Fruit Color Choices of Captive Silvereyes (Zosterops Lateralis)
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The Condor98:7&Z-790 Q TheCooper Ornithological Society 1996 FRUIT COLOR CHOICES OF CAPTIVE SILVEREYES (ZOSTEROPS LATERALIS’)’ HELEN L. PLJCKEY, ALAN LILLY AND DENNIS J. ODOWD~’ Department of Ecology and Evolutionary Biology, Monash University,Clayton, Victoria 3168, Australia Abstract. Fleshy fruits occurin many colorsin nature,but red and blackpredominate. One popularhypothesis to explain the adaptivesignificance of fruit colorationis that it attractsfrugivorous birds that disperseseeds. We presentedSilvereyes (Zosterops lateralis), importantfruit consumersin southernAustralia, with choicesin the aviary betweenboth artificialfruits (made from gelatin)and actualfruits (Rhagodia parabolica) of threedifferent colors(red, yellow and white).Silvereyes exhibited a strongoverall preference for redamong both artificialand real fruits.Although individual birdsvaried in their colorpreferences for artificial fruits, all preferredthe red fruits of R. parabolica. The consumptionrate of real fruits was much greaterthan that of artificial fruits, which was probablyattributable to differencesin the characteristicsof the two fruit types.Exposure of Silvereyesto a main- tenancediet of a particularcolor for 12 daysalso failed to alter their collectivepreference for red fruits. Further, responsesto artificial infmctescencesoffering color choiceseither betweenor within infmctescencesshowed that Silvereyespreferred red fruits regardlessof the spatialformat of presentation.The overall fruit color preferencesof Silvereyeswere basedon selectionof huerather than brightness,but within the samehue, some individuals exhibitedpreferences for particularbrightness levels. These results are consistentwith the hypothesisthat fruit color is relatedto avian frugivoryand suggestthat birds can act as strongselective agents on fruit color. Key words: brightness;color; frugivory; fruit-color polymorphism;fnrits; hue;preference; Silvereye;Zosterops lateralis. INTRODUCTION in their initial color preferences,and in the tran- Red is one of the most common colors of bird- sitivity and temporal stability of these prefer- dispersed fruits (Ridley 1930). Surveys of re- ences. Often, however, inferences about the im- gional floras, despitetheir taxonomic differences, portance of color in determining fruit preferences support this generalization (Turcek 1963, Knight in aviary studieshave been confounded by cross- and Siegfried 1983, Gautier-Hion et al. 1985, speciescomparisons of fruits that introduce oth- Wheelwright and Janson 1985, Lee et al. 1988, er, uncontrolled variables suchas their size,shape, Willson et al. 1989). Willson and Whelan (1990) taste and nutritional value (Turcek 1963, Mc- proposeda set of hypothesesto explain the evo- Pherson 1987, Moermond et al. 1987). Further, lution of fruit color. Of these, perhaps the most in studies involving artificial “fruits”, it is un- intuitively appealing hypothesis is that certain clear whether their use reflects the birds’ pref- colors are more attractive to frugivorous birds erencesfor real fruits (Willson et al. 1990). Field- and promote seeddispersal. However, only a few based studies on the influence of color on fruit experiments have examined whether birds dis- selection are complicated by other factors such criminate among fruits on the basis of color (Mc- as crop size, accessibility, relative abundance, Pherson 1987, 1988, Willson et al. 1990, Willson and differences in plant morphology (Wheel- and Comet 1993, Willson 1994). Collectively, wright and Janson 1985, McPherson 1987, Will- these studies showedthat individual birds differ son and O’Dowd 1989, Whelan and Willson 1994). Silvereyes (Zosterops lateralis, Zosteropidae) ’ Received19 March 1996.Accepted 25 June 1996. are major consumers of fleshy fruits in southern z Correspondingauthor. Address for all correspon- Australia, consuming fruits of just under half of dence:Department of Ecologyand EvolutionaryBi- the 100 or so plant specieswhose fruits are re- ology, Monash University, Clayton, Victoria 3 168, ported to be taken by birds in temperate Aus- Australia, e-mail: [email protected] 3Current address: Institute of Pacific Islands For- tralia (Forde 1986, French 1990). We used aviary estry,Forestry Research Laboratory, 1643Kilauea Ave., experiments to determine whether Silvereyes ex- Hilo, HI 96720. hibit a distinct color preferencein their selection [7801 FRUIT COLOR CHOICE BY SILVEREYES 781 of fruits. First, we examined the strength and we determined preferencesby the number of fruits consistency of their fruit-color preferencesover of each color consumed and the order in which a short time scale,using both artificial fruits and they were consumed. Once presented with the the polymorphic fruits of the shrub, Rhagodia test fruits, eachbird was observedat 2 m distance parabolica R.Br. (Chenopodiaceae).Second, be- from behind a blind. Using an event recorder, cause birds may learn quickly to change their we monitored the number of fruits of each color preferencesfor fruits, depending on their degree taken and the sequencein which they were re- of exposure to them and on their availability moved over a specified time. (Morden-Moore and Willson 1982) we exposed The use of gelatin-based, artificial fruits as well Silvereyes to a diet of a particular color to see if as the natural polymorphic fruits of Oldman their fruit color preferenceschanged as a result. Saltbush, Rhagodia parabolica, allowed us to Third, the spatial scale at which fruit color se- compare color preferences involving different lection might be made is poorly known. We ex- pigment systems and attempt to control for all amined the effect of scale of fruit presentation factors other than color itself (Appendix 1). Ge- (within and between artificial infructescences)on netically polymorphic fleshy fruits occur where the color preferences of Silvereyes. Lastly, we different individuals of the same speciesproduce determined whether the fruit color preferences fruits of different colors. Individual plants of of Silvereyes were based on differencesin hue or Rhagodiaparabolica produce red, yellow or white brightness, the two aspectsthat make up what fruits and these color morphs do not differ sig- we commonly refer to as color (Goldstein 1989). nificantly in size, mass, pulp-seed ratio, water content or major nutrients (Willson and O’Dowd METHODS 1989). Artificial fruits were made from a sugar and EXPERIMENTAL PROCEDURES gelatin recipe modified from Levey and Grajal Twenty-seven adult Silvereyes were caught un- (199 1) and were dyed either red, yellow (using der permit in mist nets between 29 February McKormickO food dyes) or white (using titani- 1993 and 7 January 1994 at Bacchus Marsh, um white) to approximate the same color stan- approximately 53 km west of Melbourne, Vic- dards as the fruits of R. parabolica (approximate toria, Australia (37”37’S, 144”25’E). The birds Methuen colors: lOB8, 3A7 and lA1, respec- were maintained in a holding aviary (approxi- tively, Komerup and Wanscher 196 1). Fruits of mately 3 x 2 x 2 m). They were given at least R. parabolicawere collectedin January 1993 from two weeks to adjust to captive conditions and remnant eucalypt mallee at Djerriwarrh Creek were fed on a maintenance diet of FarexO baby (Myers et al. 1986) approximately 11 km south- food, apples, pears, and water. For all experi- east of Bacchus Marsh. Fruits were frozen at ments, birds were placed in individual cages(37 - 15°C to prevent deterioration and to enable cm wide x 50 cm deep x 36 cm high) and given experiments to be carried out when the fruits an additional 5 days on the maintenance diet to were not available in the field. adjust. These cages were placed together in a room such that the birds could not make visual EXPERIMENT 1: COLOR SELECTION OF contact, but could hear one another. Each cage ARTIFICIAL AND REAL FRUITS had a wire mesh front that allowed accessand Fruit-color preferences of Silvereyes were first observations to be made. Percheswere placed at examined using artificial fruits and then fruits of both ends of each cageand food was always pre- R. parabolica, with an interval of approximately sented at the front of the cage. On any test day, one week between test series.Fourteen birds were the birds were tested serially after being deprived testedonce daily for seven consecutivedays, and of food for one hour; after the trial, each bird in each trial all three fruit colors were presented was placed on the maintenance diet again. Water simultaneously. Each trial lasted for 25 minutes. was available ad libitum. The trials in any one Three (10 cm diameter) glass petri-dishes were experiment were carried out on consecutivedays placed in a row on the floor at the front of the between 06:00-ll:OO. The time at which each cage. This forced the birds to leave the perches bird was tested was varied systematically to test to feed. Each dish contained 20 fruits of a single for any possible effect of time of day on fruit- color. They were presentedon a rectangular(34.5 color preferences.Like Moermond et al. (1986), x 10 cm) background of green cardboard (Me- 782 HELEN L. PUCREY ET AL. thuen 26E8) to approximate the contrast be- the first type of presentation. The secondtype of tween the fruits and their background color in presentation was then administered in the same nature. The positions in which the different col- way, after a one-day interval. ored fruits were presented in each trial were se- EXPERIMENT 4: HUE AND lected randomly from the six different permu- BRIGHTNESS tations