1 Changes in avian communities in the San Luis Valley, Costa Rica Nicholas Kovacs Department of Wildlife Biology, University of Vermont ABSTRACT In this experiment, avifauna of transformed Costa Rican premontane moist forest was censused using fixed r adius point counts within three habitats. Species abundance for all observed birds was calculated based on these observations. Study sites included pasture and fragmented secondary forest. Species abundance was compared to previous values known for the are a from a1993 study. For the 51 species observed in the area, 31 showed frequency changes. Twenty six of the 31 were negative declines, and five showed positive increases in population. Four species declined from common to rare, three species declined from fairly common to rare and two new species were added that were not in the previous study. Almost all birds that declined relied on forest habitat. The species that showed positive increases generally were not habitat specific. Local and global changes hel p to explain differences depending on habitat requirements, altitudinal ranges and migratory status. RESUMEN En este estudio, la avifauna del bosque hÃºmedo premontano transformado de Costa Rica fue estudiada con cuentas de punto de radio fijo en tres Ã¡re as. Se determinÃ³ la abundancia de las especies basadas en estas observaciones. Los sitios de estudio fueron el potrero y el bosque secundario fragmentado. La composiciÃ³n de la comunidad se comparÃ³ con los datos recolectados para el Ã¡rea de estudio en el a Ã±o 1993. Para las 52 especies observadas en el Ã¡rea, 31 cambiaron de frequencia. VeintisÃ©is de las 31 disminuyeron y 5 de las especies mostraron aumentos positivos de poblaciÃ³n. Cuatro especies declinaron de ser comunes a raras y dos especies nuevas fueron agregadas que no estaban en el estudio anterior. Casi todas las aves que declinaron pertenecÃ an al hÃ¡bitat del bosque. Las especies que mostraron aumentos positivos no fueron, en general, especÃficas del hÃ¡bitat. Los cambios locales y globales ayudan a e xplicar las diferencias que dependen de requisitos del hÃ¡bitat, distribuciÃ³nde altitudinal y del estado migratorio. INTRODUCTION Birds throughout the Americas are constantly being subjected to increasing human pressures on their natural habitat. Threats include overhunting, pollution, and habitat alteration. Hunting usually does not have detrimental affects on populations, but when combined with habitat alteration, it can decimate rather easily (Stiles 1989). Another problem is the continued use of DDT as a pesticide in many developing nations. Habitat destruction however, is the greatest threat to bird communities. In its assessment of migratory bird populations, the U.S. Fish and Wildlife Service lists habitat loss as the primary cause of population decl ine in 24 of 28 species categorized as endangered or of Biogeography Theory suggests that a large and unfragmented area can support higher diversity (DeGraaf and Rappole 1995). It can be expected that smaller, fragmented
2 patches will support lower diversity of forest species. Even with an increase in some species that are habitat tolerant, habitat fragmentation decreases overall species richness and decreases species density (As kins et. al. 1990). Declines or increases in bird populations may be attributed to normal population fluctuations. Most populations respond to natural variations in food resources, wet/dry cycles, prey densities and climatic events (DeGraaf and Rappole 199 5). However, it is becoming more accepted that habitat loss is a key contributor to population changes. Atmospheric warming may also affect populations. Some species have been allowed to move from the lowlands to the highlands because of shifting freezing heights, more consecutive dry days, and the creation of environmental conditions suitable for lowland species (Pounds et al. 1999). Forest remnants usually do not represent the entire range of microhabitats that were present in the original site (Robbin s et al. 1989). Forest loss disrupts ecosystem interactions and function. For example, loss of primary forests in the Neotropics has created more species to use secondary habitat (Rappole 1995). This may contribute to the declines of primary forest species . The rate of habitat loss in the Neotropics is alarming. Unprecedented amounts of forest are being converted to sites of agricultural production (Myers 1980). No recent studies have documented declines in bird species in former Central American premontan e moist forest. This experiment was designed to better understand the community composition and population trends of this zone. All species in the area were documented and compared to the most recent richness and abundance of the Monteverde San Luis area p ublished by Michael Fogden et al. (1993). It is proposed that human pressures, such as fragmentation and global warming. Migratory birds are decreasing as well. Trends sh ow that 109 species of Nearctic migrants are in significant decline (Rappole 1995). Habitat loss is attributed to increased predation caused by fragmentation in both wintering (Saab and Petit 1992) and breeding grounds (Robbins 1980). METHODS Avifauna co mmunities were surveyed between October 23 rd and November 13 th , 2004, in San Luis, Puntarenas, Costa Rica. Two sites between 1000meters 1100meters were censused. The first site was a continuous pasture and disturbed secondary forest near Finca la Bella and extended from just above the Irazu road to the cliff edge. The site was bordered by continuous secondary forest and fragmented secondary forest on both northeastern and southwestern boundaries. The second site was a series of patchy fields and fragmented secondary forest located at the San Luis Eco lodge. A third site consisted of fragmented secondary forest that extended from Finca la Bella to the road in front of the San Luis Public Elementary School. Primary forest is almost non existent in San Luis, th erefore fragmented secondary forests represent the major forest habitat in the area. Unfortunately, there is not amount of fragmentation from 1993 until 2004 Communities were surveyed and a census of the number of species and individuals was taken. This was done by point counts similar to Robinson et al. (1999). Six points were randomly selected within the plot and were spaced at least 200meters to apart. Fifteen minutes were spent at each point censusing within a 20m radius, here based
3 on only visual obs ervation of birds. Audio counts were not included because I did not know all birdcalls. Both sites were surveyed each day from 6:15am until 9:15am, an hour and a half at each site. Site visitation alternated each day. Species abundance was classified based Species were said to be Common, Fairly Common, Uncommon and Rare. A Common species was one seen several times daily. A Fairly Common species was one seen daily or almost daily. An Uncommon species was one that was seen on ce or twice a week. A Rare species was one that was seen or heard less than once a week or at longer intervals (Fogden 1993). Frequency values for each species observed in this study were compared to the ones observed by Fogden and sign changes were noted. A positive change in a species was one that either increased in categorical abundance. A negative change was one that declined in categorical abundance. The change in species richness was not measured because birds that were not observed but listed by Fog den (1993) were not counted. This was done because chance indicates that in the small amount of time that the seeing all of the Rare species was low. Missing these specie s due to chance would lead to misinterpretation of data and a suggested decline in a species. Neotropical migrant frequency was also analyzed. Changes in migrant counts were compared to trends from the Breeding Bird Survey (BBS), a compilation of roadside surveys across the United States from 1966 until present in order to census breeding bird populations and measure declines. These were compared to one another to see if declines in species abundance at the study site coincided with declines in North Ameri can population counts. The time intervals of 1966 2003 and 1993 2003 were used to show declines. The first time period was used because it is over the entire length of the survey and is the most comprehensive. The second value was used in order to represen t recent decline data from the time Fogden published his results. The values for 1993 2003 were found using the BBS analytical tool, which calculates population change rates over any given period of time (Sauer, J. R., J. E. Hines, and J. Fallon 2004 ). RE SULTS After a total of 54 hours, fifty one species were observed. Thirty one species showed a change in category. Twenty six of these were negative declines (Table 1). The remaining five had positive frequency changes (Table 2). The largest decline in ca tegory was seen in four common species. These were the Steely vented Hummingbird ( Amazilia saucerrottei Catharus ustulatus Wilsonia pusilla ) and the White eared Ground Sparrow ( Melozone leucotis ) all showed decl ined 3 categories, from Common to Rare. The second largest decline in category was in the Golden browed Chlorophonia ( Cholorophonia callophrys ), which changed from Common to Uncommon and the Squirrel Cuckoo ( Piaya cayana), Swainson's Thrush ( Catharus ustu latus ), Black and white Warbler ( Mniotilta varia ), and the Rose breasted Grosbeak ( Pheucticus ludovicianus ), who declined from Common to Uncommon respectively. Eighteen species declined one category; Common to Fairly Common (n=6 out of 28 Common species), Fairly Common to Uncommon (n=9 out of 17 Fairly Common species), and Uncommon to Rare (n=3 out of 3 Uncommon individuals).
4 Overall there were 28 species that were previously Common, of which 11 decreased in category. Four of these declined from Common to Rare. These were These birds all were highly abundant in 1993 but were found to be nearly non existent in this study. The Golden browed Chlorophonia ( Cholorophonia callophrys ) declined from common to uncommon. Fourteen Fairly Common species (n=17) changed in category. Three species declined from Fairly Common to Rare. These were the Squirrel Cuckoo ( Piaya cayana ), Black and white Warbler ( Mniotilta varia ) and the Rose breasted Grosbeak ( Pheucticus ludovicianus ). Nine Fairly Common birds decreased to uncomm on. Two species increased from Fairly Common to Common. All Parulidae (n=4) species declined except for one, the Tennessee Warbler ( Vermivora peregrina ), which remained the same. Tyrannidae (n=5 of 8 species present) species had the most declining species , but also had one increase. The families that had no declines were the Cathartidae (n=2), Psittaciformes (n=1), Apodidae (n=2), Tityridae (n=1) and Corvidae (n=2) families. Not all species decreased in frequency. Five species increased in category. Two of the 5 species that increased were not present in 1993 (Fogden 1993), the Violet Sabrewing ( Campylopterus hemileucurus ) which increased two categories to Uncommon and the Scarlet rumped Tanager ( Ramphocelus passerinii ), which increased one category to Rare . The other three increasing species showed single category changes. They were Psarocolius montezuma ), which increased from Rare to Uncommon, and the Great Kiskadee ( Pitangus sulphuratus ) and Rufous tailed Hummingbird ( Amazilia tzacatl ), both of which increased from Fairly Common to Common. Of the 26 declining species, 12 were neotropical migrants who are also suffering declines in North American populations (Degraaf and Rappole 1995). These species all decreased in abundance in the study. Figure 1 shows the declines of ten of these species based on BBS data from 1966 1993 and 1993 2003. BBS showed increases in two species, the Yellow bellied flycatcher ( Empidonax flaviventris ) and the Summer Tanager ( Piranga rubra) . However, conf licting population values in the literature are shown (DeGraaf and Rappole 1995), and these species are believed to be declining and also declined in this study. The time period used by DeGraaf and Rappole may have been different than the 1966 2003 or 1993 2003 to show declines. For this reason they were excluded from figure 3.
5 Table 1. Bird Species exhibiting negative category changes in San Luis Valley, Costa Rica using Fogden values (Fogden 1993). Species are classified as C (common), F (Fairly Family and Species Average number days/ week Total number of individuals sighted Fogden value Study value Category difference Cuculidae Groove billed Ani ( Crotop haga sulcirostris ) 3.5 22 C F 1 Squirrel Cuckoo ( Piaya cayana ) .5 2 F R 2 Trochilidae Steely Vented Hummingbird ( Amazilia saucerrottei ) .25 1 C R 3 Plain capped Starthroat ( Heliomaster constantii ) .25 1 U R 1 Ramphastidae Keel billed Toucan ( Ramphastos sulfuratus ) 3.5 20 C F 1 Momotidae Blue crowned Motmot ( Momotus momota ) 2 5 F U 1 Picidae Hoffman's Woodpecker ( Melanerpes hoffmannii ) 3 15 C F 1 Tyrannidae Boat billed Flycatcher (Megarhynchus pitangua) 4 13 C F 1 Brown crested Flycatcher ( Myiarchus crinitus ) 2 4 F U 1 Western Wood Pewee ( Contopus sordidulus ) 2.5 6 F U 1 Eastern Wood Pewee ( Contopus virens ) 2 4 F U 1 Yellow bellied Flycatcher ( Em pidonax flaviventris ) 2 4 F U 1 Hirundinidae Barn Swallow ( Hirundo rustica ) 3.5 750 1000 C F 1 Troglodytidae Rufous and white Wren ( Thryothorus rufalbus ) 2 5 F U 1 Turdidae Wood Thrush ( Hylocichla mustelina ) .5 1 U R 1 Swainson's Thrush ( Catharus ustulatus ) .5 2 C R 2 Parulidae Black and white Warbler ( Mniotilta varia ) .25 1 F R 2 Worm eating Warbler ( Helmitheros vermivorous ) 1 5 F U 1 Wilson's Warbler ( Wilsonia pusilla ) .5 2 C R 3 R ufous capped Warbler ( Basileuterus rufifrons ) 1.5 6 F U 1
6 Icteridae Northern Oriole ( Icterus g. galbula ) 3.5 145 C F 1 Thraupidae Golden browed Chlorophonia ( Cholorophonia callophrys ) 2.5 200 C U 2 Summer Tanager ( Pirang a rubra ) 1 4 F U 1 Emberizidae Rose breasted Grosbeak ( Pheucticus ludovicianus ) .25 1 F R 2 Indigo Bunting ( Passerina cyanea ) .25 1 U R 1 White eared Ground Sparrow ( Melozone leucotis ) .25 1 C R 3 TABLE 2. Bird Species ex hibiting positive category changes in San Luis Valley, Costa Rica using Fogden values (Fogden 1993). Species are classified as C (common), F (Fairly methods). Family and Spec ies Average Number days/ week Total number of individuals sighted Fogden value Study value Category difference Trochilidae Violet Sabrewing ( Campylopterus hemileucurus ) 1 5 U 2 Rufous tailed Hummingbird ( Amazilia tzacatl ) 5 18 F C 1 G reat Kiskadee ( Pitangus sulphuratus ) 7 30 F C 1 Icteridae Montezuma's Oropendula ( Psarocolius montezuma ) 2 25 R U 1 Thraupidae Scarlet rumped Tanager ( Ramphocelus passerinii ) 1 1 R 1
7 FIGURE 1. Neotropical Migra nts found in San Luis Valley that showed declines in the study as well as in Breeding Bird Surveys along North American routes. Only declining populations were included in this figure. Increasing populations over either time intervals occurred in three spe cies but values were less than 1%. (Sauer, J. R., J. E. Hines, and J. Fallon 2004 ) DISCUSSION Relative species abundance changed from the previous Fogden values for several species in this study, suggesting an overall loss in diversity. Thirty two spec ies (n=51 total) observed showed a change in category. Of these 32, 26 declined, suggesting an overall loss in abundance. The major categorical changes in the Black and white Warblers and Worm eating Warblers could be explained by habitat loss. In a study by Rappole and Morton et al. (1995), these species were originally common to a six hectare rain forest site but disappeared from the site after fragmentation occurred. Habitat fragmentation affects the Black and white Warbler only when it is involved in in terspecies relationships
8 with mixed flocks of birds. In this interaction, it defends the flocks from other Black and white Warblers. However, when they live solitarily, habitat fragmentation does not affect populations (Rappole 1995). This suggests that ot her factors, such as behavior, can influence the effect of fragmentation on a population. Other species that declined mostly prefer secondary and primary forest habitat. The Squirrel Cuckoo is found in the canopy and on edges of primary and secondary fores t. This may indicate poor quality habitat. Other interior species, like the Steely Woodpecker declined in this study. Another species found in the forest interior is the Keel billed Toucan, which also showed declines. M ost of the declining species observed in this study are either totally or partially reliant on forest habitat, such as the squirrel declined were the Rufous capped Warbler ( interior forest and edge), the Summer Tanager (forest and forest edge), Rose breasted grosbeak (open woods and semi open forest edge) and the White eared Ground Sparrow (thickets and patchy woodland) (Stiles 1989). Two other interior species, the Rufous an d white Wren ( Thryothorus rufalbus ) and the Swainson's Thrush ( Catharus ustulatus ), are associated with forest and scrubby woodland undergrowth. These species were common in 1993, but now hardly occur. With the conversion of most of San Luis to agricultura l fields, these species are losing habitat and are often displaced by other disturbance loving species. Other events may have the winter (Stiles 1989). With all the d ecreases in abundance of bird species, one would expect that some species would fill the niche gaps left by the declining species. For example, Rufous tailed Hummingbirds prefer non forest habitat and showed an increase in abundance. Wherever forest has be en removed this species is the most abundant and widespread Costa Rican hummingbird (Stiles 1989). Some Violet Sabrewings descend to lower altitudes after the breeding season, which could explain their presence in this study. The only species of Tyrannidae that increased in abundance was the Great Kiskadee. On the Caribbean slope it has increased greatly because of deforestation (Stiles 1989). The increase in frequency semi open forests and banana plantations. The last species that showed increase was the Scarlet rumped Tanager. It enjoys thickets and pastures locally in the North Pacific (Stiles 1989) and therefore may be moving up to higher altitudes with similar habitat su ch as San Luis. Some declines may not be due just to local habitat degradation but rather habitat loss elsewhere. Neotropical migrants are a prime example. These species are at high risk on both their wintering, migratory and breeding grounds due to habit at loss there. If migratory birds are limited by availability of tropical wintering grounds, any destruction to the habitat will cause migratory bird populations to decline (Morton 1980a, Rappole 1974, Vogt 1970). It can be expected that many of the migran ts in San Luis are decreasing because of declines in North America. All migrants observed in San Luis that showed declines also suffer declines in BBS data. Every migratory warbler decreased in abundance except the Tennessee Warbler, which seems to be very well adapted to disturbed and non disturbed sites (Mader 1992). This study adds to the idea that population declines of Neotropical migrants in North America are reflected in wintering
9 habitats such as the San Luis study site. It also suggests that declin es in local wintering habitats may be contributing to a larger population crisis. Several families did not contain a single observed species that changed in category. This may be because of individual species lack of habitat preference. Cathartids, or vu ltures, thrive in open habitat similar to the study site (Stiles 1989). Psittaciformes are only represented by one species in this study, the Orange chinned Parakeet (Brotogeris jugularis). This species prefers open country with scattered fruit trees and f orest edges typical to the San Luis valley (Stiles 1989). Next the Apodidae, or Swift family, can be found across a range of habitats (Stiles). The Tityridae family also only consisted of one species, the masked Tityra ( Tityra semifasciata ), which is also found across a range of habitats, from forest canopy to clearings or savannahs (Stiles). The last family, which showed no change in frequency among any of its observed species was the Corvidae family, or Jay family. Brown Jays ( Cyanocorax morio ) and White throated Magpie Jays ( Calocitta formosa ) were abundant among the habitat. Brown Jays travel in flocks throughout open woodland habitats, forest edge, open country and are found in almost every habitat (Stiles). The White throated Magpie Jay is seems to be found in several habitats other than forest habitat (Stiles). Habitat conversion is a problem. Butcher et. al (1981) suggested that edge and edge interior species have a competitive advantage over interior forest species in fragmented habitats. This sugges ts that fragmentation can greatly affect community composition. Furthermore, variations of carrying capacities and long term population sustainability between the untouched and disturbed habitats have major implications for conservation (Rappole 1995). A d isturbed landscape may not be able to support as much diversity as a mature primary habitat. It is important that people act to curb the habitat loss that plagues the tropics and that ecologists manage wintering habitats as well summer breeding grounds. Th e San Luis valley is an example of diversity loss in disturbed communities. However, habitat loss cannot be blamed for all population changes. The number of premontane species at 1540 meters in Monteverde, Costa Rica, has increased, whereas the resident sp ecies have not changed (Pounds et al. 1999). No major deforestation has occurred for 60 years in this region. This suggests that changes in community composition are not changing as a result of habitat loss, but rather from the increase in atmospheric temp erature. Many things may have affected the results of this study. First, the study by Michael Fogden was done over a much longer period than this study. Populations may fluctuate, giving biased samples based on the time of year. This study may have caught a species during a normal population fluctuation, which stabilizes over a longer interval, and called it a decline. Furthermore, this study may have over estimated declines based on the lack of use of birdcalls, which Fogden used in his study. Many Rare o r forest birds may have been missed because calls were not counted. However, some declines were so large in category change that these factors should not affect the overall suggestion of relative population change.
10 ACKNOWLEDGMENTS First I would like t o thank Jenier Garro Cruz for showing me the trails and birding sites of San Luis. I also would not have gotten far without the help of Mathew Gasner for helping me ID the most difficult little birds. I would like thank Mavricio Ramirez and Franco Naebaro, and the Eco lodge folks for letting me bird on their land. Lastly I would like to thank Allan Masters for helping me maintain direction in my study and Javier Mendez for helping with the translation of the abstract. LITERATURE CITED Askins, R.A., J.F. Lynch, and R. Greenberg. 1990. Population declines in migratory birds in eastern North America. In : DeGraaf, M.R. and Rappole J.H. Neotropical Migratory Birds. Cornstock Publishing Associates. London. Butcher, G.S., W.A. Nierinf, W.J. Barry, and R.H. Go odwin. 1981. Equilibrium biogeography and the size of nature preserves: An avian case study. In: Rappole, J.H. 1995. The Ecology of Migrant birds. Smithsonian Institution Press. Washington. DeGraaf, M.R. and Rappole J.H. Neotropical Migratory Birds. 199 5. Cornstock Publishing Associates. London. Fogden, Michael. 1993. An annotated checklist of the Birds of Monteverde and Penas Blancas. Monteverde, Costa Rica Mader, T. 1992. Distributions of Wintering Neotropical Migrants and Resident Birds in a Pri mary Forest and a Series of Successional Sites in the San Luis Valley, Costa Rica. UC EAP (fall). Myers, N. 1980. Conversion of Tropical Moist forests. Washington: National Academy of Sciences. Pounds, A, Fogden M.P.L., and Campbell, J.H. 1999. Biologica l response to climate change on a tropical mountain. Nature. 398:611 615. Rappole, J.H. 1995. The Ecology of Migrant birds. Smithsonian Institution Press. Washington. Robinson, D.W. 1999. Long Term Changes in the Avifauna of Barro Colorado Island, Panam a, a Tropical Forest Isolate. Conservation Biology. 13:85 88. Robbins, C.C., D.K. Dawson, and B.A. Dowell. 1989. Habitat area requirements of breeding forest birds of the Middle Atlantic states. In: DeGraaf, M.R. and Rappole J.H. Neotropical Migratory B irds. Cornstock Publishing Associates. London. ---------1980. Effect of forest fragmentation on bird population in the piedont of Mid Atlantic region. American Naturalist. 33:31 36. Saab, V.A., Petit, D.R. 1992. Impact of pasture development on winter bird communities in Belize, Central America. The Condor. 94:66 71. Sauer, J. R., J. E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, Results and Analysis 1966 2003. Version 2004.1. USGS Patuxent Wildlife Research Center , Laurel, MD . Stiles, G.F. and Skutch, A.F. 1989. A guide to the Birds of Costa Rica. Cornstock Publishing Associates. London.
11 U.S. Fish and Wildlife Service (USFWS). 1987. Migratory nongame birds of management concern in the United States. In: DeGraaf, M.R. and Rappole J.H. Neotropical Migratory Birds. Cornstock Publishing Associates. London.
12 Appendix 1: Additional species that showed no categorical change SPECIES Fogden value Study value Cathartidae Black Vultur e ( Coragyps atratus ) C C Turkey Vulture ( Cathartes aura ) C C Psittaciformes Orange chinned Parakeet ( Brotogeris jugularis ) C C Apodidae White collared Swift ( Streptoprocne zonaris ) C C Vaux's Swift ( Chaetura vauxi ) C C Ramphastid ae Emarald Toucanet ( Aulacorhynchus prsinus ) C C Tityridae Masked Tityra ( Tityra semifasciata ) C C Tyrannidae Tropical Kingbird ( Tyrannus melancholicus ) C C Olive sided Flycatcher ( Contopus borealis ) F F Corvidae Brown J ay ( Cyanocorax morio ) C C White throated Magpie Jay ( Calocitta formosa ) F F Hirundinidae Blue and white Swallow ( Notiochelidon cyanoleuca ) C C N. Roughwinged Swallow ( Stelgidopteryx serripennis ) F F Troglodytidae House Wren ( Trogl odytes aedon ) C C Plain Wren ( Thryothorus modestus ) C C Turdidae Clay colored Robin ( Turdus Grayi ) C C Parulidae Tennessee Warbler ( Vermivora peregrina ) C C Thraupidae Yellow throated Euphonia ( Euphonia hirundinacea ) C C Ember izidae Yellow faced Grassquit ( Tiaris olivacea ) C C Rufous collared Sparrow ( Zonotrichia capensis ) C C
13 Appendix 2: On site declining Neotropical migrants and population change values for North American BBS survey. Migrant Species BBS dec line 1966 2003 P value BBS decline 1993 2003 P Value Western Wood Pewee ( Contopus sordidulus ) 0.87 0.171 1.25 0.00006 Eastern Wood Pewee ( Contopus virens ) 2.3 0.000 1.78 0.000 Yellow bellied Flycatcher ( Empidonax flaviventris ) 1.32 0.263 2.23 0.0093 1 Barn Swallow ( Hirundo rustica ) 1.72 0.128 0.93 0.000 Wood Thrush ( Hylocichla mustelina ) 2.58 0.000 1.78 0.000 Swainson's Thrush ( Catharus ustulatus ) 0.64 0.087 0.5 0.06903 Black and white Warbler ( Mniotilta varia ) 2.04 0.004 0.28 0.38214 Wor m eating Warbler ( Helmitheros vermivorous ) 1.6 0.311 0.45 0.46385 Wilson's Warbler ( Wilsonia pusilla ) 2.58 0.0006 1.41 0.00388 Rose breasted Grosbeak ( Pheucticus ludovicianus ) 0.9 0.072 0.75 0.01613 Indigo Bunting ( Passerina cyanea ) 0.14 0.437 0.5 9 0.000 Summer Tanager (Piranga Rubra) 1.03 0.02813 0.39 0.27179
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Cambios en las comunidades de aves en el valle de San Luis, Costa Rica
Changes in avian communities in the San Luis Valley, Costa Rica
In this experiment, avifauna of the transformed Costa Rican premontane moist forest was censused using fixedradius point counts within three habitats. Species abundance for all observed birds was calculated based on these observations. Study sites included pasture and fragmented secondary forest. Species abundance was compared to previous values known for the area from a 1993 study. For the 51 species observed in the area, 31 showed frequency changes. Twenty-six of the 31 were negative declines, and five showed positive increases in population. Four species declined from common to rare, three species declined from fairly common to rare and two new species were added that were not in the previous study. Almost all birds that declined relied on forest habitat. The species that showed positive increases generally were not habitat specific. Local and global changes help to explain differences depending on habitat requirements, altitudinal ranges and migratory status.
En este estudio, la avifauna del bosque hmedo premontano transformado de Costa Rica fue estudiada con cuentas de punto de radio fijo en tres reas. Se determin la abundancia de las especies basadas en estas observaciones. Los sitios de estudio fueron el potrero y el bosque secundario fragmentado. La composicin de la comunidad se compar con los datos recolectados para el rea de estudio en el ao 1993. Para las 52 especies observadas en el rea, 31 cambiaron de frecuencia. Veintisis de las 31 disminuyeron y 5 de las especies mostraron aumentos positivos de poblacin. Cuatro especies declinaron de ser comunes a raras y dos especies nuevas fueron agregadas que no estaban en el estudio anterior. Casi todas las aves que declinaron pertenecan al hbitat del bosque. Las especies que mostraron aumentos positivos no fueron, en general, especficas del hbitat. Los cambios locales y globales ayudan a explicar las diferencias que dependen de requisitos del hbitat, distribucin altitudinal y del estado migratorio.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Seleccin de hbitat
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Fall 2004
Ecologa Tropical Otoo 2004
t Monteverde Institute : Tropical Ecology