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

Natural History

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.

Ecology

Behavior

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.

Identification

Adult male

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.

Female

IMG-20190918-WA0005_edited_edited_edited

Olive-green backs with no streaking, but some individuals can have slightly blue-green rump and forehead.

Underparts are whitish with varying amounts of yellow tinge with the undertail coverts remaining white.

The streaking on their flanks tends to be olive and very faint.

Their wings are darker with two white wing bars.

They have an obvious pale supercilium and a thin, dark line running through the eye.

Adults have a completely dark bill and immatures have a bicolored bill with dark upper mandible and pale lower mandible.

One of the species that can be easily confused with Cerulean Warbler is Blackburnian Warbler (Setophaga fusca)

The overall color of the upperparts of immature/female Blackburnian Warblers is olive-brown, whereas the upperparts in immature/female CERWs is grayish-blue to greenish.

Blackburnians always show contrasting, dark auriculars whereas CERWs have a noticeable dark line running through the eye.

BLACKBURNIAN Warbler

cerulean Warbler

Conservation status

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.

In Canada

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.

A farmer is taking care of the rice patty

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.

References

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

A prime
stopover site

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.