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Distribucin de la altitud y la particin de nicho de dos especies de Reinitas (Parulidae) en Monteverde
Altitudinal distribution and niche partitioning of two Redstart species in Monteverde (Parulidae)
Global climate change is affecting habitat and species interactions globally (Sekerciouglu et al. 2008), including
tropical montane regions (Pounds et al. 1999, Holmes 2000, Pounds et al. 2006). In Monteverde, Costa Rica, two species of warblers, the Collared Redstart (Myioborus torquatus) and the Slate-throated Redstart (M. miniatus), are known to partition altitudinally with a sizeable sympatric zone of overlap (Shopland 1985, Mahan 1998, Adams
2003). As with other species, global warming is thought to cause changes in the altitudinal distributions of both Redstarts. Both species are expected to move upward. To avoid competitive exclusion, niche partitioning should occur in the sympatric altitudinal zone. The purpose of this study is to establish the current altitudinal distribution of both species, and compare that to previous studies. Additionally, this study attempts to identify the presence and
mechanism of niche partitioning between the species in their zone of overlap. Finally, it documents changes in relative species abundance of the two species over time. Altitude data, species data, and foraging height data for the two Redstart species were collected from primary and secondary forests in the Monteverde Cloud Forest Preserve, Santa Elena Cloud Forest Preserve, and trails on Cerro Amigos; all in Monteverde, Puntarenas, Costa Rica, and with
altitudinal range from 1,400 to 1,750 m. Results from this study clearly indicate increased altitudinal changes. Slate-
throated Redstarts, traditionally the lower altitude species, were observed up to 1,710 m in elevation; almost 200 m higher than previous studies done in the same forests report. The Collared Redstart was observed a minimum of 10 m higher than previous observations. More importantly, the Collared Redstart is now sympatric with the Slate-
throated Redstart in the entire altitudinal distribution observed. Next, relative abundance of the Collared Redstart was found to be drastically lower than previous studies. The Collared Redstart made up less than 25% of the observations in this study, whereas it has consistently composed greater than 50% of observations in past studies.
Finally, foraging height was found to be a significant mechanism of niche partitioning in the sympatric altitudinal zone. In this overlap zone, below 1,602 m Slate-throated Redstarts foraged at greater heights than Collared Redstarts, while above 1,602 m the opposite trend was observed. The findings of this study support the conclusion that global climate change has affected the distribution of species in Monteverde. Because the Slate-throated Redstart is now present at altitudes previously inhabited only by the Collared Redstart, a new equilibrium of relative species abundance will be reached through effective niche partitioning, or extinction will occur most likely resulting in the loss of the Collared Redstart.
El cambio climtico global est afectando el hbitat y las interacciones de especies a nivel mundial (Sekerciouglu et al. 2008), incluyendo las regiones montaosas tropicales (Pounds et al. 1999, Holmes 2000, Pounds et al. 2006). En Monteverde, Costa Rica, dos especies de reinitas, Myioborus torquatus y M. miniatus, se sabe que tienen una particin altitudinal con una zona considerable de solapamiento simptricas (Shopland 1985, Mahan 1998, Adams 2003). Al igual que con otras especies, se piensa que el calentamiento global causa cambios en la distribucin altitudinal de las dos reinitas. Se espera que ambas especies se trasladen hacia arriba para evitar la exclusin competitiva, la particin de nicho debe ocurrir en la zona altitudinal simptricas. El propsito de este estudio es establecer la actual distribucin altitudinal de las dos especies, y compararlas con estudios anteriores. Adems, este estudio trata de identificar la presencia y el mecanismo de particin de nicho entre las especies en su zona de solapamiento.
Text in English.
Partitioning, Habitat (Ecology)
Reinitas de madera
Compartimentacin, hbitat (Ecologa)
Tropical Ecology 2009
Ecologa Tropical 2009
Compartimentacin de Nichos
t Monteverde Institute : Tropical Ecology
1 Altitudinal Distribution and Niche Partitioning of Two Redstart Species in Monteverde (Parulidae) Brad Nitta Department of Biochemistry, Occidental College ABSTRACT Global climate change is affecting habitat and species interactions globally ( Sekercio uglu et al. 2008 ) including tropical montane regions (Pounds et al. 1999, Holmes 2000, Pounds et al. 2006) In Monteverde, Costa Rica, two species of warblers, the Collared Redstart ( Myioborus torquatus ) and the Slate throated Redstart ( M. miniatus ), are known to partition altitudinal ly with a sizeable sympatric zone of overlap (Shopland 1985, Mahan 1998, Adams 2003) As with other species, global warming is thought to cause changes in the altitudinal distributions of both Redstarts Both species are expec ted to move upward T o avoid competitive exclusion niche partitioning should occur in the sympatric altitudinal zone. The purpose of this study is to establish the current altitudinal distribution of both species, and compare that to previous studies. Add itionally, this study attempts to identify the presence and mechanism of niche partitioning between the species in their zone of overlap. Finally, it documents changes in relative species abundance of the two species over time Altitude data, species data and foraging height data for the two Redstart species were collected from primary and secondary forests in the Monteverde Cloud Forest Preserve, Santa Elena Cloud Forest Preserve, and trails on Cerro Amigos; all in Monteverde, Puntarenas, Costa Rica and with altitudinal range from 1,400 to 1,750 m Results from this study clearly indicate increased altitudinal changes. Slate throated Redstarts, traditionally the lower altitude species, were observed up to 1,710 m in elevation; almost 200 m higher than pr evious studies done in the same forests report. The Collared Redstart was observed a minimum of 10 m higher than previous observations. More importantly, the Collared Redstart is now sympatric with the Slate throated Redstart in the entire altitudinal dis tribution observed. Next, relative abundance of the Collared Redstart was found to be drastically low er than previous studies. The Collared Redstart made up less than 25% of the observations in this study, whereas it has consistently composed greater than 50% of observations in past studies. Finally, foraging height was found to be a significant mechanism of niche partitioning in the sympatric altitudinal zone. In this overlap zone, below 1,602 m Slate throated Redstarts foraged at greater heights than Coll ared Redstarts, while above 1,602 m the opposite trend was observed. The findings of this study support the conclusion that global climate change has affected the distribution of species in Monteverde. Because the Slate throated Redstart is now present at altitudes previously inhabited only by the Collared Redstart, a new equilibrium of relative species abundance will be reached through effective niche partitioning, or extinction will occur most likely resulting in the loss of the Collared Redstart. RESUME N El cambio climÂ‡tico global estÂ‡ afectando el hÂ‡bitat y las interacciones entre especies a nivel mundial (Sekerciouglu et al. 2008), incluyendo las regiones tropicales de montaÂ–a (Pounds et al. 1999, Holmes 2000, Pounds et al. 2006). En Monteverde, Costa Rica, dos especies de currucas, el Collared Redstart (Myioborus torquatus) y la Slate throated Redstart (M. miniatus), se sabe que la particiÂ—n altitudinal simpÂ‡trica con una zona de solapamiento (Shopland 1985, Mahan 1998, Adams 2003). Al igual que con o tras especies, el calentamiento global se cree que causa cambios en la distribuciÂ—n altitudinal de las dos Redstarts. Ambas especies se espera se muevan hacia arriba. Para evitar la exclusiÂ—n competitiva, la particiÂ—n de nicho deberÂ’a ocurrir en la zona d e altitud simpÂ‡tricas. El objetivo de este estudio es establecer la distribuciÂ—n altitudinal de las especies, y que compararlo con estudios anteriores. AdemÂ‡s,
2 este estudio trata de identificar la presencia y el mecanismo de particion de nicho entre las es pecies en su zona de superposiciÂ—n. Por Âœltimo, documentar los cambios en la abundancia relativa de especies de las dos especies en el tiempo. Datos de altitud, las especies, y altura de forrajeo para las dos especies colirrojo se obtuvieron de bosques pri marios y secundarios en la Reserva Bosque Nuboso de Monteverde, Reserva del Bosque Nuboso de Santa Elena y senderos en el Cerro los Amigos, todos en Monteverde, Puntarenas, Costa Rica, y con el rango altitudinal de 1,400 a 1,750 m. Los resultados de este estudio indican claramente el aumento de los cambios de altitud. Slate throated Redstarts, tradicionalmente la mÂ‡s baja altitud de las especies, se han observado hasta 1,710 m de altitud, casi 200 metros mÂ‡s altos que los anteriores estudios realizados en el mismo informe de los bosques. El collar colirrojo se observÂ— un mÂ’nimo de 10 m mÂ‡s alta que las observaciones anteriores. MÂ‡s importante aÂœn, la Collared Redstart simpÂ‡trico con el colirrojo de garganta roja en toda la distribuciÂ—n altitudinal observad o. A continuaciÂ—n, la abundancia relativa de Collared Redstart resultÂ— ser drÂ‡sticamente mÂ‡s bajos que los estudios anteriores. El Collared Redstart formado por menos del 25% de las observaciones en este estudio, que ha compuesto mÂ‡s de 50% de las observac iones en los Âœltimos estudios. Por Âœltimo, la altura de forrajeo resultÂ— ser un importante mecanismo de particion de nicho altitudinal en zonas simpÂ‡tricas. En esta zona de solapamiento, por debajo de 1,602 m de Slate throated Redstarts forrajea a mayores alturas que Collared Redstarts, mientras que por encima de 1,602 m de la tendencia opuesta se observÂ—. Los resultados de este estudio apoyan la conclusiÂ—n de que el cambio climÂ‡tico global ha afectado a la distribuciÂ—n de las especies en Monteverde. Debido a que el Slate throated Redstart estÂ‡ presente una altura previamente habitadas sÂ—lo por el Collared Redstart, un nuevo equilibrio de relativa abundancia de las especies se alcanzarÂ‡ a travÂŽs de un efectivo particiÂ—n de nicho, o la extinciÂ—n se producirÂ‡ muy probablemente como resultado la pÂŽrdida del Collared Redstart. INTRODUCTION Climate change in mid elevation tropical cloud forests, like those in Monteverde, Costa Rica causes dryer conditions and impacts abundance, range, and phenology of species (H olmes 2000). Extinctions and changes in species distribution associated with climate change have been observed and well documented in the Monteverde area (Pounds et al. 1999, Holmes 2000, Pounds et al. 2006). Birds are of particular interest in climate cha nge research because of their ability to move to higher altitudes in pursuit of preferable climatic conditions. The Keel billed Toucan ( Ramphastos sulfuratus ) and Green fronted Lancebill ( Doryfera ludovicae ), among other species, have recently been observe d at higher elevations than historically noted (Holmes 2000). The recent movement of birds and other species into new areas create conditions that potentially disrupt symbiotic interactions, including predator prey interactions, mutualism, and competitive interactions ( Sekerciouglu et al. 2008 ). In the case of competition, two similar species must either partition their resources effectively or one will go extinct ( Hutchinson 1961, Begon et al. 1990, Krebs 1994 ). Two species of warbler found in Monteverde, the Collared Redstart ( Myioborus torquatus ) and the Slate throated Redstart ( M. miniatus ), have been known to be partitioned elevation ally with a narrow zone of overlap around 1550m (Shopland 1985, Mahan 1998, Adams 2003, Gabrielsson 2006). In 2006 howeve r, the Slate throated Redstart was first observed with an increased altitude of about ten meters resulting in an increased overlap zone with the Collared Redstart (Gabrielsson 2006). The higher altitude species, the Collared Redstart, has been suggested t o be competitively inferior to the Slate throated Redstart in inter specific competition in the presence of flocks (Shopland 1985). Without the influence of a flock however, the slightly larger Collared Redstart may be more dominant (Shopland 1985). The Co llared Redstart joins flocks with greater propensity (Shopland 1985), thereby favoring the Slate throated Redstart in competition. Significant differences in foraging behavior in a recent study Slate throated Redstarts chased and flashed insects more, and sat less most likely indicate the competitive superiority of the Slate throated Redstart, rather than effective niche partitioning (Gabrielsson
3 2006). Although slight differences were found in the foraging height of the two species, indicating a possible mechanism of niche partitioning (Adams 2003) there was also overlap in vertical height of the two species in their zone of altitudinal sympatry (Gabreilsson 2006) Unfortunately, other observations may indicate a decrease in Redstart abundance in the Mont everde Cloud Forest Preserve (Donnelly 1998). These observations, combined with the recent altitudinal changes in distribution, indicate that the Collared Redstart may be facing local extinction if the two species are not able to effectively partition the i r available resources in the widening zone of sympatry The purpose of this study is to examine the altitudinal distribution and foraging behavior of the Slate throated Redstart and Collared Redstart in Monteverde, Costa Rica. Because past studies hav e indicated small altitudinal changes in Redstart distribution, this study will attempt to determine if the distribution of either species has continued to change. Further, this study hopes to determine if increased altitudinal changes have impacted inter specific interactions observed through differential foraging heights: a form of niche partitioning. Finally, since avian diversity including Redstart abundance may be declining, and because the Slate throated Redstart is thought to have a competitive advan tage over the Collared Redstart, species composition of the two Redstarts will be compared to previous studies to determine any change. METHODS Study Species The Slate throated Redstart ( M. miniatus ) is a neotropical species of warbler found from the southern tip of Mexico, throughout much of Central America, and into northwestern South America (Shopland 1985, Stiles & Skutch 1989). In Monteverde, it has traditionally been present below 1,580 m, and until Gabrielsson's ( 2006 ) sightings it was not recor ded above this elevation (Shopland 1985, Fogden 1993, Mahan 1998, Adams 2003). The Collared Redstart ( M. torquatus ) is also found in the Monteverde area, and is endemic to Costa Rica and western Panama (Shopland 1985, Stiles & Skutch 1989, Garrigues 2007). The Collared Redstart was formerly found predominately at elevations higher than the Slate throated Redstart, although there has been an overlap zone between 1,500 m and 1,580 m (Shopland 1985, Fogden 1993, Donnelly 1998). Both species are primarily insec tivorous, and flash their white tail feathers while gleaning or sallying to scare insects into flight ( Shopland 1985, Stiles & Skutch 1989 Mumme 2002 Garrigues 2007). Where the two species have been observed foraging in flocks, the Slate throated Redstar t appears to be dominant and requires less foraging time to meet its energetic requirements (Shopland 1985). Both Redstart s but especially the Collared Redstart join flocks with great propensity, likely creating conditions where the Collared Redstart is co mpetitively inferior (Shopland 1985). Altitudinal distributions of the two species have changed to some degree between 1985 and 2003 (Fig. 2). All data were taken from the same trail system in and around the Monteverde Cloud Forest Preserve. In 1985 the Slate throated Redstart was observed between 1,350 and 1,575 m while the Collared Redstart was observed between 1,490 and 1,630 m (Shopland 1985). In 1998, the Slate throated Redstart was found up to the same maximum elevation, however the Collared Redstar t was found between 1,560 and 1,670 m (Mahan 1998). The change in the Collared Redstart elevation between 1985 and 1998 represents a 70 m increase in the minimum elevation, and a 30 m reduction in overall altitudinal range. In 2003, the Slate throated Reds tart
4 was observed at a maximum altitude of 1,580 m, similar to previous studies (Adams). The Collared Redstart was observed at a similar minimum altitude to the 1998 study, thus indicating little or no change in altitudinal distribution between 1998 and 20 03 (Mahan 1998, Adams 2003). Study Sites Data w ere collected at three locations in Monteverde, Puntarenas, Costa Rica from April 14, 2009 until May 10, 2009 : the end of the dry season The Monteverde Cloud Forest Preserve (MVCFP) contains primary and secondary cloud forest, and ranges in altitude from 1,535 m to 1,710 m. All public trails in the MVCFP were walked, aside from the Rio Trail, which was closed. The Santa Elena Cloud Forest Preserve (SECFP) predominantly contains secondary forest in reg eneration since 1977, and ranges in elevation from approximately 1,550 m to 1,710 m. All trails in the SECFP were walked. Additionally, the road and trail from the Biological Station up to Cerro Amigos was walked, which ranges up to 1,800 m in elevation. M onteverde has an average annual temperature of 18.5 Â¡C with annual average rainfall of 2,519 mm plus 886 mm of mist per year (Clark 2000). Altitude Trails at the three locations were walked on multiple days, between the hours of 0630 and 1400. I walk ed at a normal pace until I heard or saw either species of Redstart. After visually confirming the species of Redstart, I recorded altitude data from a wrist top altimeter, as well as the species. If I heard a Redstart, but could not locate it, I played th e call from an iPod and portable speaker. This was done in order to attract the bird so that the species could be visually confirmed, and determine if it was in a pair. I performed t tests on my altitude data. Additionally, I performed a Chi squared to com pare abundance data with previous studies. Foraging Height While making altitude observations, I simultaneously observed the foraging height. When a Redstart was found, I estimated the height above the forest floor that it was foraging, by eye. Most obse rvations of foraging height were taken when the recording had not been used to find the bird. In cases where the recording was used, I waited until the bird resumed its normal foraging behavior to record the foraging height. The foraging height recorded wa s the height at which the bird spent most of its time foraging during the period of observation. Depending upon how long the individual bird spent foraging within view, I observed the bird for between 20 seconds and four minutes. I performed t tests on the data as well as a linear fit model. RESULTS Altitude A distinct altitudinal difference was observed between the two species (t test: t = 2.627, df = 34, P = 0.0128). The Slate throated Redstart was found abundantly across a larger range of altitudes, w hile the Collared Redstart was found only within a much smaller altitudinal transect (Fig. 1). Collared Redstarts were observed from 1,570 m to 1,675 m with a mean altitude of 1,600 m.
5 Slate throated Redstarts were found between 1,430 m and 1,710 m with a mean altitude of 1,568 m. Current altitudinal distributions of the species are markedly higher than in past studies, and the range of the Slate throated Redstart now essentially completely encloses the range of the Collared Redstart (Fig. 2). The Slate thr oated Redstart was approximately three times more abundant than the Collared Redstart (Fig. 3). A total of 59 Redstarts were observed: 14 Collared Redstarts and 46 Slate throated Redstarts. This change in relative abundance of the Collared Redstart (Fig. 3 ) was also in stark contrast to previous studies (Chi Square test: 2 = 20.12, df = 4, P = .00047) ( Shopland 1985, Mahan 1998, Adams 2003, Gabrielsson 2006). Foraging Height Foraging height was observed for 59 Redstarts, and compare d to the elevation at which they were observed (Fig. 4a). Over the entire altitude range, non species specific (linear fit model: F = 14.70, df = 1, P = 0.0003) as well as species specific (linear fit model: F = 9.45, df = 1, p = 0.0033) foraging height wa s significantly correlated to altitude. However, species specific foraging height compared to altitude was more strongly correlated (linear fit model: F = 41.18, df = 1, P < 0.0001) in the altitudinal overlap zone when it was examined alone (Fig. 4b). Inte restingly, in this altitudinal overlap zone (Fig. 4b), Collared Redstarts were observed foraging lower than Slate throated Redstarts from 1,570 m to 1,602 m (t test: t = 1.75, df = 15, P = 0.0003). From 1,602 m to 1,675 m however, Collared Redstarts were o bserved foraging higher than Slate throated Redstarts (t test: t = 1.80, df = 11, P = 0.00002). DISCUSSION Altitudinal census data from this study indicates that both the Slate throated Redstart and Collared Redstart have moved up in altitude. The Colla red Redstart was found down to an altitude ten meters higher than the minimum altitude of the most recent, previous altitudinal study (Adams 2003). Meanwhile, the Slate throated Redstart was found to inhabit altitudes as high as 1,710 m; about 200 m higher than previous studies recorded in the same forest ( Shopland 1985, Mahan 1998, Adams 2003, Gabrielsson 2006) Thus, the altitudinal change of the Slate throated Redstart has been much more drastic. Although the Collared Redstart has increased its minimum a ltitude, it is only a ten meter increase. Its maximum altitude however, has changed very little. As climate change has been linked to species' altitudinal change in other studies in Monteverde (Pounds et al. 1999), it is the most likely factor stimulating the altitudinal change in distribution of the Slate throated Redstart. The Collared Redstart, although it may be similarly affected by climate change, has most likely not moved up as drastically in recent years because it has nowhere else to go; it is alre ady near the highest elevation possible in Monteverde. The increase in Slate throated Redstart altitude (Fig. 1 and Fig. 2) is cause for alarm. Such rapid altitudinal change is consistent with studies by Pounds et al. (1996) that measure profound ecosystem change most likely due to global warming. In the case of the Redstarts, this rapid altitudinal increase may also lead to a change in the dynamics of competitive interactions between the species In this case, the Slate throated Redstart has increased its altitudinal distribution more rapidly than the Collared Redstart. In Monteverde, t he observed range of the Collared Redstart is now completely within the range of the Slate throated Redstart. This greatly enlarged zone of overlap increases pressure on bot h species, and ultimately will result in niche partitioning of resources or the extinction of one species. Studies have shown that a small
6 geographic range is the best predictor of species extinction ( Manne et al. 1999, Harris & Pimm 2007) Therefore, bas ed on species range alone, the Collared Redstart appears to be far more likely than the Slate throated Redstart to go extinct. While past studies have observed a relatively even abundance of both species, the low relative abundance of Collared Redstarts o bserved in this study should serve as a warning that the Collared Redstart is possibly in danger of local extinction (Fig. 3). In this study, surprisingly low numbers of Collared Redstarts were observed in proportion to Slate throated Redstarts. This obser ved reduction in abundance in Collared Redstarts can be explained in three ways. First, the increased overlap with the Slate throated Redstart has put too much pressure on the Collared Redstart. The two species are very similar in their use of resources, a nd the Slate throated Redstart is likely to be the competitively superior species in general (Shopland 1985). Therefore, the Collared Redstart may be heading towards local extinction. Alternatively, partial niche partitioning may occur, but with the Collar ed Redstart at a lower equilibrium abundance Therefore, the relative abundance of the Collared Redstart observed here may indicate biased niche partitioning favoring the Slate throated Redstart. Finally, it is possible that the Collared Redstart populatio n has remained constant while the Slate throated Redstart population has increased. This third possibility is unlikely because a previous study has already indicated decreasing Redstart abundance (Donnelly 1998). Additionally, equal or greater sampling eff ort was exerted in this study, resulting in a sampling size similar to previous studies. In light of increased altitudinal overlap between the two species, observations of foraging height indicate that niche partitioning likely occurs Differential foragin g height was suggested as a possible mechanism for niche partitioning between the two redstarts (Adams 2003), although the author seemingly incorrectly speculated that it was the Collared Redstart who more often encroached into the Slate throated Redstart territory New altitudinal data from this study however, makes it clear that it is the Slate throated Redstart moving into the Collared Redstart 's territory. Further, the trends in foraging height observed in thus study indicate that outside of the sympatr ic altitude zone, the Slate throated Redstart forages at different heights without significant preference (Fig. 4a). Inside the overlap zone however, the Slate throated Redstart forages higher than the Collared Redstart below 1,602 m and lower than the Col lared Redstart above 1,602 m (Fig. 4b). Insid e the overlap zone, opposite species behavior above and below 1,60 0 m creates an apparent paradox. Both biotic and abiotic factors may combine to explain this foraging trend. At high altitudes more habitat is e xposed to high winds and blowing mist (Haber et al. 2000). Assuming that the Slate throated Redstart is competitively superior to some extent, at higher altitudes it may choose to forage closer to the ground, where exposure to the elements is reduced. The slightly larger size of the Collared Redstart may make it able to handle increased exposure. Thus, when above 1,600 m the Slate throated Redstart chooses to forage higher. Additionally, the Collared Redstart has always been present at altitudes above 1,600 m and may be suited to a more exposed environment. A small set of data indicates that foraging height was positively correlated with altitude for the Collared Redstart, above the previous overlap zone (Adams 2003 ). This trend still appear s to occur in the presence of the Slate throated Redstart (Fig. 4b). Therefore, although the Collared Redstart is believed to be competitively inferior in most circumstances, the trend in foraging heights found in this study may partially represent a preference of the Slat e throated Redstart to be shielded from the elements, and partially represent a remnant foraging preference of the Collared Redstart. Still, the foraging trend observed in this
7 study indicates a clear mechanism for niche partitioning in the sympatric altit udinal zone, potentially allowing the two species to continue to exist in the same environment. Species extinctions and habitat transformation are important issues facing tropical montane regions such as Monteverde. This study has demonstrated that altit udinal distributions of the Collared Redstart and especially the Slate throated Redstart have changed markedly over the past decade. Next, this study when compared to past studies, clearly demonstrates a reduced abundance of Collared Redstart in proportion to Slate throated Redstarts. Finally, this study has shown that niche partitioning in the sympatric altitudinal zone is accomplished through differential foraging heights. Still, many facets of the topic remain unknown. It has yet to be seen if the decrea sing Collared Redstart abundance will ultimately lead to extinction, or will eventually come to a new equilibrium due to effective niche partitioning. With such a small range compared to the Slate throated Redstart, the Collared Redstart is much more likel y to face extinction ( Manne et al. 1999, Harris & Pimm 2007) Future study is warranted in this area to see if the trend in niche partitioning continues amid changing altitudinal distributions, and to try to determine more definitively a biological justifi cation behind the mechanism. These drastic changes within the past decade should serve as a warning. Global climate change greatly affects tropical species. ACKNOWLEDGMENTS I would like to acknowledge Yi men Araya, first for inspiring me to study birds, and subsequently for his endless help with my project: birding on multiple early mornings, helping me to interpret my data with statistics, and always being enthusiastic and encouraging. Of course, I would like to thank Alan Masters for all of his laidback guidance and wisdom in helping me choose a project, and guiding me through data collection and the obligatory complications. Thank you also to JosÂŽ Carlos CalderÂ—n for graciously helping me to find papers, to find previous data, and guiding me in writing my paper. Thanks to my fellow student Taz for going birding with me, and taking pictures. Finally, I wish to thank the MVCFP, the SECFP, and the local guides for sharing their knowledge with me. LITERATURE CITED Adams, E. M. 2003 Niche partitioning and overlap in a cloud forest between two Neotropical warblers. Tropical Ecology and Conservation Spring '03, CIEE, Monteverde. pp. 126 133. Begon, M., Harper, J.L., and Townsend. C.R. 1990. Interspecific Competition. Ecology: Individuals, Populations and Communities Blackwell Scientific, Cambridge, MA. pp. 256 258. Clark, K. L., Lawton, R. O., and Butler, P.R. 2000. The Physical Environment. Monteverde: Ecology and Conservation in a Cloud Forest Nadkarni, N.M. and Wheelwright, N.T., eds. Oxford Univers it, Oxford. pp. 17. Donnelly, E. 1998. Changes in Bird Species Composition in Four Habitat zones in Monteverde. Tropical Ecology and Conservation Spring '98, CIEE, Monteverde. pp. 318 327. Fogden, M. 1993. An Annotated Checklist of the Birds of Montev erde and Penas Blancas Michael Fogden, Monteverde, Costa Rica. Gabrielsson, E. 2006. Behavioral changes of the Slate throated Redstart ( Myioborus
8 miniatus ) and the Collared Redstart ( Myioborus toquatus ) along an altitudinal gradient in the Monteverde C loud Forest Preserve. In Tropical Ecology and Conservation Fall '06, CIEE, Monteverde. pp. 162 167. Garrigues, R. and Dean, R. 2007. The Birds of Costa Rica Cornell University Press, Ithica, NY. pp. 272 273. Haber, W.A., W. Zuchowski, and E. Bello. 20 00. An Introduction to Cloud Forest Trees: Monteverde, Costa Rica Mountain Gem Publications, Monteverde, Costa Rica. pp. 11 12. Harris, G., and S. L. Pimm. 2007. Range size and extinction risk in forest birds. Conservation Biology 798. pp. 1523 1739 Holmes, B. 2000. The Case of the Dwindling Cloud Forest. International Wildlife 30. Hutchinson, G.E. 1961. The Paradox of the Plankton. American Naturalist 95, 145 159. Krebs, C.J. 1994. Distribution and Abundance at the Community Level. Ecology: Th e Experimental Analysis of Distribution and Abundance Harper Collins, New York, NY. pp. 176 180. Mahan, J. S. 1998. Altitudinal abundance of Collared Redstarts and Slate throated Redstarts in the Monteverde Cloud Forest Preserve. Tropical Ecology and C onservation Fall '98, CIEE, Monteverde. pp. 51 56. Manne, L. L., Brooks, T. M., and S. L. Pimm. 1999. Relative risk of extinction of passerine birds on continents and islands. Nature 399. pp. 258 261. Mumme, R. L. 2002. Scare tactics in a neotropical warbler: white tail feathers enhance flush pursuit foraging performance in the Slate throated Redstart ( Myoborus miniatus ). The Auk Vol 119(4), pp. 1024 1035. Pounds, A. J., Fogden, M. P., Cambell, J. H. 1999. Biological response to climate change on a tropical mountain. Nature 398. pp. 611 614. Pounds, A. J., Bustamante, M.R., Coloma, L.A., Consuegra, J.A., Fogden, M.P.L., Foster, P.N., LaMarca, E., Masters, K.L., Merino Viteri, A., Puschendorf, R., Ron, S.R., Sanchez Azofeifa, G.A., Still, C.J., a nd Young, B.E. 2006. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature, 439. pp. 161 167. Sekerciouglu, C. H. 2008. Climate Change, Elevational Range Shifts, and Bird Extinctions. Conservation Biology 22. pp. 14 0 150. Shopland, J. M. 1985. Facultative following of mixed species flocks by two species of Neotropical warbler. Ph.D. dissertation, University of Chicago, Chicago. Stiles, F. G. and Skutch, A. F. 1989. A Guide to the Birds of Costa Rica Cornstock P ublishing, Ithica, NY.
9 FIGURES FIGURE 1. The number of individual Slate throated Redstarts and Collared Redstarts encountered at various altitudes in the MVCFP, SECFP, and Cerro Amigos. A significant difference was observed between the two species (t test: t=2.627, df=34, p=0.0128). The Slate throated Redstart was observed in a much greater range of altitudes than the Collared Redstart, making the entire range of the Collared Redstart an overlap zone. Figure 2. The altitudinal range observed in t he MVCFP for the Slate throated Redstart (left) and Collared Redstart (right) from Shopland 1985, Mahan 1998, Adams 2003, and this study in 2009.
10 Increased altitudinal distribution is apparent for both species, however the change is the most striking with the Slate throated Redstart between 2003 and 2009. FIGURE 3. The relative distributions of species observations in 1985, 1998, 2003, 2006, and 2009. Collared Redstarts have traditionally made up a more significant proportion of the observations (Chi Squ are test: 2 = 20.12, df = 4, p = .00047). From 1985 to 2006 Collared Redstarts consist 54%, 54%, 53%, 59% of the observations respectively. In this study they
11 consisted of less than 25% of the total observations. (Shopland 1985, Mahan 1998, Adams 2003, Gabrielsson 2006) (a) (b) FIGURE 4. (a) Individual foraging height observed compared to the altitude for Collared and Slate throated Redstarts in all altitudes of the MVCFP, SECFP, and Cerro Amigos. (b) Individual foraging height compared to altitude for Collared and Slate throated Redstarts observed in the overlap zone only. The line with positive slope represents linear regression of
12 Collared Redstart data ( y = 0.34 x 523.63 R 2 = 0.65 ). The negative slope re presents a linear regression of the Slate throated Redstart data ( y = 0.37 x + 607.23 R 2 = 0.60 ).