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The Current Status of the Cerulean Warbler on its Winter Range
View the entire report "The Current Status of the Cerulean Warbler on its Winter Range" (PDF 22 pages; 542KB)
Fundación ProAves and American Bird Conservancy (ABC) assisted the U.S Fish and Wildlife Service with an assessment of the current status of the Cerulean Warbler and its associated habitats in its wintering range. Project objectives included compilation of all available relevant information from South America and establishment of a database with all sources of information and contacts.
Major results include:
Available relevant literature in Spanish on Cerulean Warbler and its habitat was compiled and translated into English. Biologists from countries where the species has been recorded in South America (Colombia, Venezuela, Peru, and Bolivia) were asked to contribute additional written information and government documents. The contacts included representatives from governmental agencies and non-governmental organizations. Information gleaned in this manner was recorded in a database.
Working with the Cerulean Warbler Technical Group, Fundación ProAves and ABC compiled all historical and recent data on Cerulean Warbler in Latin America for the analysis presented in this document.
Distribution and Ecology on the Wintering Range
The Cerulean Warbler (Dendroica cerulea) is one of the most threatened (Vulnerable by IUCN 2006) Nearctic migrant passerines that winters in South America.
The Cerulean Warbler has been considered to range at low densities across large areas of the Northern Andes. However, our data analysis, predicted range modeling, and target follow-up research has revealed that the species is actually quite selective in its ecological preferences. It appears to prefer specific climatic conditions, such as cooler lower subtropical clines with moderate rainfall levels. These conditions are highly correlated with coffee production areas of the Northern Andes, particularly the intermontane valleys of central Colombia, source of more than 50% of all non-breeding records.
For over a century, these intermontane valleys in Colombia have been intensively farmed. Little or no intact native forest remains, and what does remain is unprotected. Traditionally, the dominant agriculture in this region has been Arabica coffee grown as an understory crop under agro-forestry systems using a diversity of canopy tree species to create shade conditions, These shade coffee conditions have suited wintering Cerulean Warblers. Unfortunately, however, shade coffee agro-forestry practices currently are being rapidly converted to higher-yielding sun-coffee varieties following a market collapse in retail prices and national policy aimed at maximizing production. While no comprehensive figures exist on shade to sun conversion rates, one key Cerulean Warbler stronghold has lost over 80% of its shade canopy cover. It is our conclusion that the lack of native forest in valleys preferred by Cerulean Warblers and the rapid pace of changing agricultural practices place the species at increasing risk on its wintering grounds in the northern Andes.
Winter Range and Habitat Requirements
Throughout its entire range in South America, Cerulean Warbler is an uncommon winter resident (October-February) of northern Andean forests. This habitat consists of tall forest canopy and forest borders, particularly in broadleaf evergreen forests and in shade coffee plantations in the lower subtropics and premontane forest of the Andes from Colombia to Peru, possibly to Bolivia, and, at least hypothetically (based on a single record), in the tepui region of Venezuela. The species typically occurs at elevations from 500-1500 m, reaching higher in the northern portion of the range than in the south (Hilty and Brown 1986, Robbins et. al 1992, Jones et. al 1992, Hamel 2000).
Cerulean Warbler is a canopy and subcanopy dweller with insectivorous feeding habits and behavior (Colorado et. al 2006). The species uses primary forests as well as mature shade coffee and other agroforestry crops like cacao and cardamom, especially in areas where the remnant forest is scarce or absent in its restricted elevation range (Colorado & Cuadros 2004, Hamel & Jones 2005, Moreno et. al 2006). Intensive surveys have shown that the species is not equally distributed across its potential range (Moreno et. al 2006). The probability of detecting a larger number of individuals is correlated with the amount of available habitat and its quality (type, structure, and area) within specific conditions of elevation, temperature (19-23 ºC), and precipitation (Moreno et. al 2006). In its preferred habitat, it is possible to find Cerulean Warbler with reasonable regularity (Moreno et. al 2006). In dense forest habitat the birds may be overlooked because they are mostly silent and it is more difficult to observe them here than in a shaded plantation. In a few core areas the Cerulean Warbler showed intra- and inter-seasonal site fidelity to a particular wintering location (Colorado & Cuadros 2004, Moreno et. al 2006).
Cerulean Warbler is usually a member of mixed species flocks, although it is not restricted to this behavior. It exhibits few interactions with other flock members (Colorado et. al 2006). Some ecological studies suggest that it is more sedentary and solitary during the rainy season (October – November) when there is a greater abundance of food available and more mobile and gregarious later (January – March) when available food resources diminish and the birds have to move and join flocks to increase foraging success (Ramoni-Perazzi 2005).
Non-breeding Season Distribution Models
Grupo Cerúleo members of the Cerulean Warbler Technical Group (including ProAves and ABC) have studied Cerulean Warbler winter distribution and habitat records since 2003. The winter range of Cerulean Warbler includes the Northern Andes in Colombia, Venezuela, Ecuador, and northern Peru. There are also a few records from southern Peru and northernmost Bolivia. The species occurs primarily at middle elevations on humid forested slopes of the Andes (Figure 1), with 79.4% of 220 records between 800 and 1600 meters (median 1,350 m). The species is absent from the wettest and driest slopes (e.g., Chocó and Tumbesian). This elevation range is highly correlated with the principal areas of coffee production.
We used Cerulean Warbler records from October to February and MAXENT 2.2 (Phillips et al. 2006) to develop a climatic model of the potential non-breeding distribution of the species. Data layers with a resolution of 1 km2 came from the Grupo Cerúleo CERW Latin America Database. We created 10 bioclimatic layers of temperature and precipitation (WorldClim—http://www.worldclim.org) for the winter period and combined these with a discrete vegetation cover layer (GlobalVegetationMonitoringUnit—http://www-gvm.jrc.it/glc2000/) for the model. The program was set to use the automatic features option and default regularization values (Phillips et al. 2006). We call the resulting modeled potential distribution of Cerulean Warbler the “original” distribution. We then clipped the original non-breeding distribution with a vegetation layer of current forest within the study area to create a model of “current” distribution. Model performance was evaluated using receiver operating characteristic (AUC) curves (Fielding & Bell 1997).
Winter sites at which Cerulean Warblers have been recorded most frequently were characterized by warm temperatures (winter mean temperature), low seasonality, and intermediate mean diurnal and annual temperature ranges (Figure 2) and precipitation at the lower end of the precipitation range (Figure 3) with intermediate seasonality. Broadly, these variables coincide with conditions that the species finds in its breeding range. With a high AUC value (0.970±0.010), the model was a successful predictor of the presence and absence of Ceruleans (Figure 4). The most important predictors for its winter distribution were the minimum temperature of the coldest month and seasonal precipitation.
The climatic model for Cerulean Warbler winter distribution predicted presence of the species across the northern Andes. However, it also predicted occurrence in areas outside its known winter range, including an area from southern Mexico to northern Panama, the tepuis of Venezuela, and the Caribbean from eastern Cuba, Jamaica, Haiti and Dominican Republic throughout the Lesser Antilles. These out-of-range predictions comprise commission errors (over-prediction). For our subsequent analyses, we restricted the area of study to the countries of the Northern Andes ecoregion.
Changes in Distribution
Comparison of the “original” (potential range inferred from climatic variables of record locations) and “current” (portion of the original range on which appropriate vegetation still occurs) model distributions suggests a loss of nearly 60% of suitable Cerulean Warbler habitat in the Northern Andes (Table 1, Figure 5). GIS analysis comparing the “original” and “current” coverages revealed an increase in fragmentation across the species range: number of patches more than doubled (increased by 117%), total edge increased by 29%, and edge density more than tripled (increased by 229%) while mean patch size decreased by 81.5% (Table 1). Eighty-five percent of the species’ modeled current distribution occurs in the remaining dense evergreen and closed-canopy evergreen tropical forests (Table 2).
We feel that the modeled distribution is actually far too general, however, and fails to take into account Cerulean Warbler’s specific ecological preferences. Source data may also have included sporadic, occasional, and passage records rather than only records from core wintering locations. Those of us with extensive experience with Cerulean Warbler in the field believe that it is more likely that less than 10% of its original intact natural forest and coffee agro-forestry habitats remain and that these remaining areas are being rapidly lost to habitat conversion. In addition, there are currently no state protected areas in the core of the wintering range of the species.
Present and Potential Winter Habitat Loss
The original vegetation cover in the Northern Andes has been drastically transformed. Documentation of landscape transformation varies among countries, but there is general agreement on the causes behind the transformation, including colonization/human settlement, infrastructure and development projects, mine and/or petroleum exploitation, and wood cutting for fuel. There are few figures that accurately reflect annual Andean deforestation rates, in part because existing analyses have been performed at different levels of detail and under different vegetation classification systems and therefore are not comparable. The most reliable estimates come from Venezuela, Ecuador, and Colombia and range from 200 to 600 thousand hectares per year in recent time periods. In general, reforestation does not keep pace with deforestation.
Fires are the most important natural cause of transformation. Lack of appropriate management plans, inability to influence management on non-public land, and lack of capacity to control the boundaries of protected areas have resulted in unsustainable use of natural resources (FAO.2006, FAO & INRENA 2005, FAO & IDEAM 2005, Rodríguez 2005, Rodríguez et al 2004, IDEAM 2001, FAO 1999).
In the Colombian Andes, only 9.6% of the 2,763,523 ha of original forest within the 1500–2000 m elevation range remains (Rodriguez et. al 2004). Approximately 870.000 hectares between 800–2000 m on the three Andean cordilleras are currently devoted to coffee production on coffee farms averaging 2 ha in size.
The valley of the Cauca River between the Central and Western cordilleras is Colombia’s main coffee region. In 2000, only 26% of this entire coffee region comprised natural ecosystems, and much of that natural area was forest above 2,000 m in elevation. Traditional agriculture in the Colombian Andes has transformed mountain ecosystems into rural landscapes. In most of this area, elements of agricultural landscapes are the only alternative for biodiversity conservation, as there are very few fragments of natural ecosystems remaining (Armenteras et. al 2005).
In the inter-Andean region of Ecuador, only 7% of original forest cover (805,000 ha) survived in 1999. The forest cover in the Sierra region of Peru has been reduced by 32% in the last 20 years (FAO & INERENA 2005). In Venezuela, a great percentage of remnant forests are restricted to regions with pronounced slopes since these slopes were a physical obstacle to transformation. Otherwise, flat and moderately sloped areas were all transformed. This pattern results in a drastic decrease in habitat heterogeneity that is evident throughout the Andean system (Rodríguez 2005).
Very little of the original forest preferred by Cerulean Warbler survives intact, and many current records occur within agro-forestry systems (human-altered landscapes) that are rapidly undergoing conversion to more intense non-canopy production unsuitable for the species. Since the species’ preferred wintering areas traditionally have been the most sought-after agricultural farmland areas of the Andes (coffee production, livestock, etc), those areas are almost entirely unprotected by state or regional reserves.
The growing level of awareness and worldwide interest in generating forest protection mechanisms is encouraging. However, current land protection is largely restricted to areas of low commercial value and wilderness areas. The particular requirements of the Cerulean Warbler suggest alternative approaches:
The modeled distribution of the Cerulean Warbler suggests that as much as 15% of its potential range falls within protected areas (Appendix 1), most of which are in Colombia and Venezuela (Figure 1). However, surveys and records do not indicate occurrence of the species within those areas (despite the fact that surveys are usually biased toward protected areas). Hence the suggestion that 15% of Cerulean habitat is under state protection is misleading. In addition, we have no evidence that there is anything resembling a population stronghold within any existing state protected area.
The species is predicted to occur in 47 Important Bird Area (IBA) and 6 Alliance for Zero Extinction (AZE, www.zeroextinction.org) sites, although many of those sites remain unprotected or inadequately protected (Appendix 2).
A partial version of the “Forest Law for Natural and Protected Areas” is being applied in Ecuador. This law still contains various weaknesses in need of improvement (Zúñiga 1999). In Venezuela, wood extraction is controlled by the government and requires a special permit on both private and public land. Wood extraction from natural reserves is based on management plans that are given to managers through long-term agreements (Ministerio del Ambiente y de los Recursos Naturales Renovables 1999). In Colombia, various plans, programs, and laws address protection of forest resources, ranging from the “National Plan for Forest Development,” the “Management Plans for Protected Areas,” the “Watershed Recovery,” and a “Green Plan” for restoration and establishment of forests.
Achievement of sustainable development remains one of the greatest challenges in tropical countries. Recent increases in conservation activities in South America suggest that new generations of conservationists are maturing with sustainability in mind. The extent of protected areas in South America has increased (FAO 2006). Initiatives such as those of Fundación ProAves Colombia in establishing the first protected area for a neotropical migrant in Latin America—the Cerulean Warbler Natural Bird Reserve—need to be expanded and replicated in other countries. There is a close relationship between wintering migrants like Cerulean Warbler and a large portion of the endangered resident biodiversity. Likewise, there should be continued support and promotion of agro-forestry initiatives (such as supporting consumption of “bird friendly” shaded coffee) that benefit preservation of migrant and resident species.
View the entire report "The Current Status of the Cerulean Warbler on its Winter Range" (PDF 22 pages; 542KB)