Birds cause serious damage to orchards and vineyards and growers to utilize various techniques in a bid to control them ( Tracey & Mary, 2006). Eyespot balloons, which are usually suspended above crops, are usually used by growers to control bird damage to fruits and other horticultural crops because the technique is not only humane but also environmentally friendly ( Inglis, Huson, Marshall, & Neville, 1983; Tipton et al., 1989). However, the cost-effectiveness of the use of eyespot balloons to protect ripening fruits from bird damage is uncertain. The purpose of this literature review is to assess foraging under fear. Particularly, the review presents information on birds’ reactions to round marks. Eyespots have been utilized as antipredator mechanisms for a long period. Published studies on eyespots have invoked two main hypotheses to explain the occurrence. Some researchers argue that when the eyespots are large and centrally located, they intimidate predators in that they resemble the eyes of the predator's enemies. Other researchers have concluded that when the eyespots are small and peripherally located, they act as markers for deflecting the predators’ attack to the non-vital body parts ( Stevens, 2005). Therefore, from the above, it is apparent that the two aforementioned hypotheses are not necessarily conflicting. However, recent researches on birds’ reaction to round marks indicate that eyespots are utilized to scare predators for the fact that they are either novel or infrequently encountered striking features. A study conducted by Fukuda, Frampton, &Hickling (2008) highlights the importance of assessing the various bird-scaring technologies presently available to growers quantitatively. In their study, Fukuda, Frampton, &Hickling (2008) investigated experimentally the effectiveness of a visual bird scarer, particularly a Peaceful Pyramid, and a cheaper eyespot in two vineyards in New Zealand. Basically, visual scarers have been categorized into four broad categories; predator decoys which are natural replicas of the predators and can either be static or inertia, laser bird dispersal system which employs the use of laser technology generated by diodes to scare away birds, reflective light bird repellers which uses the wind to move reflective surfaces that reflect sunlight that scares away birds and the Ersatz Decoy Seagull Eggs which are used to replace real eggs in a Herring Gull's or Black-Backed Gull's nest, the gull adopts the inert plastic egg as its own, preventing it from breeding and controlling numbers in a humane, non-lethal way ( Tracey & Mary, 2006). The peaceful pyramid used in this study falls under the predator decoy category. In their first trial, the researchers found that the rate of damage of natural bunches of Riesling grapes by Sturnus vulgaris reduced significantly with the use of eye-spot balloons as compared to the use of a visual bird-scarer. However, a change occurred after twenty-four days of monitoring the mutilation of the natural Riesling grapes, in that the rate of damage decreased by 75% and 84% with the use of eyespot balloons and visual bird scarer respectively. The conflicting results rendered the researchers to carry out a second trial whereby they attached both balloons and a pyramid to a vineyard wire in clusters of table grapes. In their second trial, the researchers noted that although both the balloon and the pyramid reduced the birds’ damage to the grapes within fifteen minutes, they did not have a further effect. After the second trial, Fukuda, Frampton, and Hickling (2008) concluded that both the balloon and the pyramid do not offer an economically considerable reduction in grape damage. Also, the University of Canterbury Wildlife Management class of 1993 conducted a fieldwork study to assess the effectiveness of a small group of eyespot balloons on damage to grapes at varying distances from the balloons and at the time prior to harvesting. During the field trial, the students hung six balloons above a ripening crop of pinot noir grapes just five weeks before harvest time. The damages to the bunch of grapes were assessed visually at distances of up to 40 meters from the balloons. The visual assessment was done at weekly intervals (Hickling Graham, 1995). The findings of the study indicated that the damage to grapes by birds increases as harvest approaches, particularly as the grapes ripen and become colored. According to the obtained results, bird damage was only 55% to the grapes that were directly below the balloons. Further, the work established that the deterrence impact of balloons protracted to about twenty meters in the course of the first week. However, this changed over the subsequent weeks. Eye spots on balls scare birds because they mimic the eyes of a large raptor. Further detailed experiment shows that the eyespot technique is more effective if the balls used are big and arranged in a manner that they appear as if they are staring and they have a three-dimensional appearance (Fukuda et al., 2008). The results of this study complement two prior studies conducted in Japan and North America in 1983 and 1989 respectively. The Japan study, by Shirota et al (1983), revealed that the scaring of birds was by the eyespot itself rather than the balls used. Additional report by modern-day farmers in New Zealand shows that the use of eyespot balloons is effective and fairly economical more so in small fields since only about eight balloons are required for every acre of land (Fukuda et al., 2008). However, this technique is only 90% effective since it does not fully deter birds from invading crops and only works for about two weeks. Researchers have suggested that the technique works efficiently if used within the last fortnight before harvest since this is the period that crops such as grapes are more susceptible to attacks (Fukuda et al., 2008).
Conclusion
Published researches differ regarding the reasons why balloons are suspended above ripening crops to control bird damage to the fruits. Some scientists argue that when the eyespots are large and centrally located, they intimidate predators in that they resemble the eyes of the predator's enemies. Other scientists highlight that when the eyespots are small and peripherally located, they act as markers for deflecting the predators’ attacks. Therefore, it is suggested that there is a great need to use eyespot balloons in conjunction with other methods of bird control.
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References
Fukuda, Y., Frampton, C. M., & Hickling, G. J. (2008). Evaluation of two visual bird scarers, the Peaceful Pyramid®, and an eye-spot balloon, in two vineyards. New Zealand Journal of Zoology , 35 (3), 217-224.
Hickling Graham, J. (1995). Evaluation of eye-spot balloons in a Canterbury vineyard.
Inglis, I. R., Huson, L. W., Marshall, M. B., & Neville, P. A. (1983). The feeding behavior of starlings (Sturnus vulgaris) in the presence of ‘eyes.' Ethology , 62 (3), 181-208.
Stevens, M. (2005). The role of eyespots as anti-predator mechanisms principally demonstrated in the Lepidoptera. Biological Reviews , 80 (4), 573-588.
Tipton, A. R., Rappole, J. H., Kane, A. H., Flores, R. H., Johnson, D., Hobbs, J., & Palacios, J. (1989). Use of monofilament line, reflective tape, beach balls, and pyrotechnics for controlling grackle damage to citrus. In Great Plains Wildlife Damage Control Workshop Proceedings (p. 413).
Tracey, J. B., & Mary, B. (2006). Managing bird damage to fruit and other horticultural crops . Commonwealth Government-Department of Agriculture, Fisheries, and Forestry-Bureau of Rural Sciences.
