Cerulean Warbler

(Setophaga cerulea)

Status: Threatened (ESA 2011), Endangered (SARA 2005)

Table of Contents:

Species Identification
Diet
Habitat and Range
Biology and Behaviour
Similar Species
Threats/Reasons for Being Endangered
Conservation and Recovery Strategies
How You Can Help

A male Cerulean Warbler

Species Identification:

The Cerulean Warbler is a small warbler, weighing only about 8-10 grams. They have long wings and a short tail. Adult males have deep blue colouring on top, with a distinct blue-black band across their throat. Females have light blue-green colouring on top, yellow-white eyebrows, and a whitish underbelly that has a yellow tint to it. They are more likely to be heard than seen- their call is buzzy and rapid, and ends a high note.

Diet:

They eat a range of insects from the upper canopy of forests, hopping from branches to pick their meal off of the leaves. They feed mainly on insects during breeding season, and feed their young primarily butterfly larvae. Their diet includes flies, beetles, weevils, and caterpillars. During migration they will forage in groups with other birds, and they have been known to eat plant matter in the winter to supplement their diet.

A female Cerulean Warbler. Picture by: Always a birder! https://flic.kr/p/UFYYje

Image taken from: https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/cosewic-assessments-status-reports/cerulean-warbler-2010.html

Habitat and Range:

The Cerulean Warbler breeds within the northeastern United States including the Appalachian Mountains and the Great Lakes region, extending up to southern Ontario and Quebec. The two main concentrations of Canadian populations are in the Carolinian region of southwestern Ontario and the Frontenac Axis, on the eastern edge of The Land Between. There are also scattered populations around the Trent-Severn waterway. An estimated 4% of their global breeding territory is within Canada, and about 1,000 individuals breed here. Overwintering territory is in the northwestern parts of South America, especially in the northern Andes Mountains.

Ideal habitat in their breeding range is large patches of undisturbed deciduous forests with tall, well spaced trees and sparse undergrowth. They are especially drawn to forests that are near wetlands, but may also be found in upland areas.

Biology and Behaviour:

The males arrive at the breeding territory by mid-May, and the females will arrive a week or two later. They often return to the same nest site, and will nest in loose colonies. The female Cerulean Warbler builds her nest high in a deciduous tree, using materials such as bark strips, grass, lichen, fur, moss and spider webs. She will lay 2-5 eggs, incubate them for 11-12 days, and then nestlings will be ready to fly 10 days after hatching, which usually starts in July. Cerulean Warblers can be aggressive towards other song birds, especially during the breeding season.

When migration begins, Cerulean Warblers have a long journey south. It takes them about four months to fly from Ontario to the Andes Mountains. While the spring journey north only takes about two months, their long migration periods make them more susceptible to exposure.

Similar Species:

Black-throated Blue Warblers are similar but have black extending across their eyes, cheeks, and throat, and neither sex has double wing bars
The Blackpoll Warbler, Bay-breasted Warbler, and Pine Warbler look like the female Cerulean, but they have more yellow than green along their backs

Threats/ Reasons for being at Risk:

1. Forest fragmentation in breeding grounds: This is mainly due to agricultural expansion and intensive logging. Populations of Cerulean Warblers were much higher before European settlement in North America. Because of this, we have lost >80% of the original hardwood forests in Ontario. Forest fragmentation also exacerbates the effects of predators and brood parasites, since the Cerulean Warbler is generally safer in large tracts of forest where they can remain hidden.

2. Forest loss and fragmentation in wintering grounds: The forests in the Northern Andes where Cerulean Warblers overwinter are being harvested for fuel, and the land is being converted into fields and pastures for agricultural production The main related industries are coffee, coca, tea, and rice.

3. Climate change: There has been an increase in both cold snaps and extreme weather events over the years, which can have an impact on breeding and nesting if Cerulean Warblers migrate too early in the spring. Their nests are also located high up in trees on the ends of branches, where extreme storms can knock them off. These unpredictable weather events are especially dangerous during the Cerulean Warbler’s long migrations.

4. Brood parasites: The Brown-Headed Cowbird will lay their eggs in the nests of other species to avoid parenting duties. Brown-Headed Cowbirds often destroy the host bird’s eggs before replacing them with their own, and if not the offspring will generally bully the Cerulean Warbler’s offspring into starvation. This threat is also made worse by forest fragmentation, making it easier for the Brown-Headed Cowbird and other parasites to find Cerulean Warbler nests.

Photo by: US Fish & Wildlife Services https://flic.kr/p/tEYSQS

Conservation and recovery strategies:

Habitat suitability maps have been created for the Thousand Islands Ecosystem to ensure that Cerulean Warblers have the proper breeding and nesting grounds
Several surveys have taken place in Ontario Parks across the province in order to get an idea of where our Cerulean Warbler populations are able to thrive
Ontario has developed Best Management Practices for forestry operations, giving specific practices to improve habitat for Cerulean Warblers. However, these practices are optional for forest managers

How can you help?

Buy Fair Trade, sustainable, shade-grown coffee to help protect the Andean forests from development
Leave tall trees and intact forest habitats on your property, with plenty of large, old growth trees
Tell your local government officials to better enforce laws and bylaws
Take steps to make your windows visible to birds using window decals or blinds

Quick Facts:

Cerulean Warblers will re-nest if the first one fails, and they will often recycle the materials from their old nest, especially the coveted spider webs
This bird is very hard to study because they live high up in the tree canopy and are often hard to spot
Female Cerulean Warblers have a unique way of leaving their nest. They will often “bungee jump” off the side, only spreading their wings once they have passed the nest

Additional Resources:

Cornell Lab of Ornithology page.
Bird Life International blog.
Colombia’s Cerulean Warbler Bird Reserve.

Photo by: Julio Mulero https://flic.kr/p/2kXs6ny

Sources:

Cadman, M.D, D.A. Sutherland, G.G. Beck, D. Lepage, and A.R. Couturier (eds.). 2007. Atlas of the Breeding Birds of Ontario, 2001–2005. Bird Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature, Toronto, xxii + 706 pp.

COSEWIC. 2010. COSEWIC assessment and status report on the Cerulean WarblerDendroica cerulea in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. x + 40 pp.

COSSARO. 2011. https://www.ontario.ca/page/cerulean-warbler-evaluation

Environment Canada. 2011. Management Plan for the Cerulean Warbler (Dendroica cerulea) in Canada. Species at Risk Act Management Plan Series. Environment Canada, Ottawa. iii + 19 pp.