The Cerulean Warbler
A pasage migrant in Costa Rica
Scientific name: Setophaga cerulea
(Previous Drendroica until 2011)
Family: Parulidae (Wood Warblers)
English name: Cerulean Warbler
Common names in Spanish: Reinita Cerúlea, Chipe Cerúleo, Reinita Cielo Azul, Bijirita Azulosa, Verdín azulado
Cook, G. (1812). A View on the River Schuylkill Near Philadelphia [Steel engraving], Philadelphia Museum of Art.
The description of the Cerulean Warbler is attributed to the Scottish poet and naturalist Alexander Wilson, the father of American ornithology, whose unique work in its time and nature served as a precedent and inspiration for those who continued to do science in this field.
The data shared on this site focuses on the Cerulean Warbler's behavior during its stopover in Costa Rica. Although this species has been studied more extensively on its breeding and wintering grounds, its natural history during stopover is poorly known.
Cerulean Warblers are very active and spend most of their time looking for their food in the foliage on the outer part of the branches.
They search for their prey on the underside of the leaves, oftentimes sallying out from another branch to pick the invertebrate from the leaf. This sallying behavior is actually a good clue to the presence of a CERW.
Unmistakable with their contrasting light-blue back and stark-white underparts, black streaks on the back and flanks and dark necklace.
The wings are darker with blue edging to the primaries and two white wing bars.
Their facial pattern can vary with some individuals showing a strong white supercilium or broad, black streaks on either side of the crown.
The bill on adults is completely black.
The Cerulean Warbler is a species of special conservation concern and considered Near Threatened by the International Union for Conservation of Nature (IUCN) because its population has decreased by more than 70% between 1970 and 2014. This steep population decline is attributed mainly to the loss, degradation and fragmentation of their habitat on both their breeding and wintering grounds, as well as stopover sites used during migration.
Agriculture as well as the logging industry have affected the habitat quality of the Cerulean Warbler and the greatest long-term challenge seems to be the lack of knowledge and studies about this warbler’s basic ecology and this is an obstacle for conservation efforts.
In the United States
Open-pit mining for coal extraction represents one of the most significant threats to habitat loss, especially in the Appalachian region. By 2001, almost 1000 km² were designated for mining in West Virginia (Wells, 2007), a state whose territory is like that of Costa Rica; this is equivalent to approximately half of Costa Rica's Greater Metropolitan Area. In addition to mining, deforestation, changes in land use in habitats of importance to warblers and nest parasitism by the Brown-headed Cowbird (Molothrus ater) also add to the list of threats.
In the wintering sites
In the tropics the threats to CERWs also involve the loss and fragmentation of habitat due to deforestation and the change in land use for livestock and agricultural activities such as intensive banana and pineapple plantations.
COSEWIC (2003). COSEWIC assessment and update status report on the Cerulean Warbler Dendroica cerulea in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa.
Wells, J. V. (2007). Cerulean Warbler (Dendroica cerulea). En Birder’s Conservation Handbook (329-332). Princeton University Press.
Las Brisas Nature Reserve
Las Brisas Nature Reserve has been identified as an important stopover site for Cerulean Warblers and other migratory species as well. It is located on the lower northeast slopes of the Turrialba Volcano and has two different sections, one at 250 m.a.s.l. and the other between 650 - 1030 m.a.s.l., overlooking the Caribbean Sea.
Part of the reserve is old-growth forest, but the majority of its area has been drastically transformed from poor pasture-land to a diverse forest through an exemplary reforestation project. It is here that we have found the greatest concentrations of CERW. A unique combination of factors -precipitation, temperature and maybe even topography- combine to make a favorable habitat preferred over other regions of Costa Rica.