J. Field Ornithol., 58(3):345-349

FLOWER DESTRUCTION AND NECTAR DEPLETION BY AVIAN NECTAR ROBBERS ON A TROPICAL , CORDIA SEBESTENA

ROBERT A. ASKINS Departmentof Zoology ConnecticutCollege New London, Connecticut 06320 USA

KAREN M. ERCOLINO Departmentof ConnecticutCollege New London, Connecticut 06320 USA

JEI•ImE¾D. W^I•LEI• Departmentof Zoology ConnecticutCollege New London, Connecticut06320 USA

Abstract.--Visitors to the blossomsof a scarletcordia tree (Cordia sebestenaL.) were observed on St. John, U.S. Virgin Islands.The blossomswere visitedby birds 191 timesduring 5.3 h of observation.Two speciesof hummingbirdsfed exclusivelyfrom the open ends of blossoms,possibly effecting in the process.Coereba flaveola and Tiaris bicolor removednectar by piercingthe baseof the corollaand drinking the nectar,but without pollination.Loxigilla noctis also extracted nectar without pollinatingthe . It plucked the corolla,grasped the basein its bill, and tilted the corollaupward as if drinking from a bottle. Nectar depletionand corolla plucking may have a major impact on the reproductive success of scarlet cordia.

DESTRUCCIONDE FLORESY DEPLECIONDE NECTARPOR PJ•JAROS LADRONES DE NECTAR DEL •RBOL TROPICAL CORDIA SEBESTENA Sinopsis.--En St. John, Islas Virgenes Estadounidenses,se estudiaronlos visitantesa los capullosde Cordiasebestena L. En 5.3 h de observaci6n,los capullosfueron visitados por avesen 191 ocasiones.Dos especiesde zumbadores,Eulampis holosericeus y Orthorhynchus cristatus,se alimentan exclusivamentedesde la parte abierta de la flor y durante el proceso probablementepolinizan. Coerebaflaveola y Tiaris bicolor obtienen nectar perforando la base de la corolay bebiendonectar sin polinizar la planta. Loxigillanoctis tambi•n obtieneel nectarde la planta sin polinizarla. Este arrancala corolla,luego toma la baseen su pico, y la elevahacia arriba, comosi estuvierabibiendo de una botella. Las avesque agotanel nectary extraenla corolade la planta,muy bien podriantener impacto negativo en el •xito reproductivode C. sebestena.

Bird-pollinatedflowers are subjectedto heavyselection pressure from nectar-robbingbirds and insects(McDade and Kinsman 1980, Roubik 1982, Roubik et al. 1985). Nectar robbersnot only depletenectar supplies, but mayalso damage floral parts.We observedtwo typesof nectarrobbing on scarletcordia (Cordia sebestena L.; family Boraginaceae):one relatively nondestructivetype that resultedprimarily in nectardepletion, and another type, to our knowledgenot previouslydescribed, that resultedin destruc- tion of the flower. Scarlet cordia is a small tree of coastal habitats in the West Indies

345 346] R. A. Askinset al. j. FieldSummerOrnithol. 1987

(Johnston1949). The blossomshave many characteristicsassociated with hummingbirdpollination (Faegri and van der Pijl 1979): bright reddish- orange corolla and calyx; a long corolla tube (averaging32 mm) with basal nectaries;and flower clustersthat protrude horizontally or hang down obliquely. Percival (1974) observedStreamertail (Trochiluspolytmus) feeding at scarletcordia blossoms in Jamaica.The blossomsalso have adaptationsassociated with preventionof nectar-rob- bing (Kodric-Brownet al. 1984): the petalsare completelyfused, and the fusedsepal tube firmly encasesthe base of the corolla, forming an ad- ditional barrier to protect the nectar. All visitors to the blossoms of an 8-m tall scarlet cordia were recorded during a total of 5.3 h at Little LameshurBay, Virgin IslandsNational Park, St. John, U.S. Virgin Islandson 29-30 May 1986. Observations were made between 06:10 and 10:00 on both days, and between 16:30 and 18:30 on 29 May. Frequent additionalbrief observationsfrom 27- 31 May indicatedvery little nectar-feedingat the tree by either birds or insectsat midday (10:00-16:30), a pattern that is typical of somehum- mingbird-pollinatedplants (Brown et al. 1981). Birdswere frequentvisitors to cordiablossoms, but only oneinsect was observedtaking nectarfrom a blossom(Table 1). Of 191 blossomsvisited by birds, 44% were robbed(i.e., nectar was taken without touchingthe reproductivestructures of the flower) and 56% were fed uponlegitimately (i.e., nectartaken by dipping the bill into the corolla,touching the anthers and stigmasin the processand potentially effectingpollination). Two speciesof hummingbirds(Green-throated Carib, Eulampis ho~ losericeus,and Antillean CrestedHummingbird, Orthorhynchuscristatus) accountedfor almostall of the legitimatefeeding (Table 1). Bananaquits (Coerebafiaveola)were the most frequent nectar robbers(Table 1). They

TABLE1. Feedingtechniques used by birds feedingon the nectarof a Cordiasebestena tree during a total of 5.3 h of observation.

Nectar robbing Legitimate (numberofblossoms) feeding (number of Flower Corolla Species blossoms) piercing plucking

Green-throated Carib Eulampis holosericeus 97 0 0 Antillean Crested Orthorhynchuscristatus 7 0 0 Bananaquit Coerebafiaveola 3 41 0 Black-facedGrassquit Tiaris bicolor 0 16 0 Lesser Antillean Bullfinch Loxigillanoctis 0 0 27 Carpenter bee Xylocopasp. 1 0 0 v,,i.•s, No. • Nectar-robbingBirds [347 piercedthe sepalsand the base of the corolla, creating an oval slit into which they insertedtheir bills to feed on nectar. In most casesthey appearedto use holesthat were already present,as did the Black-faced Grassquits(Tiaris bicolor)that visitedthe tree. There were 4 aggressive encountersbetween Bananaquits, and they chasedGrassquits away from the tree twice. No aggressionwas observedbetween Bananaquits and hummingbirds;in fact, Bananaquitsand Green-throated Caribs some- times fed peacefully at the same flower cluster. Also, Lesser Antillean Bullfinches(Loxigilla noctis) and Bananaquitsfed closetogether without aggression. A surveyof the lower half of the tree revealedthat 54% of the intact flowershad been pierced.Nectar levels(measured by extractingnectar at the base of the corolla with a i cc syringeat 09:45) were lower in piercedflowers than in unpiercedflowers. The meansand 95% confidence intervals are as follows: pierced--12 ___8 t•l; unpierced--24 ___12 t•l. This differencewas not significant(t = 1.8, df -- 74, P = 0.08). LesserAntillean Bullfinchesmay have a disproportionateeffect on the reproductiverate of the tree becausethey destroyflowers when robbing nectar (Table 1). Bullfinchesreached the nectar by graspingthe corolla at the baseand plucking it out of the calyx(Fig. i). They sometimesfed on nectar while pressingthe corolla down againsta branch with a foot. More often they graspedthe narrow, basal end of the corolla in the bill

Julie Zickefoose

FIGURE1. Nectar-feedingbehavior of LesserAntillean Bullfinch (Loxigillanoctis) on a Cordiasebestena tree. (a) The bullfinch plucksthe corollaout of the fusedsepal tube. (b) The corollais then held againsta branchwith the foot while the baseof the corolla is graspedin the bill. (c) Nectar is extractedby tilting the corolla up to a vertical position.Illustration by Julie Zickefoose. 348] R. A. Askinset al. J.Field Ornithol. Summer 1987 and releasedthe foot holding the corolla.The head was then tilted back, raising the corollavertically as it was graspedin the partially openbill, as if the bird were drinking from a bottle. The corollawas then dropped. In the lower half of the tree 34% of the calyceshad no corolla, but we do not know how many of thesehad been removedby bullfinches.The removal of the corollaprobably preventspollination, but the pistils were not damagedso fruit developmentmight occurif the flower had already beenpollinated. Various species of birdshave been recorded eating (Riley and Smith 1986) and two speciesof icterids in Mexico often knockedoff corollaswhile searchingfor insectsor nectar in Ceiba acu- minata flowers (Baker et al. 1971), but we know of no other reports of birds plucking and drinking nectar from corollas.Previously, Lesser An- tillean Bullfinches have been describedas feeding primarily on fruit (Raffaele and Roby 1977, Adolph and Roughgarden1983). C. sebestenaapparently is an introducedtree to St. John (Little and Wadsworth 1964), and the Green-throated Carib and Lesser Antillean Bullfinch have only recently colonizedthe island (Lack 1976, Raffaele and Roby 1977). C. sebestenaand the five speciesof birds that visited it occurtogether on many islandsin the LesserAntilles (Johnston1949, Bond 1974), however, so interactionsamong these speciesmay have oc- curredfrequently during their evolution.Our preliminary resultssuggest that nectar-robbingbirds may have an important impact on reproduction in scarlet cordia.

ACKNOWLEDGMENTS

We wish to thank Glenn Dreyer, Peter Feinsinger,John Kricher, and William Niering for helpful commentson an early draft of this paper,and GeorgeBarrowclough for permitting us to use specimensfrom the collectionof the American Museum of Natural History. Also, we thank Noel Pachta,superintendent of Virgin IslandsNational Park, for permissionto do this project in the park.

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