Pollen and the Nitrogen Requirements of the Lesser Double&Hyphen;Collared Sunbird

826 ShortCommunications [Auk, Vol. 117 The Auk 117(3):826-830, 2000 Pollen and the Nitrogen Requirements of the LesserDouble-collared Sunbird IAN G. VAN TETS • AND SUSAN W. NICOLSON Departmentof Zoology,University of CapeTown, Rondebosch 7701, South Africa Nectarivorousbirds typically are small, often Tets and Whelan 1997, van Tets 1998). However,the weighingless than 10 g, and havehigh basalmeta- amino acidcomposition of pollenvaries among plant bolic rates.To meet their high energyneeds, many species,and many hummingbird-pollinatedplants nectarivorousspecies spend a large proportion of in North Americaproduce pollens that are deficient their time foragingon nectar.Like all birds,they also in two essentialamino acids: methionine and lysine requirenutrients such as protein, vitamins, essential (Martinez del Rio 1994). fatty acids,minerals, and trace elements.Amino ac- The relative importanceof insectsand pollen in ids occur in floral nectar, and their concentrations the dietsof nectarivorousbirds varies considerably. are thoughtto be related to pollinatortype, being Anna'sHummingbirds (Calypte anna) appear to de- low in nectarof bird-pollinatedplants (Bakerand pendon insectsfor nitrogen.They thrive in captivity Baker 1982). However, the correlation between ami- on a diet of fruit flies and nectar (Brice 1992), but no acid concentrationand pollinatortype has been when fed pollenthey digestonly 5% of the pollen disputedby Gottsbergeret al. (1984),who suggested grains (Briceet al. 1989).In contrast,Purple-crowned that most of the variation in nectar amino acids re- Lorikeets (Glossopsittaporphyrocephala) feed exten- suitsfrom flowerdamage or contaminationwith pol- sivelyon pollenand digestup to 50%of the pollen len. In any event,the amountsare too low to satisfy grainsthey ingest (œ = 37%;Wooller et al. 1988,Rich- the protein requirementsof nectarivorousbirds ardsonand Wooller1990). New Holland Honeyeaters (Martinez del Rio 1994).Other nitrogensources are (Phylidonyrisnovaehollandiae) fall somewhere between insectsor pollen(Richardson and Wooller 1990, Brice thesetwo extremes.The role of pollen in their diet 1992),both of whichpresent birds with a numberof hasbeen studied in two siteswith stronglyconflict- potential problems. ing results (Paton 1981, 1982;Wooller et al. 1988). It In insects,a large proportionof the nitrogenis in is important to note that all of thesestudies are based the formof chitin,a polymerthat may be difficultfor primarily on measures of full and empty pollen a smallnectarivorous bird to digest.Although some grains under a light microscope.Although these seabirdsare capableof producingchitinases (Jack- measurementsprovide an estimateof a bird's diges- son et al. 1992), no nectarivorous birds are known to tive capabilities,they are not accurateand can seri- do so. Accordingly,the major nitrogensources ob- ously underestimatepollen digestibility (van Tets tainedfrom insectsare likely to be the proteins,pep- and Hulbert 1999). tides, and free amino acids found in their tissues. Lesser Double-collared Sunbirds (Nectariniachal- However, the amino acid compositionof insects ybea)are commonin muchof SouthAfrica. They are varies greatly and often is dominatedby nonessen- importantpollinators of many plantsin the Cape tial amino acidssuch as proline (Tomlinet al. 1993). FloralKingdom of the southwesternCape, a region As a result, insectsare not always as good a source with a highfloral diversity characterized by thefam- of proteinas is widelybelieved. Furthermore, it may ilies Proteaceae, Ericaceae, and Restionaceae. In ad- be energeticallyvery expensivefor a small nectari- dition to the indigenousflora, N. chalybeafeeds ex- vorousbird to catchenough insects to meetits nitro- tensivelyon introducedspecies of Eucalyptus,which gen requirements. may haveled to increasesin the numbersand range Pollenhas long been recognized as a majorsource of this group (Fraserand Crowe1990, Parker 1994). of nitrogen for various insect pollinators,most no- In this study,we fed Eucalyptuspollen to LesserDou- tably honeybees(Apis mellifera; Schmidt and Buch- ble-collared Sunbirds to determine whether it was a mann 1985,Dobson and Peng1997), and it has re- viablesource of nitrogenfor this species. centlybecome clear that pollen alsois an important Methods.--Wecaptured eight sunbirds (seven elementin the diet of vertebratepollinators (Howell males and one female) in KirstenboschBotanic Gar- 1974, Richardson and Wooller 1990, Law 1992, van densin February1998 and housedthem separately in holdingcages (70 x 80 x 40 cm)in the University of Cape Town Animal House.Except during experi- •Presentaddress: Mitrani Department of Desert ments, birds were fed a mixture of 0.7 M sucrose and Ecology,Jacob Blaustein Institute for Desert Re- Ensure© (3 g per 100 mL solution)presented in com- search,Ben-Gurion University of the Negev,Mid- mercially manufacturedfeeders and renewedtwice reshetBen-gurion 84990, Israel. daily. The sunbirdsmaintained body mass and con- E-mail: [email protected] dition on this diet. July2000] ShortCommunications 827 TABLE1. Percentageof intactand emptypollen grains in Eucalyptuscalophylla pollen (n = 8), in the experimentaldiet (n = 15 batches,5 eachwith low,medium, and high pollendensity), and in the cloacalfluid of Nectariniachalybea (n = 16). Valuesare œ_ SE. Pollen E. calophylla Experimental Cloacal state pollen diet fluid Intact 88 + 1.5 85.2 _+_1.0 68.6 -+ 2.9 Empty 12 --_1.5 10.9 +_0.9 29.8 + 2.7 Partially empty 0 0.3 + 0.1 1.7 + 0.3 Germinating 0 3.6 + 0.9 0 Duringthe feeding trials, birds were kept in cylin- empty, partially empty, and germinating pollen dricalcages (diameter 36 cm,height 50 cm) in a con- grainsin pollenthat had beenin the diet solutions stant-environment room at 20øC, 50% relative hu- for 24 h. The delayensured that any effectson the midity, and a 12L:12D light cycle.Birds were fed pollen of immersionin sucrosesolution, such as in- from glass feeders constructedfrom modified bu- ducedgermination, were takeninto account.Similar retteswith the bottomend curvedupward to pre- measurementswere alsomade using fresh pollen. ventspillage and the openingenlarged .to a diameter We conductedthree trials,with a 72-h restperiod of 3 to 4 mm. We recordedbody massof eachbird at between each trail, so that each bird was tested on the beginningand end of eachfeeding trial. eachdiet. The diet givento an individualduring a For the first 48 h of each trial, birds were fed ad trial wasdetermined randomly, with the soleproviso libitum a maintenancediet of 1.2 M sucrosesupple- that no bird was giventhe samediet twice.When all mentedwith 3 g of Ensureper 100 mL solution.For three trials were completed,the samplesof cloacal the next48 h, theywere fed oneof threediets: 1 g, 3 fluid were thawed and diluted to 50 mL with dis- g, or 5 g of bee-collectedpollen suspendedin 100mL tilled water.The level of organicnitrogen in eachwas of 1.2 M sucrosesolution. We useda relativelyhigh thendetermined using Kjeldahl analysis. The nitro- concentrationto minimize settlingof the pollen;N. gen contentof all threeexperimental diets was de- chalybeareadily feeds on sugarsolutions of thiscon- terminedat thesame time. Using these values, we de- centration(Lotz and Nicolson1999). The pollen was termined nitrogen intake and nitrogen output of obtained commercially(Ridley Bee Products,West- eachbird. By regressingnitrogen balance (the dif- ern Australia)and was collectedby beeskept in an ferencebetween intake and output) againstnitrogen Eucalyptuscalophylla plantation. The pollen con- intake,we determinedthe dietary maintenanceni- tained95% Eucalyptus pollen (measured using light trogen requirements of N. chalybeaon E. calophylla microscopy),almost all of whichwas from a single pollen.We alsocalculated the standarderror of the species,presumably E. calophylla.A singledrop of a dietary maintenancenitrogen requirement. commercialmultivitamin supplement(Abbott Lab- Results.--Alleight birds maintainedbody mass oratories)was added to each 100 mL of sucroseso- and conditionduring the threefeeding trials. At the lution. The mixture was kept at 4øCand a freshso- start of the three trials, meanbody masswas 8.0 -+ lution madeevery 24 h. SE of 0.2 g, 8.0 -+ 0.2 g, and 8.2 _%0.1g, respectively; To minimizethe amountof pollenthat settledout, at the endof the trials,body mass averaged 8.1 + 0.2 the experimentaldiet availableto eachbird did not g, 7.9 ___0.1 g, and 8.0 -+ 0.2 g. exceed2 mL. Small volumesof fresh food were pro- In the pollen-sucrosemixtures fed to the birds, videdat intervalsof 2 to 3 h throughoutthe day,and 10.9%of the pollengrains were empty and 3.6%had the added volumes recorded. At the end of each trial, begunto germinateafter 24 h (Table1). In thecloacal the remainingsolution was filtered and the dry mass fluid, 29.8% of the pollen grainswere empty and of pollen determined. none was germinating(Table 1). This suggeststhat During the second48 h of the trial, traysof liquid 18.9%of the pollengrains had beenemptied in tran- paraffinwere placed beneath the cagesto collectthe sit, aswell as all thosethat had begunto germinate. cloacal fluid. Cloacal fluid was then removed from Becausethe latter were less than 4% of the total, and the traysusing Pasteur pipettes. During the second becausemost pollen was eaten in lessthan 24 h, 19% and third feedingtrials, three subsamples,each of is a reasonableestimate of the ability of N. chalybea less than 0.1 mL, were removed from the cloacalfluid to digestE. calophyllapollen grains. of eachbird

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