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Changes in bird species distribution in four altitudinal zones in Monteverde, Costa Rica Grant Connette Department of Biology, Davidson College ABSTRACT Theimpact of climate change on ecosystems has been demonstrated by studies of upper elevation amphibians (Pounds 1997). In spite of this, the degree to which climate change affects birds is not as clear. Two previous studies have found the upward movement of many species of birds in the Monteverde area (Donnelly 1998, Palm 2003). I surveyed birds in four altitudinal zones on the less studied Atlantic slope of the Monteverde Cloud Forest Preserve and compared my findings to the bird distributions reported by Michael Fogden in 1993. I found significant upward movement of bird species (sign test, p < 0.05) as well as a general decline in the abundances of certain bird species (x2 = 27.65, df = 3, p < 0.001). The majority of these declines are probably linke d to the reforestation of the Peas Blancas valley but climate change is likely responsible for the upslope movement and increased abundance of higher elevation birds. RESUMEN El impacto del cambio climtico en los ecosistemas ha sido demostrado por varios estudios en anfibios que habitan las zonas altas (Pounds, 1997). A pesar de esto, el hecho que el cambio climtico afecte de alguna manera a las aves, no est claro. Dos estudios anteriores han encontrado que muchas especies de aves de zonas bajas estn incrementa sus poblaciones en Monteverde (Donnelly, 1998 y Palm, 2003). Examin 4 zonas altitudinalmente diferentes en el lado Atlntico de la Reserva del Bosque Nuboso Monteverde y compar mis resultados a las distribuci ones de especies de av es reportadas por Michael Fogden en 1993. Encontr que algunas especies aume ntaron su distribucion en areas de mayor altura (Prueba de Muestra, p<0.05) as co mo una declinacin general en los a bundancia de ciertas especies de aves (x2=27.65, df=3, p<0.001). La mayora de stos declines estn relacionados probablemente a la reforestacin del valle de Peas Blancas pero el cambio climtico es probablemente responsable del movimiento ascendente y de la abundancia creciente de aves propias de zonas bajas en zonas ms altas. INTRODUCTION Global climate change is now a welldocumented phenomenon but its effects on plant and animal diversity are still being studied. While bird s may be able to cope with some change in temperature or moisture, it is possible that their di stributions are highly dependant on changing food availability. Frui ting and flowering periods may respond to climate change due to the link between their phenology and abioti c triggers like rainfall. Birds relying on these plants for food could be affected by the change in fruit or nectar availability. Insects are also highly re sponsive to rainfall patterns, cloudiness, temperature fluctuation, humidity and other environmental factors (Janzen 1983). Insect density peaks at the lower edge of middle elevation cloud forests, where the dry season is less severe and overall productivity is higher as plants have lower maintenance costs on cool nights (Janzen 1973). As global temperatur es rise, it is possible that the lower edge of the these cloud forests will move progr essively higher, driving insects and insectivorous birds upslope in pursuit of abundant food.
While food abundance could affect altitudi nal distribution of bird species, the additional effect of competitive interactions must be considered. Robinson and Terborgh (1995) found that interspecific territoriality is widespr ead among Amazonian birds of similar niches with adjacent territories. In every case, the less dominant species was forced to accept a less productive habitat. Tropical birds forced into less productive highlands have developed adaptations to cold or rainy weather which allow them to be competitively superior at higher elevations. A good example of such a bird is the WhiteBreasted Blue Mockingbird, which forages on the ground and does better when rain drives insects out of the soil (Skutch 1950). This bird also significantly increases the time it spends incubating eggs or nestlings during cold or wet conditions (Skutch 1950). Warmer, drier, conditions coupled with increas ed food availability could put an end to facultative niche partitioning between elevat ions by allowing lower elevation birds to move into the previously inhos pitable highlands and displace resident species. Highland birds also may have simply evolved with less competition because few species are cold weather adapted. If the highlands were to warm and make cold weather adaptations unnecessary, many new and better competitors could move into the area and displace resident species. I also expect that the arrival of nest predators, such as the Keel-Billed Toucan, could cause further damage to highland bird populations already facing constricted habitats. A survey done in 1993 defined seven habitat zones for Monteverde area birds and the abundances of species were determined for each zone (Fogden 1993). Both Donnelly (1998) and Palm (2003) have repeated the Fogden study on the Pacific slope and found upslope movement of bird species, though to a lesser extent in the wet season. Pounds (1997) found that open country birds, which prefer fields, gaps, edges or young second growth, are losing local species diversity in area s of regenerating forest but that birds as a whole are losing little species diversity as a result of temperature changes. The goal of my study was to define current bird distribut ions on the more aseasonal Atlantic slope of the Monteverde area. I predicted that my study would show both the decline of open country birds from the Fogden study and a gene ral upslope movement of bird species due to climate change. MATERIALS AND METHODS I spent twelve days identif ying every bird encountered between the Monteverde Cloud Forest Preserve entrance and the Re fugio Aleman (Appendix A). I used A Guide to the Birds of Costa Rica (Stiles and Skutch 1989) to id entify every bird encountered. Some more secretive birds were identified by sound. I also re corded the altitude at which each bird was spotted. I began around 5:45 AM every day and made observations on the El Camino and Peas Blancas trails. Each time I walked down, I sp ent the night at the shelter and walked back on the same trail. My study covered much of zones 3-6 from Fogdens study, which have approximate altitudinal ranges. Zone 4 wa s the highest (above 1600 m on the Pacific and above 1400 m on the Atlantic Slope) while zone 3 (roughly 1500-1600 m) was below on the Pacific slope. Zones 5 (1000-1400 m) a nd 6 (800-1000 m) were lower on the Atlantic Slope. Zones 3 and 4 are cloud forests that receive around 3 m of rainfall annually while zones 5 and 6 are rainforest that receive be tween 4 and 8 m of rainfall annually. The cloud forest also receives much precipitation in the form of mist during the dry season 2
and therefore has higher epiphyt e abundance than the lower Atla ntic rainforest (Haber et al., 2000). The Atlantic slope rainforest, zone s 5 and 6, is particularly rich in ferns, aroids and large-leaved he rbs (Haber et al 2000). I created a list of the numb er of individuals of each bird species I found in each zone. I then classified them into cate gories of abundance based on the number of individuals of each species found in a particular zone (Table 2). Categories were roughly based on Fogdens descriptions. I called any bird found at least twice daily a common species. A fairly common species was one se en or heard daily or almost daily, usually in small numbers. An uncommon species was anything seen once or twice a week or less. I used Fogdens exact definitions fo r fairly common and unc ommon but did not use Fogdens rare designation as the distinction between a rare and an uncommon species is impossible to make over a two-week period. I also used a slight ly different definition of a common species to draw a clearer line than Fogden between a fairly common and a common species. Table 1. Abundance designations. The numb er of individuals of a species that must be seen for that species to be considered Common, Fairly Common or Uncommon in a particular zone. (The numbe r of individuals required for a species to fit into a cate g or y varied between zones due to uneven sam p lin g time. ) Zone 3 Zone 4 Zone 5 Zone 6 Common (C) 4+ 6+ 8+ 4+ Fairly Common (F) 3 3-5 4-7 3 Uncommon (U) 1-2 1-2 1-3 1-2 I did not include certain birds in analysis for various reasons. Parrots and some pigeons were flocking birds that could be ex tremely common certain days but absent the rest of the time. Raptors and Vultures were not included because they were relatively uncommon in the closed forest. I also made no effort to sample nocturnal birds. For these groups of birds I felt that I was not able to reliably define th eir distributions and excluded them from analysis. Fogden also may have used different criteria for classifying Hummingbirds and I also excluded them from anal ysis. Many are listed as common and yet are generally solitary and are never seen frequently enough in the forest to fit Fogdens definition of a common species (one seen several times daily in moderate to large numbers ). To determine whether or not abundances have changed since 1993, I compared my observed species abundances fo r the remaining birds in each zone with those reported by Fogden (Chi-squared test). I isolated those species that appeared to have moved up or down slope and determined whether these changes were significant (sign test). RESULTS I identified 114 different species of bi rds in zones 3-6. Of these, 29 showed noticeable differences in their elevational range since Fogdens 1993 study (Table 2). Nine of these species moved down in elevation while 20 moved up. Fourteen of the 20 that moved up had expanded into zones where Fogden had not found them. A sign test 3
suggested that birds have made a significant movement upslope since 1993 (sign test, n = 29, p < 0.05). Many birds have also changed in ove rall abundance since Fogdens study, though these changes were not distributed equally acro ss zones. Zones 3 and 6 showed far more species declines than species increases (13 vs. 4 in zone 3, 20 vs. 0 in zone 6). Zone 5 also showed more declining species than increasing species (34 vs. 12). Zone 4, however, had more species in creasing in abundance than de creasing in the time since Fogdens study (15 vs. 13). Groups that seem to show the greatest decline in abundance were the flycatchers, antbirds, sparrows, finches and some tanagers. Woodcreepers, warblers and wrens showed no noticeable change in abundance. The majority of species increasing in abundance within any given z one were the result of up or downslope expansion into territory where they prev iously were not found. Changes in bird abundance showed a significant difference between my data and Fogdens data (Chisquared goodness of fit test, x2 = 27.65; df = 3; p < 0.001). DISCUSSION It is not surprising that birds have changed in abu ndance over the 13 years since Fogdens study. In 1993, the Bosque Eterno de los Nios had been in existence for less than ten years. At this time, clearings we re probably just returning to shorter, second growth forest cover. Many bird species that thrive in open country or forest clearings were very dominant and have showed a larg e-scale decline over the past 13 years. Nearly all of the flycatchers are more pr evalent in open areas, forest clearings, or young secondary growth (Stiles and Skutch 1989) and showed a large decline in abundance. The Tufted Flycatcher was the onl y exception to the trend in the data, yet I typically found it in small clea rings, treefall gaps and along st reams. The Tanagers that showed decline are reported to frequent the canopy and edge of dense forest (Stiles and Skutch 1989). They were said to come cl oser to the ground in second growth or in nearby clearings with scattered trees. Ta nagers, which are primarily frugivorous, now feed in a much higher canopy or more isolat ed clearings and are less common and harder to spot than they were 13 years ago. Th is expectation was supported by my study, as a large percentage of Tanagers spotted were in the cleari ng around the Refugio Aleman. Daily altitudinal movements were not a factor as the Tanagers were in the clearing around the Refugio both in the early and late morning. Common Bush Tanagers were still extremely abundant, yet they are ge neralists known for leading mixed foraging flocks through all types of forest. Spa rrows, Seedeater, Finches and Grosbeaks also typically frequent clearings, fields and shr ubby second growth in search of seeds (Stiles and Skutch 1989) and have decreased in abundance along with these sorts of habitats in the Peas Blancas valley. The decline of th e Antbirds is more puzzling, as they are usually found in dense forests. It is possible that Antbirds are very sensitive to changes in habitat and have still not r ecovered from declines following historical deforestation. It is also possible that two weeks was not su fficient time to come across enough groups of Army Ants followed by Antbirds. From experience, Antbirds were encountered following large Army Ant raids and large Army Ant columns were simply not seen often enough to make Antbird sightings frequent. With the exception of the Antbirds, all groups of birds showing decline seemed to have a likely habitat change as an explanation. 4
Some of these perceived decreases in a bundance may be due to the greater ability of Fogden, a long time researcher, to spot birds. It is probably the case that, while many species appear to have changed in abunda nce, only the groups showing the largest differences in abundance compared to other grou ps have experienced an actual change. The increase in abundance of higher elevation bi rds is also an importa nt change, as this showed up in spite of overall declin e in sightings compared to Fogden. When comparing the number of bird sp ecies that obviously moved up or down in distribution since 1993, there is more certaint y. Birds found in a higher zone than they were ever found in before have definitely sh ifted their actual ranges upslope. The twenty birds moving upslope had either moved into an entirely new habitat zone or were more common than before at high altitudes and much less common at lower altitudes. Furthermore, three of the nine bird species moving downslope were Tanagers that were only found in the clearing around the Refugio Al eman. These shifts were probably due to changing habitat and do not accurately reflec t a new altitude preference. Overall, however, there was a general trend of upslope movement. While many birds may move upslope for breeding or for food, birds found in entirely new habitat zones are more than likel y moving outside of their traditional ranges, even for breeding or migration. Perhaps the best examples of such movement are the Redstarts. The Collared Redstart is now found only in the higher parts of Zone 4 and the Slate-throated Redstart, which was not f ound in zone 4 by Fogden, is now common in zone 4. This movement was also noted by both Palm (2003) and Donnelly (1998). While there are many examples of birds movi ng upslope, the impact of these movements is not yet known. One possible indicator sp ecies is the Sooty-capped Bush Tanager, which is found only in zone 4. This speci es was fairly common in Fogdens study yet was never observed in my study, even at al titudes as high as 1700 m. The Common Bush Tanager, on the other hand, was extremely abunda nt at all levels of zone 4. Discerning the effects of climate change on bird distributions is complicated by forest regeneration at lower elevations, yet the Monteverde Cloud Fore st Preserve has been protected for a long time and movements into zone 4 cannot be ascribed to habitat changes there. The reliability of Fogdens data also ensures that changes in bird ranges are recent and significant. Climate change is implicated where all ot her possibilities fail. Amphibians were an early indicator of global climate change but for current levels of temperature and rainfall change to affect birds there probabl y needs to be a much larger effect on the ecosystem as a whole. Endothermic organism s are unlikely to be directly affected by fairly small temperature changes and are mo re likely to move upslope in response to changes in the biotic systems they depend on. The upward movement of birds coupled with the effects of competitiv e interactions suggest that further changes in bird abundance and distribution are likely to take place in th e near future. Research on changing species interactions due to upslope movement is n eeded to complete the picture of climate changes effects on birds. Only further st udy on a much larger scale can alert people to the fact that there is a lot more to lose than a few highland amphibians as a result of global warming. 5
ACKNOWLEDGEMENTS I would like to thank Tania Chavarria Pi zzaro and Alan Masters for their constant advice and support with this project. I w ould also like to thank Karen Masters, Tom McFarland and Camryn Pennington for everythi ng they helped with along the way. Thanks also to the Monteverde Cloud Fore st Preserve for letting me use their land. Finally, thank you to Kaitlin Dunn, Josh Me tten and the three men from the power company who kept me comp any during my research. LITERATURE CITED Donnelly, E. 1998. Changes in Bird Species Composition in Four Habitat Zones in Monteverde, Costa Rica. CIEE Spring. Fogden, M. 1993. An Annotated Checklist of the Birds of Monteverde and Peas Blancas. San Jos, Costa Rica. Haber, W. A., W. Zuchowski and E. Bello. 2000. An Introduction to Cloud Forest Trees; Monteverde, Costa Rica. Mountai n Gem Publications, Monteverde de Puntarenas, Costa Rica. Janzen, D. H. 1973. Sweep samples of tropica l foliage insects: Effects of seasons, vegetation types, elevation, time of day, and insularity. Ecology 54:687-708. Janzen, D.H. 1983. Insects Introduction. In Costa Rican Natural History D.H. Janzen (ed.), The University of Chicago Press, Chicago, IL. Palm, E. 2003. Patterns of Seasonal and Altitudinal Change in Monteverde Bird Communities. CIEE Fall. Pounds, A. J., M. P.L. Fogden, J. M. Sa vage and G. C. Gorman. 1997. Tests of Null Models for Amphibian Declines on a Tropical Mountain. Conservation Biology, Vol. 11, No. 6. Skutch, A. F. 1950. Life History of th e White-Breasted Blue Mockingbird. The Condor, 52 (5): 220-227. Stiles, G. F. and A. F. Skutch. 1989. A Guide to the Birds of Costa Rica. Cornell, NY. Robinson, S. K. and J. Terborgh. 1995. Interspecific Aggression and Habitat Selection by Amazonian Birds. The Journal of Animal Ecology, 64 (1): 1-11. 6
TABLES Table 2. Twenty-nine bird species w ith ranges that moved up or down in elevation. + refers to upslope movement refers to downslope movement C=Common, F=Fairly Co mmon, U=Uncommon, R=Rare, *=Vagrant (outside normal range) Numbers in parenthe sis are the actual numbers of individuals found in this study for each zone. SPECIES FogdenConnetteFogdenConnetteFogdenConnetteFogdenConnetteBlack Guan FU (2)FC (16)FC (17) + Great Curassow U (2) + Black-breasted Wood Quail FC (12)F FU (3)F Chiriqui Quail-Dove R U (1)R + Buff-fronted Quail-Dove U F UU (3)R Hoffman's Woodpecker U (1) + Plain Xenops U (2)U + Ruddy Treerunner *U (2)FC (9) Wedge-billed Woodcreeper *U (1) U (1)FF (5)F + Spotted Woodcreeper CC (6)C CC (12)C Buff-throated Foliage-gleaner U (3)F + Striped Foliage-gleaner U (1)F + Dull-mantled Antbird U (1)R + Striped-breasted Wren U (1)C + Tennessee Warbler U U (2)CC (13)C + Buff-rumped Warbler FU (1)FC (7)Tropical Parula U U (2)CF (5)C + Black-and-white Warbler FU (1) U (1)FC (15)FU (1)+ Wilson's Warbler C UC (6)CC (13)CC (5)+ Slate-throated Redstart CC (11) C (17)CC (52)UU (1)+ Olive-crowned Yellowthroat U (2)CU (1)CU (2)+ Northern Oriole U U (2)UF (4)F + Tawny-capped Euphonia U (2)FU (2)F + Scarlet-thighed Dacnis C U C CC (5)Black-and-Yellow Tanager U (1)FU (1)+ Crimson-collared Tanager C CC (8)Scarlet-rumped Tanager CU (1)CC (17)Yellow-thighed Finch FC (6) U (2) Variable Seedeater U (1) U CU (2)+ Zone 3 Zone 4 Zone 5 Zone 6 7
APPENDIX Appendix A: Map of Zones 3-6 in the Monteverde Cloud Fore st Preserve and surrounding area (Fogden 1993) Appendix B: Full data sheetNumber of indi viduals of each species found in each zone. SPECIES Zone 3 Zone 4 Zone 5 Zone 6 Black Guan 18 17 Great Curassow 2 Black-breasted Wood-Quail 4 8 3 Turkey Vulture 5 5 Black Vulture 1 Bicolored Hawk 1 Broad-winged Hawk 4 Black Chested Hawk 2 Band-tailed Pigeon 71 Short-billed Pigeon 1 8
Chiriqui Quail-Dove 1 Buff-fronted Quail-Dove 3 Brown-hooded Parrot 5 White-collared Swift 14 Vaux's Swift 18 Green Hermit 1 4 1 Green Violet-ear Violet Sabrewing 3 Purple-crowned Fairy Green-cowned Brilliant 2 White-bellied Mountain-gem Purple-throated Mountain-gem 3 Steely-vented Hummingbird 1 Striped-tailed Hummingbird 1 Coppery-headed Emerald 1 Magenta-throated Woodstar 2 Orange-bellied Trogon 2 7 Rufous Tailed Jacamar 4 Emerald Toucanet 3 Prong-billed Barbet 4 9 7 Red-headed Barbet 5 Golden-olive Woodpecker 1 1 Smoky-brown Woodpecker 1 1 Hoffman's Woodpecker 1 Plain Xenops 2 Ruddy Treerunner 1 10 Spotted Barbtail 9 10 Wedge-billed Woodcreeper 2 5 Olivacious Woodcreeper 2 1 Brown-billed Scythebill 1 Spotted Woodcreeper 3 3 12 Buff-throated Foliage-gleaner 3 Striped Foliage-gleaner 1 Lineated Foliage-gleaner 1 1 Streaked-breasted Treehunter 1 Gray-throated Leaftosser Tawny-throated Leaftosser 1 1 Red-faced Spinetail 2 Russet Antshrike 1 Dusky Antbird Immaculate Antbird 3 Dull-mantled Antbird 1 Slaty Antwren 2 Plain Antvireo 1 9
Golden-bellied Flycatcher 1 Torrent Tyrannulet 2 Olive-sided Flycatcher 1 Black Phoebe 1 Eastern Wood-Pewee 1 Tufted Flycatcher 2 54 11 Yellowish Flycatcher Yellow-bellied Flycatcher Olive-striped Flycatcher 1 2 White-throated Spadebill 1 3 Scale-crested Pygmy-Tyrant 2 Common Tody-Flycatcher 2 Mistletoe Tyrannulet 1 5 Mountain Elaenia 1 1 Striped-breasted Wren 1 Gray-breasted Wood-Wren 9 83 92 4 Ochraceous Wren 6 9 2 Nightengale Wren 2 Slaty-backed Nightengale-Thrush 3 1 Swainson's Thrush 1 1 Wood Thrush 1 Mountain Robin 1 Clay-colored Robin 1 8 Black-faced Solitaire 3 70 19 Azure-hooded Jay 12 29 1 Lesser Greenlet 1 2 1 Golden-crowned Warbler 2 1 Three-striped Warbler 5 64 26 Tennessee Warbler 2 13 Buff-rumped Warbler 1 7 Bananaquit 1 2 1 Tropical Parula 2 5 Golden-winged Warbler 3 4 Blue-winged Warbler 2 Black-and-white Warbler 2 15 1 Wilson's Warbler 6 13 5 Collared Redstart 32 Slate-Throated Redstart 4 24 52 1 Olive-crowned Yellowthroat 2 1 2 Cerulean Warbler 2 Chestnut-sided Warbler 4 3 Black-throated Green Warbler 8 1 Louisiana Waterthrush 1 Northern Oriole 2 4 10
11Tawny-capped Euphonia 2 2 Golden-browed Chlorophonia 3 1 Blue-and-Gold Tanager 3 Common Bush-Tanager 121 137 2 Scarlet-thighed Dacnis 5 Silver-throated Tanager 12 14 4 Green Honeycreeper 2 Bay-headed Tanager 2 6 Spangled-cheeked Tanager 3 Black-and-yellow Tanager 1 1 Blue-gray Tanager 2 Crimson-collared Tanager 8 Scarlet-rumped Tanager 1 17 Hepatic Tanager 2 Black-throated Saltator 1 Black-thighed Grosbeak 2 Chestnut-capped Brush-Finch 1 4 Sooty-faced Finch 5 Yellow-thighed Finch 6 2 Variable Seedeater 1 2 Blue Seedeater 1 Rufous-collared Sparrow 4 Black-striped Sparrow 1 Zone 3 total Zone 4 tota l Zone 5 total Zone 6 total Total=1490 35 558 783 114
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Cambios en la distribucin de especies de aves en cuatro zonas de altitud en Monteverde, Costa Rica
Changes in bird species distribution in four altitudinal zones in Monteverde, Costa Rica
The impact of climate change on ecosystems has been demonstrated by studies of upper elevation amphibians (Pounds 1997). In spite of this, the degree to which climate change affects birds is not as clear. Two previous studies have found the upward movement of many species of birds in the Monteverde area (Donnelly 1998, Palm 2003). I surveyed birds in four altitudinal zones on the less studied Atlantic slope of the Monteverde Cloud Forest Preserve and compared my findings to the bird distributions reported by Michael Fogden in 1993. I found significant upward movement of bird species (sign test, p < 0.05) as well as a general decline in the abundances of certain bird species (x2 = 27.65, df = 3, p < 0.001). The majority of these declines are probably linked to the reforestation of the Peas Blancas valley but climate change is likely responsible for the upslope movement and increased abundance of higher elevation birds.
El impacto del cambio climtico en los ecosistemas ha sido demostrado por varios estudios en anfibios que habitan en las zonas altas (Pounds, 1997). A pesar de esto, el hecho que el cambio climtico afecte de alguna manera a las aves, no esta claro. Dos estudios anteriores han encontrado que muchas especies de aves de las zonas bajas estn incrementando sus poblaciones en Monteverde (Donnelly, 1998 y Palm, 2003). Examine 4 zonas altitudinalmente diferentes en el lado Atlntico de la Reserva del Bosque Nuboso de Monteverde y compare mis resultados con las distribuciones de especies de aves reportadas por Michael Fogden en 1993.
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