By Casey Jones
One of the sweetest spring songs that I am always excited to hear is that of the Cerulean Warbler (Dendroica cerulea; hereafter, “cerulean”). The buzz and ascending trill has, in fact, made me leap for joy during last year’s weekly surveys that were being conducted for the Middle Eel River Watershed Initiative. As I see it, the excitement was appropriate since it was the only time our group heard a cerulean during that month-long survey of more than 100 square miles. The reason for their uncommon status is the same as it has been for most species in peril — habitat loss at the hands of humans.
Though I began with an auditory anecdote, ceruleans are equally easy on the eyes — that is, if you can manage to spot them in the mid- to high-canopy. Male ceruleans have a dark collar and streaking on its breast sides. Males and their more drab female counterparts both have an eye stripe and bold wing bars. However, it is the blue pigmentation of the male’s dorsal side that gives this bird its namesake. The color, cerulean, seems to naturally be reserved by the sky and those winged creatures that took to it millions of years ago. A Google search lists only birds with naturally occurring cerulean coloration; and it is not surprising that the United States Air Force has also used this color in their regalia.
Identifying a cerulean is more easily accomplished by listening for their song — a three-parted tzeedl tzeedl tzeedl zizizi tzeeee — ascending in tempo with a more slight increase in pitch. It is somewhat similar to the song of a Northern Parula (Setophaga americana), except cerulean song is not slurred. And the cerulean’s song may be confused with a Blackburnian Warbler (Dendroica fusca), except the cerulean’s song has more of a buzzy quality.
Ceruleans are neo-tropical migrants that nest in Indiana and throughout eastern deciduous forests in riparian zones. The nest is an open cup of bark fibers, grass stems, and hair bound together with spider silk, placed on a lateral limb of a deciduous tree in the mid- to upper-canopy. The nest is usually concealed from above by leaves or twigs on the nest branch2. If a female must abandon a nest to build a new one, she will leave behind the fibrous materials but salvage the spider web for her new nest
Some of the most interesting facts about ceruleans concern their nesting behavior. As a safety precaution while on a nest, the female will perform a peculiar tumbling act when leaving the nest3. She falls from the nest with her wings tucked in, only to open her wings near the forest floor; perhaps an attempt to disguise herself as a falling leaf. This technique would certainly make it difficult for any predators to determine frequent activity to and from a nesting location.
Another interesting fact about nesting behavior of ceruleans is that they practice kleptoparisitism — meaning that they will steal bits and pieces from nests of other birds that use similar materials. Ceruleans, Black-throated Green Warblers (Setophaga virens) and Red-eyed Vireos (Vireo olivaceus) all use similar nesting materials and will commonly steal from each other’s nests. Blue-gray Gnatcatchers (Polioptila caerulea) and American Redstarts (Setophaga ruticilla) have been the most common thieves observed2.
Obviously, nesting is not as easy as the House Sparrows (Passer domesticus) nesting in eaves and gutters might lead one to believe. Unlike this aforementioned generalist, the process of building and maintaining a nest can be rather cumbersome and consume a great deal of time and energy for ceruleans. And it can be even more complicated when the particular habitat the cerulean depends upon is being degraded at a record pace.
Cerulean populations, over the last 40 years, have declined quicker than any other warbler species in the United States3. Ceruleans have been reported to have a directly positive relationship between their abundance and the tree diameter4. Mature hardwood forests in both the breeding and wintering grounds are being paved, plowed and pastured. If the stratification of the canopy becomes homogeneous in those few remaining forest fragments, it is safe to assume that biodiversity will also suffer. Unfortunately, we are not all privileged enough to maintain a healthy forest stand on our property but, that is why these next people are working extra hard.
Two notable groups that are directly focused on monitoring cerulean populations throughout their range are The Cerulean Warbler Atlas Project (CEWAP) and the Cerulean Warbler Technical Group (CWTG). They have prepared many resources and much of this work is available online. Google it. A third institution that is dedicated to monitoring Indiana’s cerulean population is Ball State University — particularly Kamal Islam and Kirk Roth.
On the sunny side, there is a slight degree of optimism to be had. Evidence suggests that the central point of their breeding population is drifting northeast1. We hope that the cerulean population will not be left a depleted northeastern Indiana landscape when that time comes. Experience the cerulean firsthand — study the song and join us on our next May count (May 12th). The Tippecanoe Audubon Society has participated in the spring survey since 1976 and has amassed a great deal of local data to speculate about those ceruleans that take up summer residency in our own region.
1Hamel PB, Dawson DK, Keyser PD. 2004. How we can learn more about the Cerulean Warbler (Dendroica cerulea). Auk 121(1):7-14.
2Jones KC, Roth KL, Islam K, Hamel PB, Smith CG. 2007. Incidence of nest material kleptoparasitism involving Cerulean Warblers. The Wilson Journal of Ornithology 119(2):271-5.
3Mitch Waite Group. IBird Pro. Computer software. Apple App Store. Vers. 5.1. 2006. Web. 10 Mar. 2012.
4Rosenberg et al. 2000. An atlas of cerulean warbler populations Final report to USFWS 1997-2000 breeding seasons. Accessed January 20, 2011. Downloaded from Cerulean Warbler atlas project homepage. http://www.birds.cornell.edu/cewap/