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Crane, Hannah G.
Effect of resource manipulation on aggression in the Steely-vented hummingbird (Amazilia saucerrottei)
Amount of resource available for individuals within a species strongly influences levels of intraspecific competition. Aggression is a direct result of this competition. Aggressive behavior was studied in the
Steely-vented Hummingbird (SVH, Amazilia saucerrottei) in normal and reduced resource treatments. The purpose of this study was to determine if reduction in resource availability affects amount of aggression in
SVHs. Half of a resource (field of flowers) was covered with bug nets for an experimental condition. Number of chases between two or more hummingbirds was observed to quantify aggression. SVHs were
significantly more aggressive with a limited resource, with anywhere from 1.5-3 times as many chases observed in experimental trials. This circumstantial increased aggression allows us to conclude benefits of
territoriality (resource access) outweigh energetic costs when resource is limiting. Organisms with more aggression will have more resource and therefore higher fitness.
Text in English.
Aggressive behavior in animals
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Tropical Ecology Spring 2011
t Monteverde Institute : Tropical Ecology
E ffect of resource manipulation on a ggression in the Steely vented Hummingbird ( Amazilia saucerrottei ) Hannah G. Crane Department of Biology, Gustavus Adolphus College ABSTRACT Amount of r esource available for individuals within a species strongly influence s levels of intraspecific competition. Aggression is a direct result of this competition. Aggressive behavior was studied in the Steely vented Hummingbird (SVH Amazilia saucerrottei ) in normal and reduced resource treatments. The purpose of th is study was to determine if reducti on in resource availability affe cts amount of aggression in SVHs Half of a resource (field of flowers) was covered wit h bug nets for an experimental condition Number of chases between two or more hummingbirds was obse rved to quantify aggression. SVH s were significantly more aggressive with a limited resource, with anywhere from 1.5 3 times as many chases observed in experimental trials This circumstantial increased aggression allows us to conclude benefits of territ oriality (resource access ) outweigh energetic costs when resource is limiting. Organisms with more aggression will have more resource and therefore higher fitness. RESUMEN La cantidad de recursos disponibles por individuos entre un especie afecta niveles de competencia intraspecifico. Agresin es un resulta directa de este tipo de competencia. Estudiaba comportamiento agresivo en el Steely vented Hummingbird (SVH, Amazilia s aucerrottei) en dos tratamientos, normal y con los recursos reducidos. El propsito de este estudio era para entender si la reduccin de disponibilidad de recursos afecta el nivel de agresin en SVHs. La mitad del recurso (flores) estaba cubierto con mosqu iteros por un tratamiento experimental. La cantidad de persecuciones entre dos o mas colibrs era observado para cuantificar agresin. SVHs eran mas agresivas con un recurso limitada, con entre 1.5 3 veces mas persecuciones en pruebas experimentales. Esta aumentacin en agresin permite que los beneficios de territorios (acceso a recursos) es mas importante que costos energticos cuando el recurso esta limitada. Los organismos con mas agresin tendr mas recursos y entonces un fitness mas alta. INTRODUCTI ON The natural world is centered a round competition for resources, and this competition is a major driver of ecological diversity ( Svanbck & Bolnick 2007 ). Interspecific competition over reso urces is a major force of evolution, as certain species develo p niches and locate resources. Intraspecific competition can be observed to lead to change in a similar way, as organisms within a species compete for resources, encouraging natural selection. Aggressive behavior is one result of resource competition in i ntraspecific relationships (Stamps 1977). Territoriality is observed in taxa of all shapes and sizes, a result of the importance of an individual need to survive and natural selection (Stamps 1977) Organisms are
most territorial ones, consuming resources and passing on their genes more often. Hummingbirds come in two varieties. Hummingbirds are either trapliners, meaning they fly distances to feed at more spread out flowers, or territorial hummingbirds that occupy a smaller area of resource (Gass & Garrison 1999). These two types of hummingbirds have very different lifestyles, and for this study we wil l focus on territorial hummingbirds. A hummingbird known for its extreme territoriality, both inter and intraspecific, is the Steely vented Hummingbird ( SVH, Amazilia saucerrottei) A ggression is exhibited by both sexes (Stiles & Skutch 1989 Tiebout 1 991, Tiebout 1996 ). Tiebout ( 1991 ) also found that A. saucerrottei will only spend 10 20% of its time foraging while in its territory. The remainder of that time consists largely of territory defense. Also in ( 1991 ) study, it was found that A. saucerrottei had lowest flower visitation but highest flight ti me of all hummingbirds compared, such as the Chlorostilbon canivetii which had significantly less flight time. In order to invest such large amounts of energy in territory defense A. saucerro ttei must compensate for reduced feeding time. One option for these hummingbirds is to seek flowers with higher sucrose content. Lobelia laxiflora family Campanul aceae is a small tubular orange flower found in many tropical habitats especially forest c learings and roadsides. They grow between 1,000 2,600 m elevation in Costa Rica and flower year round. L. laxiflora do not succeed in extremely dry environments, but are very sun loving (Baker 1975). They average 22% sucrose nectar content slightly higher than the 21% average for native mountain flowers found in L. laxiflora are exceedingly abundant around Monteverde, Costa Rica due to their hardy, shrublike nature. SVH have been observed to feed on flowers such as L. laxiflora (Stiles & Skutch 1989). Intraspecific competition among A. saucerrottei occurs due to large resource need and natural territorial nature. Tiebout ( 1991 ) examined responses to foraging constraint and found that flight time was higher in situations when less food was available. Much of this extra flight time was to defend territory. So, will change in resource availability affect territoriality and aggressive behavior among the Steely vented Hummingbird? This study tests if limiting res ources by 50% will increase aggression in the SVH. MATERIALS AND METHODS Study Site This study was conducted in San Luis Arriba Puntarenas, Costa Rica. San Luis is in the Premontane Moist Forest Holdridge Life Zone (Holdridge 1966). The study site was a 48 m 2 field of Lobelia laxiflora The field was surrounded on two sides by trees that SVH frequently perched on. The field was 300 m off the road in La Finca Bella on Mario next to a banana field Half of the field (24 m 2 ) was u sed for actual observations For experimental data, the unobserved half of the field was covered in bug nets, and the other half (previously observed) was used for data collection.
Study Organisms The Steely vented Hummingbird ( Amazilia saucerrottei ) was used for this study. An estimated 10 20 individuals inhabited the study site SVH are green with a copper rump and gray blue tail. They are found from Nicaragua to Costa Rica, and in Colombia and Vene zuela (Stiles & Skutch 1989). Within Costa Rica, they are found most commonly in the northwestern Pacific up to 1,200 m in elevation. The SVH average 9 cm in lengt h and 4.5 grams in weight (Stiles & Skutch 1989, Garrigues & Dean 2007). Data Collection Data were taken for eight days from late April to early May during late dry season On the first two days, data were taken from 6 am to 10 am, and 10 am to 2 pm respectively to get an idea of when the hummingbirds were most active The next six days (three baseline, three experi mental) data were taken from 8 11 am. Number of chases every 15 minutes was recorded. A ch ase was quantified as at least two hummingbirds flying one chasing the other. Often there were more than 2 birds involved in a single chase, however it was still c ounted as one chase. At the beginning of each 15 minute period, I scanned the half of the field and recorded the numbers of hummingbirds either perching, an idea of how many active hummingbi rds there were at that moment. Number of chases and hummingbirds were averaged and values were analyzed with Repeated Measures ANOVA Analysis. RESULTS Normal conditions: Steely vented Hummingbirds exhibited acti vity until the early afternoon (Fig. 1) 8 am to 11 am was chosen as study time range because of stable observed activity levels. Three factors, Number of Chases, Number of Hummingbirds, and Time were compared with Pearson Correlation Tests. N umber of ch ases and time of day for baseline data, were not correlated (n=32, R=0.07, p=0.70). There was also no correlation found between number of hummingbirds and ti me of day for baseline data (n=32, R= 0.11, p=0.57). A positive correlation was found between num ber of chases and hummingbirds for baseline data (n=32, R=0.48, p=0.005). For experimental data no correlation was found between number of chases and hummingbirds (n=12, R= 0.14, p=0.66; Fig. 2 & Fig. 3).
Figure 1. Relationship of number of Steely ven ted H ummingbirds and number of chases over time in normal condition s, in a patch of Lobelia laxiflora in San Luis, CR. There was no relation between number of chases and time, and time and number of SVH. There was a positive correlation between number of SVH and number of chases (n=32, R=0.48, p=0.005). Hummingbird count in normal vs. experimental conditions : Hummingbirds prese nt during normal and experimental treatments were not significantly different, demonstrating that hummingbird presence did not change during the experiment or over time in either treatment (Repeated Measures ANOVA, F=0.09 df=1,4, p=0.59 ; Fig. 2 ). During normal periods number of hummingbirds present from day to day was much more variable than during the experimental phase ( Fig. 2). 0.0 2.0 4.0 6.0 8.0 10.0 12.0 600 630 700 730 800 830 900 930 1000 1030 1100 1130 1200 1230 1300 1330 Count Time Chases
Figure 2. SVH presence over time in a patch of Lobelia laxiflora under normal and experimental conditions, San Luis, CR. Points represent averages ( + SD) of 3 days of observations. A repeated measures analysis found no differences of count s over time or between treatments ( F=0.09, df=1,4, p=0.59). Hummingbird aggression in normal vs. experimental conditions : H ummingbird activity was much greater when resources were cut by (Repeated Measures ANOVA F=3.23 df=1,4, p=0.02; Fig. 3) Hummingbird chases were anywhere from about 1.5 3 times more frequent durin g partitioned treatments (Fig. 3). For both covered and uncovered trials, time was not a significant factor on total number of chases (Repeated Measures ANOVA, F=3.23, df=2.47, 9.8 9, p=0.61). For experimental data no correlation was found between number of chases and hummingbirds (n=12, R= 0.14, p=0.66). 5 4 3 2 1 0 1 2 3 4 5 6 800 815 830 845 900 915 930 945 1000 1015 1030 1045 Number of Hummingbirds Time Baseline Experimental
Figure 3. Number of SVH chases observed in a patch of Lobelia laxiflora in a study in San Luis, CR in baseline and experimental conditions Points represent averages (+SD) of 3 days of observations. A repeated measures analysis a significant difference between chase number in normal and experimental conditions (F=3.23, df=1,4, p=0.02). DISCUSSION Number of SVH chas es increased in times of less resource availability. When resources were limited by 50% hummingbird count remained the same, supporting the notion that increase in chases was caused by an increase in aggression, not merely an increase in number of humming birds. A. saucerrottei chase count increased with less available resource up to three times as much as during normal resource availability, also showing aggression increase. A. saucerrottei is a hummingbird known for its aggressive behavior defending it s flower patches often spending more time in resource defense than in actual resource use (Tiebout 1991). Intraspecific competition is high among all territorial breeds of hummingbirds, and incr eases with resource depletion (Tiebout 1991). The SVH especially will show more aggression towards other SVH s than other species (Stiles & Wolf 1970). This intraspecific competition is driven by individual need and is an important influence in natural selection. Hummingbirds that more successfully defend th eir territory have access to more resource, providing them more nutrients and increasing individual fitness (Stamps 1977). There is also evidence that aggressive behavior in hummingbirds may provide other benefits. Stiles ( 1982 ) discusses connections betw een aggression and courtship. Breeding males will display aggression in patches of flowers considered their territory (Stiles 1982). In this study, the observed increase in resour ce territoriality may have contributed to breeding success because the SVH nesting season is Decem ber April (Stiles & Skutch 1989 ), and observations were taken in late April and May. 10 5 0 5 10 15 20 800 815 830 845 900 915 930 945 1000 1015 1030 1045 Number of Chases Time Baseline Experimental
The results of this study support the predi ction that resource depletion will increase aggressi ve behavior in the SV H The experiment does not identify which hummingbirds are participating in the chases. Because individual hummingbirds were not identified and labeled, it is possible a small number of hummingbirds were doing all the chasing. In future research, a system of tagging chasing hummi ngbirds could be used Additionally, it would be interesting to look at continuous hummingbird count during resource treatments. This would require another individual to spot p erched hummingbirds who were often hard to see because of absence of movement Lastly, comparing SVH behavior in late dry season with other seasons may produce relevant results, this being due to L. more notice able flowering (Zuchowski 2007) in the dry season. Should aggression change in different seasons, more behavi oral conclusions could be drawn. Additional research on seasonality of behavior would shed light on hummingbird territoriality patterns and connections to feeding sources. ACKNOWLEDGEMENTS Thanks to Pablo Allen my advisor, for assisting me with my project. Thank you for coming down to San Luis to help out and for answering all my questions Thanks to Gisella Fern ndez for helping me with supplies. Also a special thanks to Mario Picado Perez for the temporary use of his land for hummingbird observ ations. And finally, thanks to my familia tica. Thank you for making me feel like a part of the family, for asking about and having interest in my project. Thank you for sharing your house hold with me and for fostering my slow, slow Spanish. Thank you Gaudy, Ral, and Loandry for everything I had an amazing month with you LITERATURE CITED B AKER H. G 1975. Sugar concentrations in nectars from hummingbird flowers. Biotrop. 7: 37 41. G ARRIGUES R. AND R. D EAN 2007. The Birds of Costa Rica Cornell Univ. Press, Ithaca, NY, pp.126 127. G ASS C. J. AND J. S. E. G ARRISON 1999. Energy regulation by traplining hummingbirds. Func. Ecol. 13: 483 492. H OLDRIDGE L. R. 1966. The life zone system, Adansonia 6: 199 203. S TILES F. G. 1982. Aggressive and ummingbird Condor 84: 208 225. S TILES F. G. AND A. F. S KUTCH 1989. A Guide to the Birds of Costa Rica. Cornell Univ. Press, Ithaca, NY, pp. 221. S TILES F. G. AND L. L. W OLF 1970. Hummingbird territoriality at a tropical flowering tree. The Auk. 87: 467 491. S TAMPS J. A. 1977. The relationship between resource competition, risk, and aggression in a tropical territorial lizard. Ecol. 58: 349 358. S VANBCK R. AND D. I. B OLNICK 2007. Intraspecific competition drives increased resource use diversity within a natural population. Proc. Biol. Sci. 274: 839 844. T IEBOUT III H. M 1991. Daytime energy management by tropical hummingbirds: responses to foraging constraint. E col. 72: 839 851. T IEBOUT III, H. M 1996. Costs and benefits of interspecific dominance rank: are subordinates better at finding novel food locations? Anim. Behav. 51: 1375 1381. Z UCHOWSKI W 2007. Tropical Plant of Costa Rica Cornell Univ. Press, Ith aca, NY, pp. 85 86.