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Migratory species have long captivated scientific communities. What mechanisms enable these animals to travel such extreme distances and the mystery behind which routes they take have been studied across a range of creatures, from the blue whale to the monarch butterfly. More recently, however, migration studies have turned their attention to the ecological relevance of migrants. Given their unique potential to link distantly separated communities, migratory species such as the White-crested Elaenia facilitate a greater degree of ecological resilience in the biomes they interact with. The longest migratory bird within the Southern Neotropical system, the Elaenia makes its way from Patagonian breeding grounds in the South to Brazil’s rainforests in the more northern tropical latitudes. The amount of fruit-eating species within Patagonian species is low, so that biological interactions in these ecosystems depend on very few species, which includes the Elaenia. Since elenias are so populous in the Patagonian forests during summer fruiting periods, they are massive contributors to the health of Patagonian forests. Serving as the main seed disperser, they are necessary for the regeneration of these forests, especially in light of recent fires. In order to better understand the migration patterns and ecological relevance of elaenias, Susana Bravo, Victor Cueto, and Cristian Andres Gorosito conducted a study on the unique bird. The objective of Bravo and her colleagues was to determine the annual cycle of the White-crested Elaenia, specifically, whether individuals use different migratory routes and wintering areas or the same ones, having come from the same breeding population.
Field work was conducted over the course of three breeding seasons from 2013 to 2016 at the “Cañadón Florido” cattle ranch in Argentina. Three methods of capture were employed. The first utilized nets where birds could be captured passively, while the second and third involved actively searching for elaeis that were exhibiting territorial displays or residing in nests, followed by capture with a species-specific model and vocalizations made by a portable speaker. 45 adults were fitted with a light-level geolocator using a leg-loop backpack harness; a to relatively light load that amounted to less than four percent of the birds’ body mass. The geolocators determined the birds’s relative location by measuring light intensity and calculating times of sunrise and sunset as well as sun elevation angle and position. To positions a day were estimated, and movement and stationary periods were distinguished. Latitudinal estimates near the equinoxes were rejected; thus the routes of each individual were evaluated using longitude information.
Of the birds fitted with geolocator and harness, 33% were recaptured, representing 15 of the 45 birds deployed in total. Interestingly, only one geolocator was returned of those dispersed among the females, thus the results of this study are predominately limited to male elaenias. Upon surveying the data, researchers concluded that all individuals passed through the same general regions, and no birds crossed the Equator or traveled to western South America (i.e., Peru, Ecuador, or Columbia). Three main fall migration routes were described following the elaenia’s departure from the Patagonian Forest heading northward. Five birds traveled over the Yungas forest, then crossed Paraguay and southern Brazil, arriving near Rio de Janeiro (called the Yungas Route). Another five followed a diagonal path, traversing the Patagonian desert and Espinal biomes in an easterly fashion before flying north along the South American coast (appropriately dubbed the Coastal Route). The final five birds followed the Paraná and Paraguay Rivers northwards until Misiones Province in Argentina when they turned east and crossed southern Brazil to Rio de Janeiro (the Parana-Paraguay River Route). The average migration rate of the eleanias was 175.0 to 246.5 km/day! Two wintering areas in eastern and central Brazil were used, and during spring migration, all elaenias studied returned to the breeding area in Argentina along the same route. Over the course of the year, these birds traveled around 9800 kilometers.
One prominent observation on eleania migratory tactics was the use of successive wintering areas where elaenais resided for close to three months. This phenomenon may be driven by climate or resource availability, since the most abundant plant species of the forest understory have ripe fruits during the dry seasons in the Atlantic Forest and Cerrado when these birds make their layover. The shrubs and small trees that produce these fruits aid in the regeneration of forest ecosystems. High quantities of these fruits ensures that eleanis are provided the nutrition they seek while the forest is rewarded by seed dispersal.
Though it can be said that the migration rates of White-crested Elaenias are among the fastest studied, different fall migration strategies were used among individuals that complicate matters somewhat. While some birds fly between 500-600 km/day over deserts or grassland, opting for the more direct route, others choose to fly through areas with potentially better options for refueling, limiting their strides to 100-200 km/day. Strong tailwinds known as the “Pampero” would aid birds crossing central Argentina; however, at this point, some birds may choose to follow the grasslands along the coast whereas others fly through humid forests along the Paraná River, where fruit is plentiful.The spring migration route frome the Cerrado/Pantanal region in the north to the Patagonian Forest in the south was consistent among all individuals. For a few reasons, only the Yungas route was utilized: aside from being the shortest route to Patagonia’s breeding grounds, elaenias could additionally avoid head winds and likely refuel.
In the Patagonian forest region that elaenias frequent, human activity has caused a high increase in fire frequency, and maintaining an abundant elaenia population is crucial for forest regeneration. Though a 50% reduction in elaenias would not threaten the existence of the species itself, given the fact that an average of eight individuals occupy every hectare, it could have a negative impact on forest regeneration capacity. The Yungas forest biome of northern Argentina that this species passes through during fall and spring migration has recently experienced severe habitat deforestation, driven by expansion of Soybean plantations, mining, and cattle production. The eleania’s overwintering site in the Atlantic Forest is another highly-threatened biome. The presence of this bird and its associated seed-dispersing role provides hope that these ecosystems will be able to recover.
Source:
Bravo, S. P., Cueto, V. R., & Cristian Andrés Gorosito. (2017). Migratory timing, rate, routes and wintering areas of white-crested elaenia (elaenia albiceps chilensis), a key seed disperser for patagonian forest regeneration. PLoS One, 12(2) doi:http://dx.doi.org/10.1371/journal.pone.0170188
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