Fruit Species Preference in Mountain Robins ( Turdus plebejus ) Leah Bomberger Department of Natural Resources; University of Nebraska Lincoln ABSTRACT The Mountain Robin ( Turdus plebejus , Turdidae ) (in the Monteverde region in Costa Rica) is an opportu nistic feeder and an important disperser for many plant species. This study examined fruit preference of the Mountain Robin between three species of plants: Viburnum costaricanum, Chamaedorea costaricana , and Chamaedorea pinnatifrons . A significant prefe rence was found for C. costaricana over C. pinnatifrons (P = 0.040). These results indicate that C. costaricana offers a higher reward and may attract more dispersers than C. pinnatifrons or V. costaricana . RESUMEN El yigÃ¼irro de montaÃ±a ( Turdus plebeju s, Turdida e) (en la regiÃ³n de Monteverde en Costa Rica) es un comedor oportunista y un dispersor importante para muchas especies plantas. Este estudio examinÃ³ la preferencia por la fruta del yigÃ¼irro de montaÃ±a entre tres especies de plantas: Viburnum cos taricanum, Chamaedorea costaricana , y Chamaedorea pinnatifrons. Se encontrÃ³ una preferencia significativa para C. costaricana por encima de C. pinnatifrons (P = 0.040). Estos resultados indicar que C. costaricana ofrece una mayor recompensa y puede atra er mÃ¡s dispersores que C. pinnatifrons o V. costaricana. INTRODUCTION food resources attracting different bird guilds. Fruvigous birds play an important role as dispersers for many tropical plant species (Stiles et al. 1989). In Costa Rican wet regions, more than half of the tree species, particularly in the understory, have fr uits adapted for seed dispersal by birds (Stiles et al. 1989). Mountain Robins ( Turdus plebejus, Turdida e) are a common species of birds in the Monteverde area and are important dispersers of cloud forest species. The Mountain Robin commonly feeds on 36 s pecies of fruits in the Monteverde area(Wheelwright et al. 1984). Most of the observed plant species being eaten were canopy to sub canopy species. The preference for certain fruits over others can help in understanding dispersal strategy and effectiven ess of a certain plant. Disperser effectiveness is defined by the contribution a disperser makes to the future reproduction of a plant (Shupp, 1993). This study evaluated the fruit preference of the Mountain Robin between three plant species, Viburnum co staricanum, Chamaedorea costaricana , and Chamaedorea pinnatifrons . V. costaricanum is a open area sub canopy species. C. costaricana and C. pinnatifrons are both light gap understory species. Fruit characteristics such as size, sugar content, and pulp am ount were analyzed to understand the level of reward received upon fruit consumption. It was hypothesized that the Mountain Robins would show a preference for
the V. costaricanum because a previous study has shown that they have been observed occasionally feeding on them with no observations of them feeding on the Chamaedorea species (Wheelwright et al. 1984). METHODS This study was conducted from July 24 to August 1, 2004 in the forest surrounding the EstaciÃ³n BiolÃ³gica in Monteverde, Costa Rica. To cat ch the Mountain Robins, three mist nets were set up on the road adjacent to the EstaciÃ³n and opened for approximately two and a half hours at dusk and dawn every day depending on weather conditions. The birds were then held in one cubic meter net cages, co vered to darken and calm birds. Three fruit species were collected from the forest surrounding the EstaciÃ³n and were continually offered to each bird; Viburnum costaricanum, Chamaedorea costaricana , and Chamaedorea pinnatifrons . These fruits were selecte d to test Mountain Robin preference based on availability and accessibility of ripe fruits. Number of fruit taken and time was recorded for each fruit selection in approximately one hour time sets. Overnight observations were also made; a known amount of the three fruit species was left for the birds and the number of remaining fruits was counted and recorded in the morning. The birds were kept for approximately three days, if eating habits were healthy, and released in the morning or early afternoon. Pu lp weight was calculated by weighing the wet fruit and then subtracting the weight of the pulp free seeds. Wet fruit diameter was measured using calipers. Sugar content was measured by inserting the pulp of each fruit species into a syringe and squeezing the fruit juice onto a refractometer to obtain a reading. A one way ANOVA test was used to determine if there was a statistically signigicant preference for certain fruit species. RESULTS A significant difference was found in the preference of fruit s pecies (one way ANOVA, P = 0.015, N = 17). C. costaricana was preferred significantly over C. pinnatifrons C. costaricana (9.5%) than in C. pinnatifrons (7%). Pulp per fruit of C. costari cana (.245g) was over double that of C. pinnatifrons (.107g)(Table 1). A difference in fruit preference between individual birds, was also found Bird (3) preferred V. costaricanum , while the three other birds all preferred C. costaricana (Fig. 2). DISCU SSION A significant preference of C. costaricana over C. pinnatifrons (Fig. 1) is interesting because the two fruits, both in the genus Chamaedorea , share many physical characteristics. They are both a fairly large fruit, which contains one large seed. C . costaricana is a bigger fruit than the C. pinnatifrons (Table 1). The significant preference for C. costaricana must be a result of factors other than physical appearance.
Chamaedorea costaricana was found to have higher sugar content, indicating it p rovided a greater reward. The C. costaricana also had more pulp per seed. Mountain Robins appear to be selecting C. cosatricana as a result of it offering a greater reward. Although Black Guans ( Chamaepetes unicolor , Crassidae) have been observed freque ntly feeding on Chamaedorea species (Wheelwright et al. 1984), the primary dispersers of the two species observed are unknown. Though Mountain Robins have not been observed feeding on the Chamaedorea species used in this study in the wild it is expected that, their preference C. costaricana over C. pinnatifrons may also be true in the wild. This preference for C. costaricana could have negative effects for the long term reproductive success of the very similar looking C. pinnatifrons . One possibility is that C. pinnatifrons is using a mimicry strategy to attract dispersors while using less effort to produce fruits with a lower reward than the C. costaricana . Further studies on this subject could include amount of fruit produced per each plant species, a higher variety of fruit species offered, a higher number of feeding repetitions, analysis of lipid and fat content in fruit species, and dispersers of the palm species. ACKNOWLEDGMENTS First, I must thank Ollie Hymen and Maria Jost for sacrificing many nights at La Taverna in order to get up at dawn and open the mist nets and also for their statistical and editorial wisdom. Much, much thanks, kids! I also want to thank Carlos Guindon for attending early morning and after dusk sessions, for teaching us how to untangle the stupid birds once caught and for his constant guidance with every aspect of this project. I would also like to thank Seanizzle for taking me on as his partner in berry and poo and for talking life in our second home in the lower lab . I would also like to thank Sonia for her ever sweetness and constant patience. LITERATURE CITED Shupp, E. W, 1993. Quantity, quality and the effectiveness of seed dispersal by animals. Vegetatio 107/108:15 29. Stiles, G. F. and A. F. Skutch, 1989. A guide to the birds of Costa Rica. Cornell Publishing. Wheelwright, N. T, W. A. Haber, K. G. Murray, and C. Guindon, 1984. Tropical fruit eating birds and their food plants: A survey of a Costa Rican Lower Montane forest. Biotropica 16: .
Table 1: Fruit characteristics of plant species Species % Sugar Fruit Weight (g) Wet fruit diameter (mm) C. costaricana 9.5 0.245 10.68 C. pinnatifrons 7.0 0.107 6.67 V. costaricanum a 17 0.12 6.0 a ( Wheelwright et al. 1984) Figu re 1: Comparison of mean number of fruit taken by Mountain Robin of Viburnum costaricanum, Chamaedorea costaricana , and C. pinnatifrons during feeding sessions of the three fruits offered using the ANOVA test. The mean of costaricana is 22.8, pinnatifron s 7.4, and costaricanum 16.2. The error bars represent the first standard deviation from the mean. Mean Number of Fruit Taken per Feeding Mean Number of Fruit Taken per Feeding Mean Number of Fruit Taken per Feeding of costaricana, pinnatifrons, and costricanum species
Figure 2: Individual Mountain Robin preferences of fruits from three species of plants; Note that bird 3 preferred V. cos taricanum over the C. costaricana .
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Preferencia de las especies de frutas en el yigirro de montaa (Turdus plebejus)
Fruit species preference in mountain robins (Turdus plebejus)
The Mountain Robin (Turdus plebejus, Turdidae) (in the Monteverde region in Costa Rica) is an opportunistic feeder and an important disperser for many plant species. This study examined fruit preference of the Mountain Robin between three species of plants: Viburnum costaricanum, Chamaedorea
costaricana, and Chamaedorea pinnatifrons. A significant preference was found for C. costaricana over C. pinnatifrons (P = 0.040). These results indicate that C. costaricana offers a higher reward and may attract more dispersers than C. pinnatifrons or V. costaricana.
El yigirro de montaa (Turdus plebejus, Turdidae) (en la regin de Monteverde en Costa Rica) es un comedor oportunista y un dispersor importante para muchas especies de plantas. Este estudio examin la preferencia por la fruta del yigirro de montaa entre tres especies de plantas: Viburnum costaricanum, Chamaedorea costaricana, y Chamaedorea pinnatifrons. Se encontr una preferencia significativa para C. costaricana por encima de C. pinnatifrons (P = 0.040). Estos resultados indican que C. costaricana ofrece una mayor recompensa y puede atraer ms dispersores que C. pinnatifrons o V. costaricana.
Text in English.
Monteverde Biological Station (Costa Rica)
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Summer 2004
Ecologa Tropical Verano 2004
t Monteverde Institute : Tropical Ecology