Davidson, Nick C.

Piersma, Theunis

English

Proceedings - University of Groningen

   University of GroningenConfusions of mass and sizePiersma, Theunis; Davidson, Nick C.Published in:The AukIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1991Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Piersma, T., & Davidson, N. C. (1991). Confusions of mass and size. The Auk, 108(2), 441-443.https://academic.oup.com/auk/article-abstract/108/2/441/5172737CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 11-10-2022Confusions of Mass and SizeAuthor(s): Theunis Piersma and Nick C. DavidsonSource: The Auk, Vol. 108, No. 2 (Apr., 1991), pp. 441-443Published by: University of California Press on behalf of the American Ornithologists' UnionStable URL: http://www.jstor.org/stable/4087973 .Accessed: 28/06/2013 05:17Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp .JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org. .University of California Press and American Ornithologists' Union are collaborating with JSTOR to digitize,preserve and extend access to The Auk.http://www.jstor.org This content downloaded from 129.125.63.113 on Fri, 28 Jun 2013 05:17:37 AMAll use subject to JSTOR Terms and ConditionsCOMMENTARIES Confusions of Mass and Size THEUNs PIERsMA1 AND NICK C. DAVIDSON2 Work by Rising and Somers (1989) and Freeman and Jackson (1990) has recently emphasized that mul- tiple, and preferably skeletal, measurements provide a more accurate assessment of body size in birds than do single, external measurements. We agree. It is re- grettable, however, that both papers perpetrate a con- fusion between two useful and quite distinct vari- ables: body size and body mass. In doing so they have ignored several highly relevant published studies, notably of nonpasserine and European species (the latter bias apparently part of a more general trend identified by Enckell 1988). Rising and Somers (1989) and Freeman and Jackson (1990) are but two recent examples of a widespread misunderstanding. Smith et al. (1990) provide a fur- ther instance. Another is the inclusion by Davies et al. (1988) of mass in an analysis of body size of Lesser Snow Geese (Chen c. caerulescens), which was criticized recently by Alisauskas and Ankney (1990), although in this case the overall interpretation of a size-related variation in fecundity of Lesser Snow Geese may not have been affected (Cooke et al. 1990). Even Peters (1984), in his extensive discussion of the ecological implications of body size, provides no clear definition of body size and in many instances discusses body size-related phenomena through body mass. Although not stated explicitly, both Rising and Somers (1989) and Freeman and Jackson (1990) ad- dressed problems of measuring structural size. The Ox- ford dictionary defines structural as "of the (essential) framework" and structural size accordingly as "the size of the essential framework" (Piersma 1984). Be- cause individual birds have a remarkable capacity to vary their mass and volume depending on their nu- tritional status, we feel it is biologically most mean- ingful to define structural size as the "nutrient re- serve-independent size of a bird." Our point is simple. There is an important but wide- ly ignored distinction that must be made between two concepts: (1) the reserve-independent structural size and (2) the variable amounts of nutrients (chiefly fat and protein) that permit birds to exercise and to survive periods of negative energy and nutrient bal- ance, the variable nutrient reserve mass. Body mass l Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, Netherlands, and Zoological Laboratory, University of Groningen, P.O. Box 14, 9750 AA Haren, Netherlands. 2Chief Scientist Directorate, Nature Conservancy Council, Northminster House, Peterborough PE1 1UA, United Kingdom. combines information about structural size and nu- trient reserve mass but alone cannot provide an in- dicator of structural size. It is the ease with which body mass can be measured (and not the difficulty; contra Freeman and Jackson 1990) that has made it a much used and interesting measure. Inclusion of body mass in structural size-related analyses may not al- ways distort interpretation of the data (e.g. see Cooke et al. 1990), but it is usually impossible to determine this from ambiguous results. By definition structural size is independent of the nutritional status that varies with time of day and year, reproductive status, and habitat quality. For this reason structural size is the preferred variable in stud- ies of geographical size variation and perhaps in be- havioral studies of contests. Note, however, that it may not be structural size per se but rather muscular power (possibly reflected in the protein reserve mass) that determines contest outcomes. Whereas studies of geographical size variation regard all this nutritional noise as a problem to be removed, studies of ener- getics and nutrient reserve status require precisely this information on reserve mass. For such studies body mass can be a useful variable if mass variations due to differences in structural size can be controlled (e.g. Davidson 1983, Ankney and Afton 1988, Piersma 1988, Piersma and van Brederode 1990). Although body mass is readily measured, structural size is more difficult, and there have been surpris- ingly few attempts to devise practical means of mea- suring structural size in birds. One approach is to define structural size as the body mass of starved birds (i.e. birds that have died after exhausting all their nutrient reserves [Piersma 1984, 1988; see King and Murphy 1985 for the distinction between nutrient reserves and nutrient stores]) and then see which body dimensions statistically "explain" starved body mass most accurately. There are, however, pitfalls even in this approach. Although birds die in natural starva- tion events after substantially depleting their fat and protein reserves, there is evidence that the precise nutritional state at death is influenced by the envi- ronmental conditions during starvation. This can lead 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 Evans 1982, Davidson and Clark 1985). Skeletal measurements of the sternum have been used to index pectoral muscle size (Evans and Smith 1975, Piersma et al. 1984), but these linear measure- ments cover only part of the overall skeletal size. As suggested by Wishart (1979), (dry and fat-free) skel- etal mass is almost entirely reserve-independent, ex- 441 The Auk 108: 441-444. April 1991 This content downloaded from 129.125.63.113 on Fri, 28 Jun 2013 05:17:37 AMAll use subject to JSTOR Terms and Conditions442 Commentaries [Auk, Vol. 108 cept for small variations in bone marrow mass (see Moser and Rusch 1988), and should provide an ac- curate estimator of structural size on which linear dimensions can be regressed to provide an index of structural size. Moser and Rusch (1988) concluded that skeletal volume, measured by water displace- ment of excised bones, provides an even more precise measure of structural size. Finally, the first principal component of a set of linear skeletal measurements likewise provides an appropriate structural size mea- sure (Ankney and Afton 1988, Rising and Somers 1989). Most skeletal measurements require the sacrifice of birds and are of little help in studies of live birds. Consequently, body-size variation in live birds is usu- ally controlled in analyses of body mass by the use of one or more easily measured external measure- ments, typically the lengths of wing, tarsus, body, and bill. Choice of the appropriate external dimen- sions to indicate structural size can be made with appropriate models of regression on starved or skel- etal mass or volume, as outlined above, and their derivation may hold some surprises. For example, Moser and Rusch (1988) found that some commonly used external measures were only moderate correlates of structural size (measured by skeletal volume) in Canada Geese (Branta canadensis). Further, the most appropriate external measures may differ interspecif- ically, even between closely related species (e.g. in Podiceps grebes, Piersma 1988). Future studies must differentiate clearly between structural-size and nutritional-mass variation, and should avoid using body mass in exercises aimed at describing structural size. Failure to distinguish be- tween the two can lead to ambiguous and misleading interpretations. For example, the regional differences in flight-surface loading of Accipiter hawks described by Smith et al. (1990) may be purely structural and adaptively related to differences in the relative use of thermal updrafts, as the authors suggest, or they may be a consequence of differences in fat-reserve mass in relation to the length of the ensuing nonstop flight (see e.g. Smith et al. 1986). Although easy to define in general, structural size defies a uniform approximation in terms of a standard set of external measurements and should preferably be derived for each species under study. Clearly, there is still much to be elucidated about the relationships between structural size and body mass, but the un- justifiable inclusion of body mass in analyses of struc- tural size only serves to confuse interpretation of phe- nomena such as nutrient-reserve storage and geographical-size variation. LrrERAruRE CrrED ANKNEY, C. D., & A. D. AFrON. 1988. Bioenergetics of breeding Northern Shovelers: diet, nutrient reserves, clutch size, and incubation. Condor 90: 459-472. ALISAUSKAS, R. T., & C. D. ANKNEY. 1990. Body size and fecundity in Lesser Snow Geese. Auk 107: 440-443. CooKE, F., J. C. DAvIEs, & R. F. ROCKWELL. 1990. Re- sponse to Alisauskas and Ankney. Auk 107: 444- 446. DAVIDSON, N. C. 1983. Formulae for estimating the lean weight and fat reserves of live shorebirds. Ring. & Migr. 4: 159-166. , & N. A. CLARK. 1985. The effects of severe weather in January and February 1985 on waders in Britain. Wader Study Group Bull. 44: 10-16. , & P. R. EVANS. 1982. Mortality of Redshanks and Oystercatchers from starvation during severe weather. Bird Study 29: 183-188. DAvIEs, J. C., R. F. ROCKWELL, & F. CooKE. 1988. Body- size variation and fitness components in Lesser Snow Geese (Chen caerulescens caerulescens). Auk 105: 639-648. ENCKELL, P. H. 1988. Ecology: the (inter)national science. Br. Ecol. Soc. Bull. 19: 224-228. EvANs, P. R., & P. C. SMrrH. 1975. Studies of shore- birds at Lindisfarne, Northumberland. 2. Fat and pectoral muscle as indicators of body condition in the Bar-tailed Godwit. Wildfowl 26: 64-76. FREMANj, S., & W. M. JACKSON. 1990. Univariate met- rics are not adequate to measure avian body size. Auk 107: 69-74. KING, J. R., & M. E. MURPHY. 1985. Periods of nu- tritional stress in the annual cycles of endo- therms: fact or fiction? Am. Zool. 25: 955-964. MosER, T. J., & D. H. RUSCH. 1988. Indices of struc- tural size and condition of Canada Geese. J. Wildl. Manage. 52: 202-208. PETERS, R. H. 1984. The ecological implications of body size. Cambridge, Cambridge Univ. Press. PERSMA, T. 1984. Estimating energy reserves of Great Crested Grebes Podiceps cristatus on the basis of body dimensions. Ardea 72: 119-126. 1988. Body size, nutrient reserves and diet of Red-necked and Slavonian Grebes Podiceps gri- segena and P. auritus on Lake IJsselmeer, The Netherlands. Bird Study 35: 13-24. , N. C. DAVIDSON, & P. R. EvANS. 1984. Esti- mation of protein reserves of waders: the use and misuse of Standard Muscle Volume. Wader Study Group Bull. 42: 19-22. , & N. E. vAN BREDERODE. 1990. The estimation of fat reserves in coastal waders before their de- parture from northwest Africa in spring. Ardea 78: 221-236. RISING, J. D., & K. M. SOMERS. 1989. The measure- ment of overall body size in birds. Auk 106: 666- 674. SMITH, N. G., D. L. GOLDSTEN, & G. A. BARTHOLOMEW. 1986. Is long-distance migration possible for This content downloaded from 129.125.63.113 on Fri, 28 Jun 2013 05:17:37 AMAll use subject to JSTOR Terms and ConditionsApril 1991] Commentaries 443 soaring hawks using only stored fat? Auk 103: 607-611. SMiTH, J. P., S. W. HOFFMAN, & J. A. GEssAMAN. 1990. Regional size differences among fall-migrant ac- cipiters in North America. J. Field Ornithol. 61: 192-200. WISHART, R. A. 1979. Indices of structural size and condition of American Wigeon (Anas americana). Can. J. Zool. 57: 2369-2374. Received 15 August 1990, accepted 31 August 1990. On the Use of Tape Recorders in Avifaunal Surveys THEODORE A. PARKER IIF1 The utility of tape recorders and tape playbacks for censusing birds is widely recognized (Johnson et al. 1981), but little emphasis has been placed on their importance in faunal surveys. Tape recorders are in- dispensable for finding rare, secretive, or patchily distributed species, and for documenting the com- position of mixed-species flocks in forest canopy. Awareness of vocal differences in the field and taping has led to the discovery of several taxa new to science (Parker and O'Neill 1985, Parker and Schulenberg MS), and to the recognition of numerous species pre- viously considered subspecies (Lanyon 1967, 1978; Pierpont and Fitzpatrick 1983). One person equipped with a tape recorder and di- rectional microphone can document a surprisingly high percentage of a tropical forest avifauna within 4-7 days during the proper season. Without tape re- corders, several weeks (or even months) are required to locate most of the resident bird species in any low- land Amazonian locality, and such an effort would involve a large number of experienced observers us- ing the best optical equipment and many mist nets. On a recent Louisiana State University Museum of Zoology (LSUMZ) expedition to the Department of Pando in Amazonian Bolivia (Parker and Remsen 1987), I tape-recorded 243 species found within an area ca. 2 km2 of upland rain forest in only seven days. The "final" list of forest birds for the same area, after 54 days of intensive fieldwork (including 36,804 mist-net hours) by seven experienced ornithologists, included 287 species. I tape-recorded 85% of the avi- fauna in just one week. Ten of the species that I missed altogether were almost certainly visitors to the site, and most of the other species not found were those typically missed during brief surveys of rain forests, such as forest raptors and canopy hummingbirds. In an age when few ornithologists collect speci- mens, taping is the quickest and most practical way to build an inventory of a diverse avifauna. Locality I Present address: Museum of Natural Science, 119 Foster Hall, Louisiana State University, Baton Rouge, Louisiana 70803 USA. lists based only on sight records should be viewed with some skepticism (and are likely to be far from complete). I urge conservation organizations that fund avifaunal surveys in tropical forests around the world to require their recipients to use tape recorders sys- tematically. Copies of all recordings should be placed in a professionally maintained sound collection that provides easy access to researchers. Survey budgets should include travel funds for investigators to visit a sound collection before or after an expedition, and funds for sound specialists to identify or verify re- cordings. Those unprepared to deal with bird-song identification in the tropics are simply wasting valu- able, limited research funds that could better be spent elsewhere. Fortunately, relatively few Neotropical bird songs remain unrecorded. The Library of Natural Sounds at Cornell University contains recordings of songs or calls of 671 of the approximately 770 resident forest birds found within Amazonia below 1,000 m, or 87% of the richest avifauna on earth! Other collections, such as those in the Florida State Museum Bioacousti- cal Laboratory (Gainesville, Florida, USA), the Ar- quivo Sonoro Neotropical (Universidade Estadual de Campinas, Campinas, Brazil), and the Laborotorio de Sonidos Naturales (Museo de Ciencias Naturales, Buenos Aires, Argentina), contain recordings of many additional Neotropical species. Because field orni- thologists associated with these institutions can iden- tify the majority of bird voices recorded in the Neo- tropics, they can greatly facilitate the compilation of locality lists from tapes. The following guidelines will enhance one's chanc- es of compiling an accurate locality list in a tropical forest: 1. Get up well before dawn and be out in the area to be surveyed at least 15 min before first light. Many Neotropical species (especially tinamous, puff- birds, woodcreepers, and flycatchers) sing only 1- 3 songs during the first 5 min of light (often well before light enough to see), and they rarely vo- calize thereafter, until an even briefer period late in the day. Nocturnal species often call just before This content downloaded from 129.125.63.113 on Fri, 28 Jun 2013 05:17:37 AMAll use subject to JSTOR Terms and Conditions

Confusions of mass and size

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Confusions of mass and size

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