeegg size is the resourceflush that occursfollowingthe initial heavy
rainfall that triggersa breedingresponsein the finch
species(Grantand Boag1980,Grant and Grant 1980).
In general, small-bodied speciesshould be able to
take greaterand quickeradvantageof this than large
onesfor body-sizerelatedenergeticreasons(e.g. see
, & P. T. BOAG. 1980. Rainfall on the Galg-
pagosand the demographyof Darwin's finches.
Auk
97: 227-244.
, & B. R. GRANT. 1980. The breeding and
feedingcharacteristics
of Darwin'sfincheson Isla
Genovesa,Galgpagos.Ecol.Monogr. 50: 381-410.
LACK,D. 1968. Ecologicaladaptationsfor breeding
in birds. London, Methuen & Co.
NEWTON, I. 1972. Finches. London, Collins.
230
Commentaries
MARCHANT,S. 1960. The breeding of some S.W.
Ecuadorian
birds.
MARTIN, R. D.
metabolic
Ibis 102: 584-599.
1981. Relative brain size and basal
rate in terrestrial
vertebrates.
Nature
SCOTT, D. M., & C. D. ANKNEY. 1983. DO Darwin's
fincheslay small eggs?Auk 100: 226-227.
SMITH, C. C. &: S. D. FRETWELL.1974. The optimal
balancebetween size and number of offspring.
Amer.
293: 5740.
RAHN, H., C. V. PAGANELLI, & A. AR.
[Auk, Vol. 100
1975. Re-
lation of avian egg weight to body weight. Auk
Natur.
a Peruvian
92: 750-765.
RICKLEFS,R. E. 1968. On the limitation of brood
size in passerincbirds by the ability of adults
to nourish their young. Proc. Natl. Acad. Sci.
61: 847451.
1970. Clutch size in birds: outcomeof opposing predator and prey adaptations. Science
108: 499-506.
WILLIAMS,M.D. 1981a. Discoveryof the nest and
eggsof the CinereousFinch (Piezorhinacinerea),
endemic.
Auk 98: 187-189.
1981b. First descriptionof the nest, eggs
and young of the Tumbes Sparrow (Aimophila
[Rhynchospiza]
stolzmanni).Condor83: 83-84.
Received4 June1982, accepted26 October1982.
168: 599400.
Further
Notes on Variation
DAVID
in Leach's Storm-Petrel'
G. AINLEY 2
parisonof population sizesthey do not point out that
storm-petrelpopulationsin the ChannelIslandshave
ley 1980)at the southernend of the species'range in not been well studied and that on islandsexceptSan
the easternNorth Pacific,particularlymy mergingof Benitos, and especiallyon Guadalupe, Los CoronaO. 1. chapmaniwith O. 1. leucorhoaand my separa- dos, and the ChannelIslands,populationshavebeen
tion of populationsat GuadalupeIsland. In regard drastically reduced and forced onto adjacent rocks
to O. 1. chapmani,they attempt to substantiateits and islets by the introduction of exoticmammals to
validity as a subspecies.They give two reasonsfor main islands (e.g.R.L. DeLong and R. S. Crossin,
recognizinga sharperdistinctionbetweenit and more "Status of seabirds on Islas Guadalupe, Natividad,
northerly populations than my data indicate. First, Cedros, San Benitos, and Los Coronados,"unpubl.
they claim their data on ratios of color morphs on MS, Pacific Ocean BioL Surv. Progr., Smithsonian
Los Coronadosand San Benitos (p. 793) are closerto Inst.). Thus, most dark-rumped birds do presently
the truth than the patterns in the 103 specimens nest on San Benitos,but, if we appliedB & J's 90%
available in museums(i.e. my Fig. 2, 1980). Their rule in regardto coloras the only basisfor recognizunpublished data cannot be assessed,but, in gen- ing O. 1. chapmani,then the subspeciescouldbe ineral, they still help to illustratethe point that I em- validatedif catsand goatswere introducedsomeday
phasized, namely, that whitest morphs predominate to the San Benitos and the storm-petrelpopulation
Bourneand Jehl(1982;= B & J) disagreewith my
taxonomic treatment of Oceanodromaleucorhoa(Ain-
in the north and darkest ones in the south, while
those in between
are mixed.
The situation
there was reduced
in size as a result!
is thus
Finally, they ask that the dark birds found among
akin to that of Fulmarus
glacialisand Puffinuspacificus breedingpopulationsin the ChannelIslandsand the
of the Pacificregion, where similar color variation FarallonIslandsbe ignored,becausethesebirds were
has not been given taxonomicsignificance;B & J supposedlyvisiting from the south(p. 795). This is
should explainwhy colorvariation shouldbe treated special pleading. Even if these birds were estabdifferentlyin Leach'sStorm-Petrels.
lished immigrants from the SanBenitos,suchan unEven if color were a useful character, their second usuallyhigh rateof interchange
betweenislands(6%
point (p. 796), that 90% of all dark birds in the east- of the Farallon population would thus have to be
ern North
Pacific breed on the San Benitos and Co-
ronados, which would therefore validate O. 1. chap-
maniasa subspecies,
is pertinentonlyafterthey first
merge O. 1. willetti (Los Coronados)with O. 1. chapmani. This is contrary to Austin (1952), to the A.O.U.
(1957), and even to Bourne's earlier opinion (in Palmer 1962) and thus requires much more support with
data than they offered.Furthermore,in their comt This is ContributionNo. 134of the Point ReyesBird Observatory.
2 Point ReyesBird Observatory,4990ShorelineHighway, Stinson
Beach, California 94970 USA.
derived from San Benitos) would be evidence that
we are dealingessentiallywith only one population.
A much higher degree of philopatry, however, is
characteristicof storm-petrelsand most other species
of pelagicseabirds.Many of the dark birds caught
on the FarallonIslandshave incubationpatchesduring the appropriateseasonand at least two, which
were banded and released,were recapturedthere in
the same and in subsequentyears (PRBO unpubl.
data). These facts reduce even more the slim likelihood that these birds were visitors breeding on islands more than 800 km away.