Chapter 6 Birds

6 Birds — Update 2014 Bruce E. Young Although the pace of research on birds in following summarizes the major findings of the Monteverde may have slowed during the 14 27 publications that encompass this new years since the last synopsis (Young and research. McDonald 2000), researchers continue to make important contributions to tropical avian biology Elevational Gradients, Migration, and and other scientific disciplines. Elevational Conservation gradients and migrations of birds along them New research has shed light on the still inspire research in Monteverde, where field remarkable biotic turnovers associated with study at varying elevations is facilitated by the elevational gradients that characterize steep topography and easy access. These Monteverde and many tropical mountain slopes. gradients are also useful for studying biotic A new statistical analysis revealed that the beta responses to climate change, a growing threat to diversity (the increase in species richness across biodiversity, especially in the tropics (Colwell et habitats) of birds on the Pacific slope of al. 2008). Ornithologists completed two new Monteverde is twice as high as that at the same autecological studies and finished two long-term elevations on temperate mountains (Jankowski studies initiated in the 20th century, continuing a et al. 2009). Moisture gradients, rather than long and valuable tradition in Monteverde. Other elevation per se, best explained the rapid species scientists tackled new research topics in bird- turnover. The Caribbean slope of Monteverde, army ant interactions and behavior. Finally, with a less dramatic moisture gradient, has careful observation continues to extend our substantially lower beta diversity than that of the knowledge of natural history, such of the first Pacific slope, where a rain shadow accentuates nest description for Silvery-fronted Tapaculo moisture differences. One mechanism leading to (Scytalopus argentifrons; Young and Zuchowski narrow ranges of Monteverde birds appears to be 2003) and an observation of a Slaty-backed behavior. A series of song playback experiments Nightingale Thrush (Catharus frantzii ) foraging demonstrated that congeneric species that on a frog (Acosta and Morún 2014). The replace each other along elevational gradients show interspecific territorial behavior toward four-part migration that spans 200 km (Powell one other (Jankowski et al. 2010). and Bjork 2004). Radio-collared individuals How does abundance correlate with range migrated from Monteverde to the Caribbean size in birds occupying Monteverde’s different lowlands of northwestern Costa Rica and elevational zones? An extensive analysis using southeastern Nicaragua during the period of both mist-net and point count data revealed that September to December, then flew to species occupying fewer sites also tend to be southeastern Costa Rica where they remained less abundant in those sites than those occupying until March. Then they migrated to their more sites (Jankowsli and Rabenold 2007). At a breeding area at 1000-1800 m on the Caribbean broader scale, species with larger range sizes slope of Monteverde. Finally, in June and July, occupied more habitat zones in Monteverde than the birds migrated back across the continental those with smaller range sizes. Species endemic divide to mid-elevation Pacific slopes in to Costa Rica and western Panama tended to Monteverde, where they remained until occur at the highest elevations and have lower September. The migrations of bellbirds and abundance. These results highlight the umbrellabirds highlight pressing conservation conservation challenges posed by this and other needs. Although the breeding areas of both are endemic highland avifaunas: most species are well protected in Monteverde, each spends relatively rare at both geographical and local significant periods of time in unprotected scales. habitats. Umbrellabirds descend to the lowest Seasonal migration across elevational elevation where intact forest remains on the gradients continues to attract scientific attention. Caribbean slope of Monteverde. Loss of lowland A study of eight large frugivorous bird species, habitat may be a limitation for the Monteverde including guans, trogons, toucans, and cotingas, population as this species is known to occur at confirmed the results of earlier studies on much lower elevations elsewhere in Costa Rica seasonal movements (Chaves-Campos 2004). where lowland forests persist. Bellbirds are also The birds generally nest at higher elevations and vulnerable because they spend at least half of move to lower elevations when not breeding, their annual cycle on unprotected lands. although some individuals remain at high elevations year round. This pattern may be Climate Change partially explained by seasonal changes in fruit Ongoing climate change threatens species abundance and precipitation, but additional and ecosystems worldwide. Sensitivity of factors may also be involved. A separate study Monteverde habitats to climate and dramatic of vegetation seasonality did not detect a link climate differences over small spatial scales between vegetation changes and the migration of combine to make the area ideal for studying the one of the species that Chaves-Campos studied, effects of climate change on natural systems. the Bare-necked Umbrellabird (Cephalopterus Careful monitoring of the breeding birds in a glabricollis; Papes et al. 2012) small study plot near the entrance of the Radio-tracking of two of these large, migrant Monteverde Cloud Forest Preserve revealed that frugivores, the Bare-necked Umbrellabird and lower elevation species were colonizing and that Three-wattled Bellbird (Procnias higher elevation species were declining (Pounds tricarunculatus), revealed more details about the et al. 1999). Annual colonizations were closely annual movements of the two species. These tied to the annual number of precipitation-free species are important from a conservation days, which result from increasing temperatures standpoint because both are categorized as that raise cloud-bank levels. threatened on the IUCN Red List of Threatened A subsequent study modeled the effects of Species. The Bare-necked Umbrellabird appears future climates on the community of montane to make a straightforward migration between birds. Using data on current population sizes, breeding areas at 800-1400 m, and lower fine spatial-scale models predicted the decline of elevations (600-800 m) during the nonbreeding nearly half of 77 species currently occurring in season (Chaves-Campos et al. 2003). The the higher elevation forests of Monteverde bellbird, on the other hand, makes a spectacular during the next 100 years (Gasner et al. 2010). More sobering was the finding that seven of the Rosa National Park, 115 km away in the Pacific eight species predicted to become extirpated in lowlands (McDonald 2003). Although gene flow the region already have small ranges restricted to appears to be greater up from Santa Rosa to Costa Rican and western Panamanian highlands. Monteverde than in the reverse direction, the intense sexual selection on this lek-breeding Birds and Army Ants species appears to be insufficient to create A new area of study for Monteverde is the significant genetic isolation to lead to speciation. interaction between birds and army ants. The Brown Jays are known for their cooperative bird-army ant relationship is well known in breeding system in which multiple females and tropical lowland forests, where several specialist males contribute to the rearing of young. Use of species forage on insects and other invertebrates multi-locus DNA fingerprinting has shown that as they flee advancing army ant foraging raids. virtually all chicks in a nest are the offspring of Recent observations in Monteverde reveal that a a single primary female breeder that is able to surprising diversity of montane birds also follow suppress breeding by other females in her group army ants when they swarm. During two months (Williams 2004). Males have much less skewed of opportunistic observations at 1200-1650 m breeding success, with multiple paternity elevation, researchers identified 41 bird species occurring in 33-40% of nests, and extra-group attending army ant raids (Kumar and O’Donnell paternity occurring in at least 20% of nests. 2007). Neither the diversity nor the abundance Group size is important in determining the of ant-following birds was influenced by breeding success of a group. Larger groups are elevation, but both factors were affected by better able to defend territories with isolated forest patch size. More bird species and more nesting trees, which are key to preventing nest individuals followed army ants in continuous predation, the leading cause of nest failure forest than in small forest fragments. More (Williams and Hale 2006). Increased nesting extensive observations showed that resident success and post-fledging survival were also species are more likely to follow raids than long- correlated with group size. The benefit to non- distance migrants, and eight resident species reproductive helpers in a group appears to be regularly visit army ant bivouacs to determine access to future breeding opportunities. when raids begin (O’Donnell et al. 2010). Observation of marked individuals showed that Although no montane species rely on army ants breeding females and their social mates maintain as extensively as some lowland specialists, their bond across years and have higher nest attending army ant raids is an important

Load more