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COMMENTARIES

Confusions of and Size

THEUNIS PIERSMA• AND NICK C. DAVIDSON 2

Work by Rising and Somers(1989) and Freeman combines information about structural size and nu- and Jackson(1990) has recently emphasized that mul- trient reserve massbut alone cannot provide an in- tiple, and preferably skeletal, provide dicator of structural size. It is the ease with which a more accurateassessment of body sizein birds than body masscan be measured(and not the difficulty; do single, external measurements.We agree. It is re- contra Freeman and Jackson 1990) that has made it a grettable,however, that both papersperpetrate a con- muchused and interestingmeasure. Inclusion of body fusion between two useful and quite distinct vari- mass in structural size-related analysesmay not al- ables:body size and bodymass. In doing so they have ways distort interpretationof the data (e.g. seeCooke ignored several highly relevant published studies, et al. 1990),but it is usually impossibleto determine notably of nonpasserineand Europeanspedes (the this from ambiguousresults. latter bias apparently part of a more general trend By definition structuralsize is independentof the identified by Enckell 1988). nutritional status that varies with time of day and Rising and Somers(1989) and Freemanand Jackson year, reproductivestatus, and habitatquality. For this (1990) are but two recent examplesof a widespread reasonstructural size is the preferred variable in stud- misunderstanding.Smith et al. (1990) provide a fur- ies of geographicalsize variation and perhapsin be- ther instance.Another is the inclusionby Davieset havioral studies of contests. Note, however, that it al. (1988)of massin an analysisof body sizeof Lesser may not be structuralsize per se but rather muscular Snow Geese(Chen c. caerulescens),which was criticized power (possiblyreflected in the protein reservemass) recentlyby Alisauskasand Ankney (1990),although that determines contest outcomes. Whereas studies of in this casethe overall interpretationof a size-related geographicalsize variation regard all this nutritional variation in fecundityof LesserSnow Geesemay not noise as a problem to be removed, studiesof ener- have been affected (Cooke et al. 1990). Even Peters getics and nutrient reserve status require precisely (1984), in his extensive discussionof the ecological this information on reserve mass. For such studies implicationsof body size,provides no cleardefinition body masscan be a useful variable if massvariations of body size and in many instancesdiscusses body due to differences in structural size can be controlled size-relatedphenomena through body mass. (e.g. Davidson1983, Ankney and Afton 1988,Piersma Although not stated explicitly, both Rising and 1988, Piersma and van Brederode 1990). Somers (1989) and Freeman and Jackson(1990) ad- Althoughbody massis readily measured,structural dressedproblems of measuringstructural size. The Ox- size is more difficult, and there have been surpris- ford dictionarydefines structural as "of the (essential) ingly few attemptsto devise practicalmeans of mea- framework" and structuralsize accordinglyas "the suring structural size in birds. One approach is to size of the essential framework" (Piersma 1984). Be- definestructural size as the body massof starvedbirds causeindividual birds have a remarkablecapacity to (i.e. birds that have died after exhaustingall their vary their massand dependingon their nu- nutrient reserves[Piersma 1984, 1988; see King and tritional status,we feel it is biologicallymost mean- Ml•rphy 1985 for the distinctionbetween nutrient ingful to define structural size as the "nutrient re- reservesand nutrient stores])and then seewhich body serve-independentsize of a bird." dimensionsstatistically "explain" starvedbody mass Our point is simple.There is an importantbut wide- most accurately.There are, however, pitfalls even in ly ignored distinction that must be made between this approach.Although birds die in natural starva- two concepts:(1) the reserve-independentstructural tion eventsafter substantiallydepleting their fat and sizeand (2) the variableamounts of nutrients(chiefly protein reserves,there is evidence that the precise fat and protein) that permit birds to exerciseand to nutritional state at death is influenced by the envi- survive periodsof negativeenergy and nutrient bal- ronmentalconditions during starvation.This canlead ance, the variable nutrientreserve mass. Body mass to both the fat mass and the nonfat mass of starved birds still including some nutritional reserves,al- though absolute amounts are small (Davidson and ' NetherlandsInstitute for SeaResearch (NIOZ), Evans 1982, Davidson and Clark 1985). P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands, Skeletal measurements of the sternum have been and ZoologicalLaboratory, University of Groningen, used to index pectoralmuscle size (Evansand Smith P.O. Box 14, 9750 AA Haren, Netherlands. 1975, Piersma et al. 1984), but these linear measure- 2Chief ScientistDirectorate, Nature Conservancy ments cover only part of the overall skeletal size. As Council,Northminster House, Peterborough PE1 1UA, suggestedby Wishart (1979), (dry and fat-free) skel- United Kingdom. etal massis almostentirely reserve-independent,ex-

441 The Auk 108: 441-444. April 1991 442 Commentaries [Auk, Vol. 108 cept for small variations in bone marrow mass(see reserves, clutch size, and incubation. Condor 90: Moser and Rusch 1988), and should provide an ac- 459-472. curate estimator of structural size on which linear ALISAUSKAS,R. T., & C. D. ANKNEY. 1990. Body size dimensionscan be regressedto provide an index of and fecundity in LesserSnow Geese.Auk 107: structural size. Moser and Rusch (1988) concluded 440-443. that skeletal volumeß measured by water displace- COOKE,F., J. C. DAvIEs, & R. F. ROCKWELL.1990. Re- ment of excisedbonesß provides an even more precise sponseto Alisauskasand Ankney. Auk 107:444- measureof structuralsize. Finally, the first principal 446. component of a set of linear skeletal measurements DAVIDSON,N.C. 1983. Formulae for estimatingthe likewise provides an appropriate structural size mea- lean and fat reservesof live shorebirds. sure (Ankney and Afton 1988, Rising and Somers Ring. & Migr. 4: 159-166. 1989). ß, & N. A. CLARI<. 1985. The effects of severe Most skeletalmeasurements require the sacrificeof weather in Januaryand February 1985on waders birds and are of little help in studies of live birds. in Britain. Wader Study Group Bull. 44: 10-16. Consequently,body-size variation in live birdsis usu- ß& P. R. EVANS.1982. Mortality of Redshanks ally controlledin analysesof body massby the use and Oystercatchersfrom starvationduring severe of one or more easily measured external measure- weather. Bird Study 29: 183-188. ments, typically the of wingß tarsus,body, DAvms,J. C., R. F. ROCk'W•L,& F. COOKE.1988. Body- and bill. Choice of the appropriateexternal dimen- size variation and fitness componentsin Lesser sions to indicate structural size can be made with Snow Geese (Chen caerulescenscaerulescens). Auk appropriatemodels of regressionon starvedor skel- 105: 639-648. etal mass or volume, as outlined aboveß and their ENCKELL,P. H. 1988. Ecology: the (inter)national derivation may hold some surprises.For exampleß science. Br. Ecol. Soc. Bull. 19: 224-228. Moser and Rusch(1988) found that some commonly EvANs, P. R., & P. C. SMITH. 1975. Studies of shore- usedexternal measures were only moderatecorrelates birds at Lindisfarne, Northumberland. 2. Fat and of structural size (measuredby skeletal volume) in pectoral muscleas indicators of body condition Canada Geese (Branta canadensis).Further, the most in the Bar-tailed Godwit. Wildfowl 26: 64-76. appropriateexternal measures may differ interspecif- FromMAN,S., & W. M. JACr,SON. 1990. Univariate met- ically, even between closely related species(e.g. in ricsare not adequateto measureavian body size. Podicepsgrebesß Piersma 1988). Auk 107: 69-74. Future studies must differentiate clearly between KING, J. R., & M. E. MURPHY. 1985. Periods of nu- structural-size and nutritional-mass variation, and tritional stress in the annual cycles of endo- should avoid using body massin exercisesaimed at therms: fact or fiction? Am. Zool. 25: 955-964. describingstructural size. Failure to distinguishbe- MOSER,T. J., & D. H. Ruscrt. 1988. Indices of struc- tween the two can lead to ambiguousand misleading tural size and condition of Canada Geese. J. Wildl. interpretations.For example,the regional differences Manage. 52: 202-208. in flight-surfaceloading of Accipiterhawks described Pgrm•s,R.H. 1984. The ecologicalimplications of by Smith et al. (1990) may be purely structuraland body size. Cambridge,Cambridge Univ. Press. adaptively related to differencesin the relative use PIERSMA,T. 1984. Estimatingenergy reserves of Great of thermal updrafts, as the authors suggestßor they Crested GrebesPodiceps cristatus on the basis of may be a consequenceof differencesin fat-reserve body .Ardea 72: 119-126. massin relation to the of the ensuingnonstop 1988. Body size, nutrient reservesand diet flight (seee.g. Smith et al. 1986). of Red-neckedand SlavonianGrebes Podiceps gri- Although easyto define in general, structuralsize segenaand P. aurituson Lake IJsselmeer,The defiesa uniform approximationin termsof a standard Netherlands. Bird Study 35: 13-24. set of external measurementsand should preferably ß N. C. DAVIDSON, & P. R. EvANs. 1984. Esti- be derived for eachspecies under study.Clearly, there mation of protein reservesof waders:the use and is still much to be elucidatedabout the relationships misuseof StandardMuscle Volume. Wader Study between structural size and body mass,but the un- Group Bull. 42: 19-22. justifiableinclusion of body massin analysesof struc- ß& N. E. vAN BREDERODE. 1990. The estimation tural sizeonly servesto confuseinterpretation of phe- of fat reserves in coastal waders before their de- nomena such as nutrient-reserve storage and parture from northwestAfrica in spring. Ardea geographical-sizevariation. 78: 221-236. RISING, J. D., & K. M. Solvl•S. 1989. The measure- ment of overall body sizein birds.Auk 106:666- 674. Aur,l, mx, C. D., & A.D. AFrON. 1988. Bioenergetics SMm-I, N. G., D. L. GOLDSTEIN,& G. A. BARTHOLOMEW. of breeding Northern Shovelers:diet, nutrient 1986. Is long-distancemigration possiblefor April 1991] Commentaries 443

soaringhawks using only stored fat? Auk 103: WISHART,R.A. 1979. Indices of structural size and 607-611. conditionof AmericanWigeon (Ariasamericana). SMITH, J.P., S. W. Ho•, & J. A. GEssm•oaq. 1990. Can. J. Zool. 57: 2369-2374. Regionalsize differencesamong fall-migrant ac- cipiters in North America. J. Field Ornithol. 61: Received15 August1990, accepted31 August1990. 192-200.

On the Use of Tape Recordersin Avifaunal Surveys

Th'•Or)ORE A. P.$,RKER lip

The utility of tape recordersand tape playbacksfor listsbased only on sight recordsshould be viewed censusingbirds is widely recognized(Johnson et al. with someskepticism (and are likely to be far from 1981), but little emphasishas been placed on their complete).I urgeconservation organizations that fund importancein faunal surveys.Tape recordersare in- avifaunalsurveys in tropicalforests around the world dispensablefor finding rare, secretive,or patchily to requiretheir recipientsto use tape recorderssys- distributed species,and for documenting the com- tematically.Copies of all recordingsshould be placed position of mixed-speciesflocks in forest canopy. in a professionallymaintained sound collection that Awarenessof vocaldifferences in the field and taping provideseasy accessto researchers.Survey budgets hasled to the discoveryof severaltaxa new to science shouldinclude travel funds for investigatorsto visit (Parker and O'Neill 1985, Parker and Schulenberg a soundcollection before or after an expedition,and MS), and to the recognitionof numerousspecies pre- funds for sound specialiststo identify or verify re- viously consideredsubspecies (Lanyon 1967, 1978; cordings.Those unpreparedto deal with bird-song Pierpont and Fitzpatrick 1983). identificationin the tropicsare simply wastingvalu- One personequipped with a tape recorderand di- able, limited researchfunds that couldbetter be spent rectionalmicrophone can documenta surprisingly elsewhere. high percentageof a tropical forest avifauna within Fortunately,relatively few Neotropicalbird songs 4-7 daysduring the properseason. Without tapere- remain unrecorded. The Library of Natural Sounds corders,several weeks (or even months) are required at Cornell Universitycontains recordings of songsor to locatemost of the residentbird speciesin any low- calls of 671 of the approximately770 resident forest land Amazonian locality, and such an effort would birds found within Amazonia below 1,000 m, or 87% involve a large number of experiencedobservers us- of the richest avifauna on ! Other collections, ing the best optical equipmentand many mist nets. such as those in the Florida State Museum Bioacousti- On a recent LouisianaState University Museum of cal Laboratory(Gainesville, Florida, USA), the Ar- Zoology(LSUMZ) expeditionto the Departmentof quivo SonoroNeotropical (UniversidadeEstadual de Pando in Amazonian Bolivia (Parker and Remsen Campinas,Campinas, Brazil), and the Laborotoriode 1987), I tape-recorded243 speciesfound within an Sonidos Naturales (Museo de Ciencias Naturales, ca. 2 km2 of upland rain forest in only seven BuenosAires, Argentina),contain recordings of many days.The "final" list of forestbirds for the samearea, additional Neotropical species.Because field orni- after 54 daysof intensivefieldwork (including36,804 thologistsassociated with theseinstitutions can iden- mist-nethours) by sevenexperienced ornithologists, tify the majorityof bird voicesrecorded in the Neo- included 287 species.I tape-recorded85% of the avi- tropics,they can greatlyfacilitate the compilationof faunain justone week. Ten of the speciesthat I missed locality lists from tapes. altogetherwere almostcertainly visitorsto the site, The following guidelineswill enhanceone's chanc- and most of the other speciesnot found were those es of compiling an accuratelocality list in a tropical typicallymissed during brief surveysof rain forests, forest: suchas forestraptors and canopyhummingbirds. In an age when few ornithologistscollect speci- 1. Get up well before dawn and be out in the areato mens,taping is the quickestand mostpractical way be surveyedat least15 min beforefirst light. Many to build an inventory of a diverseavifauna. Locality Neotropical species(especially tinamous, puff- birds,woodcreepers, and flycatchers)sing only 1- 3 songsduring the first 5 min of light (often well • Present address: Museum of Natural Science, 119 before light enoughto see),and they rarely vo- FosterHall, LouisianaState University, Baton Rouge, calize thereafter, until an even briefer period late Louisiana 70803 USA. in the day.Nocturnal species often calljust before