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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.