Warblers

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Warblers

Warblers are small, perching song birds with a large number of species distributed throughout the world. There are two families of warblers, one in the New World and one in the Old World. The New World warblers (family Parulidae) comprise 126 species that occur throughout the Americas. The Old World warblers (family Sylviidae) occur in Eurasia, Africa, and Australia, and include some 350 species. Although the warblers in these two families are not closely related, having evolved from different ancestral stocks, they are of rather similar general appearance, and many species in both families are accomplished singers. These are the main reasons for their shared common name.

In addition, birds in both families of warblers are very active hunters of insects, spiders, and other small invertebrates. Most species of warblers are tropical or subtropical. However, many migratory species of warblers breed in forested and shrubby habitats at higher latitudes, and spend the non-breeding season in warmer latitudes. These migratory warblers can be regarded as essentially tropical birds that breed and raise their young in the forests of higher latitudes in order to take advantage of the seasonal abundance of arthropods.

The Old World and American warblers bear a superficial resemblance to each other. This is largely because of convergent evolutionary influences, resulting from the fact that these unrelated birds occupy rather similar niches in their ecosystems—that of small, arthropodeating birds. One difference between the families is the occurrence of 10 primary wing feathers in the Sylviidae, compared with nine in the Parulidae. The American warblers, particularly male birds, tend to be much more brightly colored than the Old World warblers.

American warblers

The American warblers are small, brightly colored, insectivorous birds. Most species of American warblers are resident in tropical forests, but some migrate to northern ecosystems to breed.

At least 52 species of American warblers breed north of Mexico. Some of these species have very wide ranges, occurring over much of the United States and Canada. The yellow warbler (Dendroica petechia), for example, is a common species found in shrubby habitats from the northern low arctic through all but the southernmost deciduous forests and western deserts. The common yellowthroat (Geothlypis trichas) is similarly widespread in marshes and

shrubby habitats, from the subarctic through the southern states and into Mexico. The yellow-breasted chat (Icteria virens) is the largest species of warbler in North America, with a body length of 15 in (38 cm). This species also ranges widely, from southern Canada through most of the United States. The yellow-rumped warbler (Dendroica coronata) is one of the most familiar species, because it breeds in many types of conifer forests, and winters throughout much of the southern United States and down into Mexico and elsewhere in Central America.

Male birds of these and many of the other North American warblers are very colorful. The prothonotary or golden swamp warbler (Protonotaria citrea) occurs in hardwood swamps and riparian forests, and has a brilliant yellow plumage, offset by its gray-blue wings. The northern parula (Parula americana) occurs in eastern hardwood forests, and has a bright blue back, and a yellow breast with a black-and-red band running across. The male blackburnian warbler (Dendroica fusca) of eastern spruce-fir forests has an orange throat framed by black and white markings. The red-faced warbler (Cardellina rubrifrons) of southern Arizona and Mexico has a crimson face and throat. The American redstart (Setophaga ruticilla) is a widespread forest species with a black body and bright, orange patches on its flanks and tail.

American warblers are a diverse component of the community of birds breeding in most forests of the Americas. In some places in North America, 10 or more species may breed in the same stand. Although all of these birds feed on small arthropods, they segregate ecologically by feeding in different parts of the tree canopy, on the surface of tree bark, or on the forest floor.

Old world warblers

About one-half of the Old World warblers are resident or short-distance migrants that breed in tropical and subtropical ecosystems of Africa. Africa is also the wintering ground for many of the other species of Old World warblers, which migrate to more northern latitudes to breed. Various species in this family are widespread throughout Eurasia, breeding as far as the northern tundra, but wintering in the tropics of Africa or Asia. Other migratory species breed in more temperate habitats.

Only a few species of sylviid warblers breed in North America. The ruby-crowned kinglet (Regulus calendula) and golden-crowned kinglet (R. satrapa) are widespread and common birds of northern conifer forests. The blue-gray gnatcatcher (Polioptila caerulea) is more southern in distribution, and prefers broad-leaf forests. The black-tailed gnatcatcher (P. melanura) occurs in desert scrub. These are all small, very active, insect- and spider-hunting birds. The gnatcatcher was named after its habit of catching small insects while hovering, or through brief, aerial pursuits.

Like other songbirds, male warblers sing to proclaim their breeding territory, and to advertise their availability as a mate to females. Some of the sylviid warblers of Eurasia are very difficult to tell apart on the basis of plumage, but they have different habitat preferences and distinctive songs.

Conservation of warblers

Warblers are small, often colorful birds, and there are many species of both the parulid and sylviid warblers. As a result, these birds are among the most avidly sought-after sightings by bird-watchers. Birding is a non-consumptive field sport rapidly increasing in popularity. Birding, in conjunction with related activities such as bird feeding, has a very large economic impact, and gives great aesthetic enjoyment to many people. Unfortunately, populations of many of the small birds that are the object of these activities, including warblers, are becoming increasingly at risk from a number of human activities.

In spite of their small size, migrating Old World warblers are commonly hunted as food in the Mediterranean region. These days, they are mostly captured with nets or using sticky perches from which the birds cannot extricate themselves. Large numbers of migrating warblers are caught in these ways, and are eaten locally or are offered for sale as a delicacy at markets. Some of the catch, generally pickled, is even traded internationally. The use of

KEY TERMS

Convergence —An evolutionary pattern by which unrelated species that fill similar ecological niches tend to develop similar morphologies and behavior. Convergence occurs in response to similar selection pressures.

Nest parasite —A bird that lays its eggs in the nests of other species. The host raises the parasitic egg, and often fails to successfully raise any of its own babies.

warblers in this way is a quite uncontrolled, free-forall exploitation, and represents a significant risk to the populations of these tiny birds.

One species of North American warbler may have become extinct quite recently. Bachman’s warbler (Vermivora bachmanii) used to breed in mature broad-leaf forests of the southeastern United States. This species has not been reported since 1988 even though there still seems to be sufficient breeding habitat, including intact stands where Bachman’s warbler used to successfully breed. If this species is extinct, its demise was probably caused by the conversion of its wintering habitat on the island of Cuba to agriculture. Once the surviving numbers of Bachman’s warbler decreased to below a critical threshold of abundance, potential mates were probably unable to locate each other in their relatively large breeding range, and the population collapsed.

Many other species of North American warblers are at risk from decreases in the area of their wintering and/or breeding habitat. This concern is especially acute for those warblers whose habitat is mature forest. Losses of natural habitat directly decrease the populations of birds that can be sustained on the landscape. In addition much of the remaining breeding habitat is fragmented into small woodlots. This means that much of the remaining forest habitat is ecologically influenced by proximity to an edge with younger habitat. This factor appears to expose warblers and other forest birds to more intense predation, and to the debilitating effects of nest-parasitism by the brown-headed cowbird (Molothrus ater). Many North American ornithologists feel that these factors are causing large declines in the populations of numerous species of migratory forest birds, including many species of warblers. The same environmental problem, along with hunting, is affecting populations of warblers and other small birds in Europe and elsewhere.

When patience and birding skills allow their close observation, warblers can be wonderfully charismatic. Unfortunately, like so many other creatures native to North America and other parts of the rapidly changing world, many species of warblers are at great risk from the environmental changes being caused by humans and their activities.

Resources

BOOKS

Baker, Kevin. Warblers of Europe, Asia and North Africa. Princeton, NJ: Princeton University Press, 1997.

del Hoyo, J., A. Elliott, and J. Sargatal. Handbook of the Birds of the World. Vol. 10, Old World Flycatchers to Old World Warblers. Barcelona: Lynx Edicions, 2006.

Ehrlich, P.R., D.S. Dobkin, D. Wheye. Birds in Jeopardy. Stanford, CA: Stanford University Press, 1992.

Elphick, Chris, John B. Dunning, Jr., and David A. Sibley, eds. The Sibley Guide to Bird Life and Behavior. New York: Knopf, 2001.

Forshaw, Joseph. Encyclopedia of Birds. 2nd ed. New York: Academic Press, 1998.

Shirihai, Hadoram, Gabriel Gargallo, and Andreas J. Helbig. Sylvia Warblers. Princeton, NJ: Princeton University Press, 2001.