Color Preferences of Frugivorous Birds in Relation to the Colors of Fleshy Fruits’
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THE CONDOR -- 1990 A JOURNAL OF AVIAN BIOLOGY SW17 Volume 92 Number 3 The Condor92545-555 0 The CooperOrnithological Society 1990 COLOR PREFERENCES OF FRUGIVOROUS BIRDS IN RELATION TO THE COLORS OF FLESHY FRUITS’ MARY F. WILLSON,* DANIEL A. GRAFF,AND CHRISTOPHERJ. WHELAN~ Department of Ecology,Ethology, and Evolution, Universityof Illinois, SherfordVivarium, 606 E. Healey Street, Champaign, IL 61820 Abstract. Aviary experimentsdemonstrated that threeNorth Americanfmgivorous mi- grantbird speciesoften exhibit colorpreferences, when factors such as taste,nutrition, and accessibilityto food sourcesare equal. Individual birds differed in initial color preferences, transitivity, and temporal stability of color preferences.There was little tendency of these birds to favor red and black, which are the most common colors of fruits of bird-dispersed plants, and a weak tendency to reject yellow. We suggestthat avian color preferencesmay not provide strong selection favoring the evolution of the common fruit colors, and that the frequency distribution of fruit hues is best explained in other ways. Key words: Color preferences;fmgivory; fruit color; Gray Catbird; Swainsons’ Thrush; Hermit Thrush;Dumetella carolinensis: Catharus ustulatus; C. guttatus. INTRODUCTION proposed,and several factors may have contrib- Fleshy fruits consumed by avian frugivores are uted to this distribution (Willson and Whelan, often colored red or black, sometimes blue, pur- in press). To begin to assessthe importance of ple, pink, or white, but rarely orange, yellow, different factors that may help account for the green, or brown (Janson 1983, Willson et al. observed distribution of fruit colors, we examine 1989). Fruit colors are commonly considered to the simple hypothesis that the frequency distri- increase the conspicuousnessof a ripe fruit crop bution of colors reflects the color preferencesof and/or attract the birds that eat fruits and dis- avian frugivores. Under this hypothesis, one perse the enclosed seeds (Darwin 1859, Ridley would expect that red and black would be pre- 1930, van der Pijl 1982). However, there is little ferred most frequently and consistently, followed documentation of the effects of fruit colors on by blue and white, and then by yellow, green, avian foraging decisions (McPherson 1988) or and brown. on variation in color preferenceseither within or Here we examine the following questionsabout among fiugivorous bird species. the attractivenessof different colors, as measured An explanation of the frequency distribution by color preferences of some frugivorous birds of fruit colors must account for the predomi- of the deciduous forest region of eastern North nance of red and black, the lesser occurrence of America: (1) What colors (of food) are preferred blue and white, and the rarity of yellow, green, (or avoided) by fruit-eating birds? (2) How vari- and brown. A number of hypotheseshave been able are color preferences among conspecific birds? Do different bird specieshave similar col- or preferences?(3) Does a given bird make “tran- ’ ’ Received2 November 1989. Final acceptance26 sitive” (i.e., internally consistent, see Methods) February1990. choices?(4) Are color preferences stable in the * Presentaddress: Forestry Sciences Laboratory, P.O. Box 20909, Juneau,AK 99802-0909. face of exposure to other food colors? (5) Does 3Present address: The Morton Arboretum, Lisle, IL the background against which the fruits are dis- 60532. played affect color preferences? 15451 546 M. F. WILLSON, D. A. GRAFF AND C. J. WHELAN METHODS purple, red, or blue. All of these colors are ren- dered as perceived by human eyes; we do not SUBJECTS know what the birds actually see. Under the ini- We used Gray Catbirds (Dumetella carolinensis), tial hypothesis, the expected outcome would be Swainson’s Thrushes (Catharus ustulatus), and a preference for red and black, followed by blue, Hermit Thrushes (C. guttatus) for these experi- and then yellow. ments. The thrushes were entirely passagemi- Because we were interested in whether color grants, but catbirds probably included both mi- preferences are transitive (e.g., if red > black, grants and summer residents. The birds were and black > blue, then red > blue, see Moer- captured locally in mist nets in the fall of 1988 mond and Denslow 1983) we used a pairwise and housed individually in six aviaries (2 x 2 design, in which the direction of preference for x 2.5 m) at the Shelford Vivarium, University each pair of colors was determined indepen- of Illinois. Birds were held lessthan 3 weeks and dently, rather than a four-way “cafeteria” design. were releasedat the end of the experiment; three The order of the pairwise comparisons was de- birds escaped during the trials. The number of termined from a random-number table, with the birds tested was constrained by the number of constraint that alternating pairwise comparisons aviaries available, the need to release birds be- on the same day used all four colors (e.g., red vs. fore their local seasonof migration was over, and blue, alternating with black vs. yellow). The first the number escaping. Captive birds were fed comparison alternated with the second, until a chiefly on a banana-based maintenance diet de- predetermined number of replicates (see below) signed for fruit-eating birds (Denslow et al. 1987) was complete. Likewise, the third comparison but augmented with eight teaspoons of glucose alternated with the fourth, and the fifth alter- (per 2 l), supplemented with mealworms (Te- nated with the sixth. nebrio molitor), wax worms (Galleria mellonel- Because catbirds reacted quite differently in la), and real fruit, of various speciesand colors, the aviaries from both thrush species,we used collectedlocally. All birds were given several days slightly different experimental protocols for the to become accustomed to aviary conditions be- different species.Catbirds adjusted quickly to the fore testing..Although some of the test birds may aviaries and readily foraged in our presence. In have hatched in 1988, all were fully fledged and a given foraging trial, two cubes (of two colors) capable of foraging independently. Two cohorts were placed in each of four separatepetri dishes of catbirds were used: Cohort 1 (birds l-6), 8- (30 mm diameter) arrayed along a wooden perch. 12 September; Cohort 2 (birds 7-12), 28 Sep- Each bird was allowed to feed from this display tember-l 1 October. Swainson’s Thrushes (n = until four of the eight cubes had been eaten (usu- 4) were tested on 19-23 September, and Hermit ally < 15 min). For each catbird, there were four Thrushes (n = 6) on 16 October-2 November. replicates of each trial; we did not attempt to All birds used in these experiments ate well in analyze any possiblepatterns of variation among captivity. replicates. Because equal numbers of cubes of each color in each comparison were available, EXPERIMENTAL PROCEDURE we defined no color preference as no statistical Initial color preferences.Experiments consisted difference (determined with the binomial exact of paired choices of maintenance diet (prepared test) in the number of cubes of each color taken. in 4-mm cubes) treated with commercial food Most experiments were conducted between coloring. Colors used were red, blue, yellow, and 07:OOand 14:O0. “black.” An approximate indication of the colors In contrast to the catbirds, neither thrush achieved is given by comparison to Smithe’s species would eat in our presence. For these (1975) color standards: red #l 10, blue #68, yel- species,therefore, we weighed out a known quan- low #18, and black #89. These colors were not tity of dietary cubes in 1O-cm petri dishes (each intended to mimic those of any particular wild color in a separate dish), which were placed in fruits, but they fall within the natural range of the middle of the aviary floors. Placement of fruit colors (to our eyes). Black was produced by dishes with different colors of food was alter- a mixture of roughly equal parts of red and blue, nated to avoid positional bias. We then allowed producing an extremely dark purple; many nat- the birds to feed until roughly half of the total urally black fruits also are, in fact, very dark food was eaten (usually <2 hr) and weighed the FRUIT COLOR PREFERENCES IN FRUGIVORES 547 remainder. The difference was the amount eaten against a background of green foliage on the par- (a small weight loss, <5%, due to evaporation ent plant. We placed each pair of dietary cubes did not affect the analysis). Experiments were on a spray of green artificial foliage attached to conducted between 07:OOand 17:OO.There were the perch. A bird on the perch could reach any three replicatesofeach pairwise comparison, and of the pairs of cubes with two or three hops. the data were analyzed by two-way (bird, color) Results of these trials were compared with those ANOVA (seeSokal and Rohlf 198 1, p. 344-348) described earlier, in which the cubes were pre- and t-tests. The t-tests were done separately for sented in petri dishes. Trials that used different each bird and each pairwise color combination backgroundswere alternated until four replicates (see Appendix 2). Although there was a direc- were completed. A significant shift of preference tional prediction for most pairs of colors (e.g., was indicated by direct comparison of the pref- red > blue), we used a conservative two-tailed erences exhibited on the two different back- test to establish whether there was a significant grounds, using x2 (P < 0.05). difference in the amount eaten. The observed direction of color preference was then deter- RESULTS mined by inspection. This procedure allowed the identification of preferences that were opposite INITIAL COLOR PREFERENCES in direction to the prediction. Catbirds. The evidence of a general preference Stability of colorpreferences. Because most in- for red and black is not strong (Table 1). Of all dividual birds of all three species exhibited 62 trials, 40 (65%) indicated no significant pref- marked preferences for certain colors, we also erence between paired colors.