Adult male cerulean warbler.
© Robert Royse

The Cerulean Warbler (Dendroica cerulea) is a Nearctic - Neotropical migrant songbird that breeds in the canopy of mature deciduous forests in eastern North America, and winters along the eastern slope of the Andes Mountains in South America.

This striking warbler has been a focus of conservation efforts during the past decade because of severe population declines (over 3% per year since 1966). So dramatic has been its decline that the United States Fish & Wildlife Service has been petitioned to list the Cerulean Warbler as Threatened Species under the Endangered Species Act; it has been designated as a “species of special concern” by the Canadian wildlife service.

Graph showing a 3% population decline per year

This graph shows a 3% decline per year. Number of warblers is depicted on the y axis, the year is on the x axis.

No one knows for sure why this bird has been quietly slipping away, but habitat loss on both its breeding and wintering grounds is certainly an important factor. Unfortunately, we have relatively little information about the ecology of this species because it spends much of its time high in the forest canopy.

Adult male cerulean warbler.
© Robert Royse

To better understand the causes of the Cerulean Warbler’s decline, researchers at Queen's University in Canada and at the Smithsonian Migratory Bird Center joined in analyzing a long-term dataset on survival rates and reproductive success collected at the Queen's University Biological Station (QUBS) in eastern Ontario. The Cerulean Warbler population at QUBS is thought to be one of the largest and healthiest known, and its numbers here have remained stable since at least the early 1990s.

Using a computer simulation model, the researchers found that the Cerulean Warbler population sampled at QUBS is actually a "sink" population, meaning it is not self-sustaining and must be receiving immigrants from other areas in order to persist over time.

Researchers have discovered that:

• Annual survival rates are lower than expected, with only 49% of birds surviving from one breeding season to the next.
• Detailed analyses of seasonal variation in survival rates suggest that events during migration or on wintering grounds are responsible for most adult mortality.
• Reproductive success is too low to compensate for the Cerulean Warbler's high mortality, with each warbler pair producing, on average, less than two young / year.

Given these grim survival and reproductive rates, the QUBS breeding population would become extinct in less than 50 years without immigration.

Adult male cerulean warbler.
© Robert Royse

Survival and reproductive rates of the elusive Cerulean Warbler are notoriously difficult to measure, so these results must be used with caution. However, this research demonstrates that Cerulean Warblers are threatened, even in relatively pristine breeding habitats. Further information on the ecology of this species, particularly on its winter grounds in South America, will enhance future conservation efforts.

The cerulean warbler photos on this page are copyrighted by Robert Royse and cannot be used for any purpose other than web viewing without prior permission. You can view more of his images at: www.roysephotos.com

This article summarizes the information in this scientific paper:

Jones, J., Barg, J. J., Sillett, Terence Scott, Veit, M. L. and Robertson, R. J. 2004. Minimum estimates of survival and population growth for Cerulean Warblers (Dendroica cerulea) breeding in Ontario, Canada. The Auk, 121: 15-22.

Download scientific paper