April 2001] ShortCommunications 519

CHASE, M. K., N. NUR, AND G. R. GEUPEL. 1997. Sur- RAPPOLE,J. H., E. S. MORTON, T. E. LOVEJOYIII, AND vival, productivity, and abundancein a Wilson's J. L. RUOS.1983. Nearctic Avian Migrants in the Warblerpopulation. Auk 114:354-366. Neotropics.U.S. Department of the Interior, Fish COOPER,B. A., AND R. J. RITCHIE. 1995. The altitude and Wildlife Service,Washington, D.C. of migration in east-centralAlaska: A radar RIMMER,C. C. 1988.Timing of the definitiveprebasic and visual study. Journalof Field Ornithology molt in Yellow Warblers at JamesBay, Ontario. 66:590-608. Condor 90:141-156. DWIGHT,J., JR. 1900. The sequenceof plumagesand SOGGE, M. K., W. M. GILBERT, AND C. VAN RIPER III moults of passerinebirds of New York. Annals 1994. Orange-crownedWarbler (Vermivoracela- of the New York Academyof Sciences13:73-360. ta). In The of North America, no. 101 (A. HUSSEL,D. J. T. 1991a. Fall migrations of Alder and Pooleand E Gill, Eds.).Academy of Natural Sci- Willow flycatchersin southernOntario. Journal ences, Philadelphia, and American Ornitholo- of Field Ornithology 62:260-270. gists' Union, Washington,D.C. HUSSEL,D. J. T. 1991b. Spring migrations of Alder WINKER, K., D. W. WARNER, AND A. R. WEISBROD. and Willow flycatchers in southern Ontario. 1992. Migration of woodlandbirds at fragment- Journalof Field Ornithology62:69-77. JOHNSON,M.D., AND G. R. GEUPEL.1996. The im- ed inland stopoversite. Wilson Bulletin 104:580- 598. portance of productivity to the dynamics of a Swainson'sThrush population.Condor 98:133- WOODREY,M. S., AND C. RAYCHANDLER. 1997. Age- 141. relatedtiming of migration:Geographic and in- KESSEL,B. 1984. Migration of Sandhill Cranes,Grus terspecificpatterns. Wilson Bulletin 109:52-67. canadensis, in east-central Alaska, with routes Received31 January2000, accepted27 November2000. through Alaska and westernCanada. Canadian Associate Editor: E Moore Field-Naturalist 98:279-282. MCINTYRE,C. L., AND R. E. AMBROSE.1999. Raptor migrationin autumn throughthe Upper Tanana River Valley,Alaska. WesternBirds 30:33-38.

The Auk 118(2):519-524, 2001

Growth Rate and Energeticsof (Turdoidessquamiceps) Nestlings

AVNER ANAVA, 1'2MICHAEL KAM, 2 AMIRAM $HKOLNIK, 3 AND A. ALLAN DEGENTM lDepartmentof Life Sciences,Ben-Gurion University of theNegev, Beer-Sheva 84105, Israel; 2DesertAnimal Adaptations and Husbandry, Wyler Department of DrylandAgriculture, JacobBlaustein Institute for DesertResearch, Ben-Gurion University of theNegev, Beer-Sheva 84105, Israel; and 3Departmentof Zoology,George S. WiseFaculty of Life Sciences,Tel Aviv University,Tel-Aviv 69978, Israel

ABSTRACT.--ArabianBabblers (Turdoidessquami- lings was 0.450,which was 18%higher than that pre- ceps)are territorial, cooperativebreeding dicted for a passerineof its body mass.Asymptotic that inhabit extreme deserts and live in groups all body mass of fledglings was 46 g, which was only year round. All membersof the group feed nestlings 63% of adult body mass,a low percentagecompared in a singlenest, and all group membersprovision at to other passerines.Energy intake retained as energy similar rates. Nestlings are altricial and fledge at accumulatedin tissuedecreased with age in babbler about 12 to 14 days,which is shortfor a passerineof nestlingsand amountedto 0.29 of the total metabo- its body mass. Becauseparents and helpers feed lizable energyintake over the nestlingperiod. How- nestlings,we hypothesizedthat the growth rate of ever, energy content per gram of body mass in- nestlingsis fast and that they fledge at a body mass creasedwith age and averaged4.48 kJ/g body mass. similar to other passerinefledglings. Using a logistic We concludedthat our hypothesiswas partially con- growth curve, the growth rate constant(k) of nest- firmed. Growth rate of babbler nestlingswas rela- tively fast comparedto other passerinespecies, but fledgling masswas relatively low. Address correspondenceto this author. E-mail: Desertsare characterizedby unpredictablerainfall [email protected] and unpredictable, often sparse, food availability 520 ShortCommunications [Auk, Vol. 118

(Evenari et al. 1982), which can lead to a relatively Doublylabelled water measurements.--Field metabol- slow growth rate of nestlings.The Arabian Babbler ic rate (FMR) and water flux of babbler nestlings (Turdoidessquamiceps) inhabits extreme deserts, were measuredfrom Februaryto August, 1996.Nes- which suggeststhat the growth rate of babblernes- tlings (n = 65) were injectedsubcutaneously with 70 tlings would be slow.However, that doesnot appear Izl/g water whose oxygen was 95% •80 and whose to be the casebecause nestlings fledge at 12 to 14 tritium produced 1.85 MBq/mL. Injections were days (Zahavi 1990), which is short comparedto pas- done between 0900 and 1100 (GMT + 2 h). One hour serinesof similar body mass(Ricklefs 1968). Conse- was allowed for equilibrationof the isotopeswith quently,nestlings have either a fast growth rate or body fluids (Degen et al. 1981), after which time a they fledge at a low body mass,or both. blood samplewas collectedfrom a brachialvein and Arabian Babblersare cooperativebreeders and the nestling was weighed on an electronicbalance parentsand helpersparticipate in the feedingof nes- (+0.2 g). Further blood sampleswere taken daily for tlings. All group membersprovision at similar rates, the next one to two daysat the samehour. irrespective of sex or dominance rank within the Blood sampleswere microdistilled under vacuum group (Wright 1997, 1998) and, therefore,we hy- until drynessto obtain pure water. Specificactivity pothesizedthat the growth rate of nestling• is fast. of tritium in the water was measuredby liquid scin- To test that hypothesis,we determinedgrowth rate tillation spectrometry(Nagy 1983).Level of •80 spe- of babbler nestlingsusing a logistic growth curve cific activity was measuredby an autogammacount- and comparedresults with other passerines.We also ing system (Packard) after converting •sO to measuredenergy use in babbler nestlingsand esti- gamma-emitting•SF by cyclotron-generatedproton mated body-energyaccumulation during growth. activation (Wood et al. 1975). Blood samplesfrom Materials and Methods.--Studyspecies and study three noninjectednestlings from differentnests were site.--The Arabian Babbler(65 to 85 g) is a treated similarly to measurebackground levels of •sO speciesdistributed in Saudi Arabia, Sinai, and in and tritium. someof the extremedeserts of Israel, and is the only Total body water (TBW) of eachnestling was cal- bird speciesin Israel that lives in groupsyear round. culated from the initial dilution volume of isotopic Thosegroups are territorialwith the numberof birds water, and water fluxes were calculated from the per group generallybetween 3 and 5 individuals,but subsequentdecline in specificactivity of tritium over the range is between 2 and 22 (Zahavi 1990). time (Degen et al. 1981).We were suspectof the exact Egg laying in eachgroup of babblerstakes place in injection volume in some nestlingsand, therefore, one nest usually between February and August. there were 48 TBW measurementsof the 65 injected Three clutchesare possible.Breeding females lay be- nestlings.In nestlingswithout TBW measurement, tween 3 and 5 eggs during each clutch and, where TBW valuesfrom the regressionanalysis of TBW on more than one female lays, the total number of eggs body masswas used (see results) to calculatewater can reach 13. Maximum number of fledglingsfrom flux and FMR (Anava et al. 2001). Total body solids one nest is about six. All membersof the group par- were calculatedas the differencebetween body mass ticipate in all phasesof nesting and rearing young and total body water.Rates of CO2production were (Zahavi 1989, 1990). estimatedfrom the declinesin specificactivities in All birds, including nestlings,were color banded. tritium and •80 over time (Nagy 1980).Eight samples To determine growth rate of nestlings,77 chicksfrom could not be measured for •80 and, therefore, there 24 nestswere used; all nestswere attendedby par- were 57 FMR measurements. ents and helpers.Five nestshad a brood of two, 10 Treatmentof data.--A logisticequation was usedto had a brood of three, 8 had a brood of four, and 1 describe the growth curve of nestlings (Ricklefs had a brood of five. All nests were observeddaily, 1968). We calculated the age of maximum growth and time of hatchingwas determinedin each.Age of rate (in days) and maximum growth rate (in grams nestlings was measured in days. Changes in body per day) at the inflection point. To compare the mass of nestlings were followed by periodically growth rate of Arabian Babblernestlings with other weighing them over the nestingperiod and pooling bird species,we determined the time required to the total of 547 measurementsthat were made (De- grow from 10 to 90% (t•0_90)of asymptoticbody mass gen et al. 1992). (Ricklefs 1968). The study was done at the Nature Reserveat Hatz- Ratesof CO2 production of babbler nestlingswere eva (30ø45'N; 35ø15'E) in the Arava, -30 km south of convertedto ratesof heat productionand energyin- the Dead Sea.That site is characterizedby long, hot, take on the basis of an insect diet: 25.7 J were ex- dry summers.It has a winter rainfall that averages pended per milliliter of CO2 produced (Nagy 1983) 35 mm annually,but there are large variationsin to- and metabolizableenergy was 0.75 of grossenergy tal rainfall and in its temporal and spatial distribu- (Robbins1983). We assumedwater influx equalled tion. Average daily air temperature for the hottest metabolic and preformed water from food. For in- (August)and coldest(January) months are 30øCand sects, a volume of 0.660 Izl of metabolic water was 15øC,respectively (Stern et al. 1986). generated per milliliter of CO2 produced (Nagy April 2001] ShortCommunications 521

40

o oo 35

30 o 5o •, 25 øøø•o 30 [ 2o

2O

0 2 4 5 8 10 12 14 10 20 30 40 50

Age (days) Body mass (g)

FIG.1. Bodymass of Arabian Babblernestlings in FIc. 2. Total body water volume of Arabian Bab- relation to age. The line representsthe logistic bler nestlingsin relationto body mass. growth curve (seetext).

body mass and mean adult mass (72 g) was 0.63. 1983). Preformed water intake was calculated as the Maximum growth rate of nestlings,5.17 g/day, was difference between water influx and metabolic water. attained at 5.74 days and the time required to grow We used FMR, growth rate, and changesin total from 10 to 90% of asymptoticbody mass(t,0_90) was body solidsto calculatethe energy budget of nest- 9.76 days lings. Metabolizable energy intake (MEI) of the Totalbody water, water flux andfield metabolicrate.- growingArabian Babbler nestlings was expressed in Total body water volume (TBW; in milliliters), esti- terms of heat production(HP) plus or minus energy mated from •80 space,increased linearly with body retention (ER) accumulatedin new tissue.HP was es- mass (mb;in grams), TBW = 0.711 mb + 1.781 (n = timated from the DLW measurements and ER from 48; Sb= 0.36;Sy.x = 1.75;r 2 = 0.89;and P < 0.001,Fig. the changesin body solidsof the growing nestlings, 2). However,TBW, as a fraction of body mass,de- assumingthat 21.3 kJwere accumulatedas body en- creasedlinearly with body mass(in grams),TBW = ergy per gram of body solidgain (Weathersand Sul- 0.861-0.0028m• (n = 48; S• = 0.001;Syx = 0.06;r 2 = livan 1991). Dry-matter intake was calculatedfrom 0.11; and P < 0.05). the MEI, assuming20.3 kJ of metabolizableenergy Both water influx and water efflux increased line- were yielded per gram dry matter (Nagy 1983). arly with body mass.The regressionequation of wa- Fresh-matterintake was the sum of dry matter and ter influx (milliliters per day) on body mass (in preformedwater of the diet. TotalER asa fractionof grams) was Water influx = 0.352 mb+ 5.221 (n = 65; total MEI was determinedon a daily basisand over S• = 0.06;S•.• = 3.56;r 2 = 0.36;and P < 0.001),and the whole nestlingperiod. water efflux (milliliters per day) on body mass (in Results.--Nestlingdevelopment and growth.--Arabi- grams) was Water efflux = 0.490 m•- 2.051 (n = 65; an Babblersat hatchingweighed 4.47 + 0.61 g (n = S• = 0.05;S•, = 2.99;r = = 0.61;and P < 0.001,Fig. 3). 37) or 6.2% of adult body mass. Hatchlings were Water efflux at 22.8 ml/day equalled water influx completelynaked and their eyelidswere fused. Their and this occurredat -50 g body mass.Field meta- eyes opened between four and six days, during bolic rate (FMR) of the nestlingsincreased linearly which time feathersemerged. As fledglings,aged 12 with body mass(Fig. 4). The regressionequation of to 14 days,their bodieswere coveredwith plumage, FMR (kilojoulesper day) on body mass(in grams) but developmentof wingsand tail werenot complete was FMR = 1.52 mb- 4.35 (n = 57; Sb= 0.37; Sy, = and, as a result, they were unable to fly. Tarsus 19.12; r == 0.24; and P < 0.001). lengthon day 10 (36.7 _+1.53 mm, n = 24) was96.6% Discussion.--ArabianBabblers attain adult body of adult size (38.8 + 0.91 mm, n = 110). massbetween 8 and 12 monthsof age (A. Anavaun- From the growth curve (n = 547) of nestlings(n = publ. data). The nestling phase (12 to 14 days) is 77) describedby the logisticcurve (Fig. 1; Appen- short comparedto other passerinesof similar body dix), the asymptoticbody massof the babblerswas mass and to passerinesin general (Ricklefs1968). 46.0 g and the constantparameters B and k were 13.3 Shorteningthe nestlingperiod can (1) allow adults and 0.450,respectively. The ratiobetween asymptote to producemore clutches per year;(2) reducethe risk 522 ShortCommunications [Auk, Vol. 118

120 '

o

ß o 25 100 q o

eee ß e o o o 80, • 20 o o

60- • •o• o.•ee • 0 40- O000 0 o oø• o ß o :o 20- 0'• 0 ""0 0

0 , , , , , , 15 20 25 30 35 40 45 50 15 20 25 30 35 40 45 50 Body mass (g) Body mass (g) F•G.4. The effectof body masson field metabolic F•G. 3. The effect of body mass on water influx rate (FMR) in Arabian Babblernestlings. (solid circles)and water efflux (open circles)in Ara- bian Babblernestlings. (Degenet al. 1992).It increasedwith age (Fig. 5) and reachedits peak (72 kJ/day) at 10 daysof age.Total of predation(Lack 1968);and (3) allow adultsand metabolizableenergy intake (MEI) per chickover the fledglings to move from the nesting site to forage nesting phase of 1 to 14 days was 759.4 kJ. This is new patches.During the nestlingperiod of babblers, only 68.5% of the predictedMEI (kJ/nestling)cal- skeletaldevelopment is fast,which is typicalof most culatedfrom an equationgenerated from fledgling passerines.However, that is not the casewith the de- body mass and fledging time (Weathers1992). The velopmentof feathersand body sizeand, asa result, ratio of ER to MEI decreasedfrom 0.62 at 1 day of fledglings are unable to fly and to forage indepen- age to 0.05 at 14 days of age (Appendix). Growing dently for about two months (A. Anava pers. obs.). nestlingsrequired more energyfor maintenanceand They are dependenton their parentsand helpersfor food. Thus, those fledglings are like nestlings,but lOO are outside the nests. The growth rate constant(k) was 0.450/day which 9O

was 18% higher than that predicted (0.381/day) for 8O a passerineof its body mass(Ricklefs 1968). The time required to grow from 10 to 90% of its body mass 7O (t7o-9o)was 9.76 dayswhich was 17% fasterthan that • 60 predicted(11.4 days) for a passerineof its bodymass 'o (Ricklefs1968). The ratio betweenthe asymptoteand adult mass, 0.63, is one of the lowest in passerine = 40 species(Bateman and Balda 1973,Degen et al. 1992, [u Weathers et al. 1990, Woolfenden 1978). 3O

Water content,as a fractionof body mass,d,e- 2O creasedwith age in babbler nestlings. A reduction has been reported in other birds (Robbins 1983, 10 Rowe 1990) and is related to an accumulation of 0 body lipid in growing nestlings (O'Connor 1977, 0 5 10 15 Robbins 1983). Water efflux and influx increased Age (days) with body massin growing nestlings.Near fledging (46 g), massspecific water influx was 46 ml g-• day-• F•G. 5. Metabolizableenergy intake (MEI); field which was similar to the 41 ml g-• day-• found for metabolicrate (FMR), heat production(HP); metab- adults (Anava 2000) and similar to Northern Shrike olizableenergy for maintenance(MEm); and energy (Laniusexcubitor) nestlings near fledging (Degen et retention(ER) of Arabian Babblernestlings. The area al. 1992). between FMR and MEm representsthe heat incre- Metabolizableenergy intake followed a sigmoidal ment of feedingfor growth.Values were based on lo- curve, as has been reported for Northern Shrikes gistic growth curve (seetext). April 2001] ShortCommunications 523

thermoregulationand, therefore,a smaller propor- Journalof Applied Physiology:Respiratory, En- tion of MEI was available for growth. Thus, even if vironmental and ExercisePhysiology 51:1183- the efficiencyof utilization of energy for growth re- 1188. mained constant,that is, the relative proportionsof DEGEN, A. A., B. PINSHOW, R. YOSEF, M. KAM, AND heat increment of feeding for growth and of ER K. A. NAGY.1992. Energetics and growth rate of stayed constant,the ratio of ER to MEI would de- Northern Shrike (Lanius excubitor)nestlings crease.However, concomittantly,as nestlings grew, Ecology 73:2273-2283. the ratio of ER per changein body massincreased DIEHL, B. 1971. Productivity investigation of two from 3.18 kJ per g body massin nestlingsof i day to types of meadowsin the Vistula Valley. XII. En- 5.65 kJ per g body massin nestlingsof 14 days. The ergy requirementin nestlingand fledglingRed- nestlings were putting on proportionately more backed Shrike (Laniuscollurio L.). Ekologia Pol- body solid and more lipid as they grew,and thus the ska 19:235-248. energy content per change in body mass increased EVENARI,M., L. SHANAN,AND N. TADMOR.1982. The Ne- with age. Averageenergy content per gram of body gev: The Challengeof a Desert,2nd ed. Harvard massover the whole nestlingperiod was 4.48 kJ per UniversityPress, Cambridge, Massachusetts. gram. LACK,D. 1968.Ecological Adaptation for Breedingin Calculated total dry matter and fresh matter in- Birds. Methuen, London. takes of each young over the 14 day nestling period NAGY,K. A. 1980. CO2production in :Anal- was 35.8 and 202.3 g, respectively.Mass-specific dai- ysisof potential errorsin the doublylabeled wa- ly dry matter intake generally decreasedwith age ter method.American Journal of Physiology238' and ranged between 0.07 and 0.11 per gram of body R466-R473. mass.Lowest fresh-matterintake per day as a frac- NAGY, K. A. 1983. The doubly labeled water tion of body mass was 0.52 at 12 to 14 days of age (3HH•80)method: A guide to its use.UCLA Pub- and highestfresh-matter intake as a fractionof body lication Number 12-1417, University of Califor- mass per day was 1.49 on the day after hatching. nia at Los Angeles,Los Angeles. Red-backed Shrikes (Lanius collurio) had an overall O'CONNOR,R. J. 1977.Differential growth and body fresh-matterintake per day of 0.56 body masswith compositionin altricial passerines.Ibis 119:147- a daily range of 0.40 to 0.78 (Diehl 1971)and North- 166. ern Shrikeshad a daily intake of 0.38 to 0.85 of body RICKLEFS,R. E. 1968.Patterns of growth in birds. Ibis mass. 110:419-451. We concluded that our hypothesiswas partially ROBBINS,C. R. 1983. Wildlife Feedingand Nutrition confirmed.Growth rate of Arabian Babblernestlings Academic Press, New York. was relatively fast comparedto other passerinespe- ROWE, R. A. 1990. Tritiated water measurements of cies,but fiedging masswas relatively low. A conse- water flux in nestling and adult Zebra Finches quenceof early fiedging could be that predation is (Poephilaguttata). Comparative Biochemistry reduced and foraging area in a harsh environmentis and Physiology95A:253-257. increased. STERN, E., Y. GRADUS, A. MEIR, S. KRAKOVER,AND H. Acknowledgments.--Wethank Amotz Zahavi for his TSOAR.1986. Atlas of the Negev. KeterpressEn- helpful discussions,Ron Mumme, Carlos Bosque, terprises,Jerusalem, Israel. and an anonymous reviewer for constructive sug- UNITED NATIONS EDUCATIONAL, SCIENTIFIC AND gestionsand personnelat Hatzeva Field Schoolfor CULTURALORGANIZATION. 1977. Map of the their help and hospitality. This researchwas sup- world distribution of arid lands. MAB, Technical ported by the Israel ScienceFoundation adminis- Note 7. UNESCO, Paris. tered by the Israel Academy of Sciences and WEATHERS,Wo W. 1992. Scaling nestling energy re- Humanities. quirements.Ibis 134:142-153. WEATHERS, W. W., M. T. STANBACK, AND W. D. KO- LITERATURE CITED ENIG. 1990. Breeding energetics and thermal ecology of the Acorn Woodpeckerin central ANAVA, A., M. KAM, A. SHKOLNIK,AND A. A. DEGEN. coastal California. Condor 92:341-359. 2000. Seasonalfield metabolic rate and dietary WEATHERS, W. W., AND K. A. SULLIVAN. 1991 intake in Arabian Babblers (Turdoidessquami- Growth and energeticsof nestling Yellow-eyed ceps) inhabiting extreme deserts. Functional Juncos. Condor 93:138-146. Ecology 14:607-613. WOOD, R. A., K. A. NAGY, S. T. MACDONALD, S. I. BATEMAN, G. C., AND R. P. BALDA. 1973. Growth, de- WAKAKUWA, R. J. BECKMAN,AND H. KAAZ. 1975. velopment,and food habitsof young Pinon Jays. Determinationof oxygen-18in water contained Auk 90:39-61. in biological samplesby charged particle acti- DEGEN, A. A., B. PINSHOW, P. U. ALKON, AND H. AR- vation. Analytical Chemistry 47:646-650. NON. 1981. Tritiated water for estimating total WOOLFENDEN,G. E. 1978. Growth and survival of body water and water turnover rate in birds. young Florida ScrubJays. Wilson Bulletin90:1-18. 524 ShortCommunications [Auk, Vol. 118

WRIGHT,J. 1997.Helping-at-the-nest in Arabian Bab- ZAHAVI,A. 1990.Arabian Babblers:The questfor so- blers: Signallingsocial status or sensibleinvest- cial statusin a cooperativebreeder. Pages 105- ment in chicks? Behaviour 54:143%1448. 130 in CooperativeBreeding in Birds (E B. Sta- WRIGHT,J. 1998.Helping-at-the-nest and group size cey and W. D. Koenig, Eds.). Cambridge in the Arabian Babblers(Turdoides squamiceps). UniversityPress, Cambridge, United Kingdom. Journalof Avian Biology29:105-112. ZAHAVI,A. 1989. Arabian Babbler.Pages 253-275 in LifetimeReproduction in Birds(I. Newton,Ed.). Received30 August1999, accepted 15 February2001. Academic Press, London. AssociateEditor: C. Bosque

APPENDIX.Parameters of the nestlingArabian Babbler'senergy budget.

Age mba Amb ERb FMR c MEI a (d) (g) (g) (kJ/day) (kJ/day) (kJ/day) ER/MEI 0 3.22 0.00 0.00 3.14 1 4.86 1.64 5.21 3.16 8.37 0.622 2 7.19 2.33 7.74 6.58 14.32 0.540 3 10.36 3.17 11.13 11.40 22.53 0.494 4 14.40 4.04 15.23 17.54 32.77 0.465 5 19.18 4.77 19.38 24.80 44.18 0.439 6 24.32 5.14 22.49 32.61 55.10 0.408 7 29.32 5.01 23.48 40.22 63.70 0.369 8 33.76 4.43 22.00 46.96 68.96 0.319 9 37.36 3.60 18.67 52.44 71.11 0.263 10 40.09 2.73 14.60 56.59 71.19 0.205 11 42.05 1.96 10.71 59.56 70.28 0.152 12 43.40 1.35 7.51 61.62 69.12 0.109 13 44.31 0.91 5.10 63.00 68.09 0.075 14 44.91 0.60 3.38 63.91 67.29 0.050 Total 41.7 219.1 543.5 759.4 0.289

Mean body mass(mb; gram) = 46/[(1 + 13.27) x Daily energyretention (ER) calculatedas: ER = fractionof body solidsx Amb x energyyield. ER = (1 - (0.865-0.0029x mb))X /ktFtb X 21.3. Field metabolicrate calculatedfrom the equation:FMR = 1.52 x mb-•3s. Metabolizableenergy intake: MEI = FMR + ER.