Creature Feature: Black-throated Green Warbler (Setophaga virens)

Black-throated Green Warbler
Black-throated Green Warbler - Photo Earl Harrison

Black-throated Green Warbler – Photo Earl Harrison

I saw the small bird out of the corner of my eye as it hovered only inches from my head, plucking insects from a budding branch. It was a ‘BTG’ or Black-throated Green Warbler; one of the more fearless migrants, and its closeness that spring morning took my breath away. Almost all local birders have some fond memories of BTGs that they found during migration in central Ohio. Their apparent fearlessness makes them one of our most easily-seen spring warbler migrants.

The Black-throated Green is one of the many brightly colored wood warblers that migrate into and through Ohio during the spring to set up nesting territories here and further north. The adult birds have a distinctive plumage, with a green back and crown framing a yellow cheek, gray wings with bright white wing bars, and a white belly with thick black streaks on each flank. The males have a striking black throat which gives the birds their common name.

Affinities. Black-throated Greens are part of the huge family Emberizidae that includes Eurasian buntings, sparrows, tanagers, and wood warblers among others. The wood warblers of the subfamily Parulinae represent nearly 100 species of brightly-colored tree-loving birds of the New World. Like much of this family, many wood warblers are basically tropical birds that have evolved a northern migration. This allows them to escape the crowded tropics for the relatively less-crowded habitats of temperate North America.  Every spring, the waves of migrant wood warblers, grosbeaks, and tanagers that flock through our newly-leafed forests bear witness to the success of this strategy.

Black-throated Green Warbler - Photo Earl Harrison

Black-throated Green Warbler – Photo Earl Harrison

Black-throated Green Warblers are part of a ‘superspecies’ of closely-related warblers that includes the Townsend’s Warbler, Hermit Warbler, Black-throated Gray, Golden-cheeked Warbler, and ‘Wayne’s’ Black-throated Green Warbler. Townsend’s is the western version of Black-throated Green, found throughout the fir forests of the northern Rockies and Pacific Northwest. Hermit Warblers are much more habitat-specific, preferring old growth conifer forests along the pacific coast. Black-throated Grays prefer broad-leaf forests in the west (maple or oaks). Golden-cheeked Warblers breed in the oak forests of the Edwards plateau area of central Texas. Wayne’s Warbler is a subspecies of Black-throated Greens that breed in the cypress swamps of coastal Virginia and North Carolina. All of these close relatives mean that Black-throated Greens are among the most recently-evolved warblers, a speculation recently confirmed with DNA evidence.

Even though they are at eye-level during migration, don’t expect that on their breeding ground. Black-throated Greens and their kin are one of the tree-top warblers of the old genus Dendroica (“tree-nesting”). They live most of the breeding season 30 to 100 feet off the ground in the canopies of conifer trees, usually those with short-needles like spruces, firs, and hemlocks. The birds’ foraging habits help it there: they creep among the branches picking bugs and eggs out of needles, and they will readily hover to get to the tip shoots where caterpillars are most likely. They even have white outer tail feathers (common to this genus), which they flick during foraging to apparently startle insects into revealing their positions.

Range. Black-throated Greens nesting range roughly corresponds to the range of small-needle conifers—hemlocks, spruce, and firs—in eastern North America. At their northern margins, they extend across the spruce belt of boreal Canada from Newfoundland to Alberta. As you follow their range further south, it narrows to the hemlock-fir montane forests of the Appalachians, stretching as far south as Georgia and Alabama. These southern birds have been stressed by the decimation of Appalachian conifer forests by acid rain and insect pests. On a backpack trip through the Great Smoky Mountains this past summer, I saw far fewer Black-throated Greens than many other warblers, principally because the hemlock forests there were severely reduced by woolly adelgids, an insect pest from China.

Black-throated Green Warbler

Black-throated Green Warbler – Photo Earl Harrison

Winter range is just as important to these Neotropical migrants as their nesting range. Black-throated Greens migrate to dry tropical forests in Mexico, Central America, and the Caribbean. This has predisposed some of them to overwinter in Florida and Texas, where locating them among foraging flocks of Yellow-rumps and kinglets in live oak hammocks has become a favorite winter exercise. It has also allowed them to become a common vagrant to the southwest and Pacific, where they are often found in oak canyons and other migrant traps.  Vagrancy and migrational plasticity seem a hallmark of this species group. Even their western counterparts—Townsends and Black-throated Grays—are famous for showing up as strays throughout the east.

This wintering and migration variability has no doubt helped Black-throated Greens and their kin to expand their nesting and winter ranges more easily than other warblers. Our Ohio populations quickly colonized the recovering hemlock groves of the Hocking Hills and Mohican areas during the 1960s to 1980s; prior to that, they were only breeders in the far northeastern part of the state. Also, expanding their foraging habitats has allowed them to colonize other areas. Wayne’s Black-throated Greens switched to bald cypresses, which allowed them to breed in coastal swamps of Virginia and the Carolinas. Other Black-throated Greens have taken to pines, which have allowed them to set up small breeding populations in southern Indiana and the Arkansas Ozarks.

Breeding. Black-throated Greens are among the earliest migrant warblers to return to their territories; it’s not unusual for the first males to be singing here in Ohio by mid-April. Their song is a slurring, distant ‘zoo zee zoo zoo zee‘ or ‘zee zee zoo zee zee‘ that is very distinctive. However, it can become hard to pick out once the bulk of Neotropical migrants starts to crowd into the morning chorus in May.

Black-throated Green Warbler - Photo Earl Harrison

Black-throated Green Warbler – Photo Earl Harrison

The birds have a strong preference for hemlocks in Ohio and the Appalachians. Locally, that means that they are best sought out in the hemlock groves of Clear Creek and the Hocking Hills. They will build a small cup nest along the branches of tall hemlocks, presumably to evade red squirrels, a potent nest predator in conifer forests. The female will lay 4-5 small eggs in the nest, and they will hatch into tiny nestlings after about 12 days of incubation. The young are fed insects for another 8-10 days until fledging, so they grow very quickly. They will continue to trail their parents, begging for another 1-2 weeks after they’ve fledged, which must be very tiresome for the adults.

Black-throated Greens can have very distinctive micro-habitat preferences. As long ago as the 1950s, the famed ecologist Robert MacArthur noted that Black-throated Greens prefer the mid-high outer branches of tall spruces, just below the sunny top of the canopy, a habit that kept them from directly competing with other conifer-loving warblers. This pattern holds in our hemlock-loving birds, and they can be the devil to lure down to view during the nesting season. Even Wayne’s Warblers prefer the high outer margins of bald cypresses, their favored tree in the Carolinas, and Townsend’s Warblers have similar habits in Douglas fir forests in the West. Many times I’ve had to settle for just hearing these birds, even when they were singing all around me.

Conservation Status. Black-throated Greens are one of the few bright spots among warblers, with a fairly stable population. Their wide range and their adaptable breeding and migration habits seem to buffer them against drastic habitat changes. This is not to say that there aren’t problems. Southern birds are having trouble adapting to the loss of hemlocks mentioned earlier, while boreal Canadian birds show wide population fluctuations tied to spruce pests like the spruce budworm. However, birds in southern Canada and the northern US seem to be slowly increasing in numbers and expanding their ranges.

Black-throated Green Warbler in full song - Photo Earl Harrison

Black-throated Green Warbler in full song – Photo Earl Harrison

Interestingly, Townsends and Black-throated Gray Warblers also seem to be slowly expanding their ranges, so much of the group is doing well. Only the isolated southern populations—Golden-cheeked Warbler and Wayne’s Warbler—are having problems, mostly to do with habitat fragmentation. This could signal problems for our more isolated Black-throated Green populations in southern Ohio and Indiana if forestry practices or pests ever imperil their hemlock and pine stands.

Here in central and southern Ohio, breeding Black-throated Greens seem to be localized to conifer patches in the unglaciated parts of eastern and southern Ohio. If you want to see them after spring migration, you can pretty much be guaranteed summer Black-throated Greens along the Hemlock Trail in Clear Creek and around Old Man’s Cave. While you can expect them in hemlock areas of the Hocking Hills and Mohican forest, they should also be looked for as breeding birds in the Shawnee and Tar Hollow forests, as well as the Flint Hills. If some of our birds make the jump to pine trees that has occurred in some other parts of their range, they could become even more widespread. In any case, Black-throated Greens appear to be one of the prime beneficiaries of the reforestation of eastern and southern Ohio over the last 80 years. It remains to be seen if they can continue to adapt to human alterations in their habitats. Hopefully, we’ll still have plenty of these fearless little warblers to amaze us for many springs to come.

The Black-throated Green is one of the many brightly colored wood warblers that migrate into and through Ohio during the spring to set up nesting territories here and further north. The adult birds have a distinctive plumage, with a green back and crown framing a yellow cheek, gray wings with bright white wing bars, and a white belly with thick black streaks on each flank. The males have a striking black throat which gives the birds their common name.

Affinities. Black-throated Greens are part of the huge family Emberizidae that includes Eurasian buntings, sparrows, tanagers, and wood warblers among others. The wood warblers of the subfamily Parulinae represent nearly 100 species of brightly-colored tree-loving birds of the New World. Like much of this family, many wood warblers are basically tropical birds that have evolved a northern migration. This allows them to escape the crowded tropics for the relatively less-crowded habitats of temperate North America. Every spring, the waves of migrant wood warblers, grosbeaks, and tanagers that flock through our newly-leafed forests bear witness to the success of this strategy.

Black-throated Green Warblers are part of a ‘superspecies’ of closely-related warblers that includes the Townsend’s Warbler, Hermit Warbler, Black-throated Gray, Golden-cheeked Warbler, and ‘Wayne’s’ Black-throated Green Warbler. Townsend’s is the western version of Black-throated Green, found throughout the fir forests of the northern Rockies and Pacific Northwest. Hermit Warblers are much more habitat-specific, preferring old growth conifer forests along the pacific coast. Black-throated Grays prefer broad-leaf forests in the west (maple or oaks). Golden-cheeked Warblers breed in the oak forests of the Edwards plateau area of central Texas. Wayne’s Warbler is a subspecies of Black-throated Greens that breed in the cypress swamps of coastal Virginia and North Carolina. All of these close relatives mean that Black-throated Greens are among the most recently-evolved warblers, a speculation recently confirmed with DNA evidence.

Even though they are at eye-level during migration, don’t expect that on their breeding ground. Black-throated Greens and their kin are one of the tree-top warblers of the old genus Dendroica (“tree-nesting”). They live most of the breeding season 30 to 100 feet off the ground in the canopies of conifer trees, usually those with short-needles like spruces, firs, and hemlocks. The birds’ foraging habits help it there: they creep among the branches picking bugs and eggs out of needles, and they will readily hover to get to the tip shoots where caterpillars are most likely. They even have white outer tail feathers (common to this genus), which they flick during foraging to apparently startle insects into revealing their positions.

Range. Black-throated Greens nesting range roughly corresponds to the range of small-needle conifers—hemlocks, spruce, and firs—in eastern North America. At their northern margins, they extend across the spruce belt of boreal Canada from Newfoundland to Alberta. As you follow their range further south, it narrows to the hemlock-fir montane forests of the Appalachians, stretching as far south as Georgia and Alabama. These southern birds have been stressed by the decimation of Appalachian conifer forests by acid rain and insect pests. On a backpack trip through the Great Smoky Mountains this past summer, I saw far fewer Black-throated Greens than many other warblers, principally because the hemlock forests there were severely reduced by woolly adelgids, an insect pest from China.

Winter range is just as important to these Neotropical migrants as their nesting range. Black-throated Greens migrate to dry tropical forests in Mexico, Central America, and the Caribbean. This has predisposed some of them to overwinter in Florida and Texas, where locating them among foraging flocks of Yellow-rumps and kinglets in live oak hammocks has become a favorite winter exercise. It has also allowed them to become a common vagrant to the southwest and Pacific, where they are often found in oak canyons and other migrant traps. Vagrancy and migrational plasticity seem a hallmark of this species group. Even their western counterparts—Townsends and Black-throated Grays—are famous for showing up as strays throughout the east.

This wintering and migration variability has no doubt helped Black-throated Greens and their kin to expand their nesting and winter ranges more easily than other warblers. Our Ohio populations quickly colonized the recovering hemlock groves of the Hocking Hills and Mohican areas during the 1960s to 1980s; prior to that, they were only breeders in the far northeastern part of the state. Also, expanding their foraging habitats has allowed them to colonize other areas. Wayne’s Black-throated Greens switched to bald cypresses, which allowed them to breed in coastal swamps of Virginia and the Carolinas. Other Black-throated Greens have taken to pines, which have allowed them to set up small breeding populations in southern Indiana and the Arkansas Ozarks.

Breeding. Black-throated Greens are among the earliest migrant warblers to return to their territories; it’s not unusual for the first males to be singing here in Ohio by mid-April. Their song is a slurring, distant ‘zoo zee zoo zoo zee’ or ‘zee zee zoo zee zee’ that is very distinctive. However, it can become hard to pick out once the bulk of Neotropical migrants starts to crowd into the morning chorus in May.

The birds have a strong preference for hemlocks in Ohio and the Appalachians. Locally, that means that they are best sought out in the hemlock groves of Clear Creek and the Hocking Hills. They will build a small cup nest along the branches of tall hemlocks, presumably to evade red squirrels, a potent nest predator in conifer forests. The female will lay 4-5 small eggs in the nest, and they will hatch into tiny nestlings after about 12 days of incubation. The young are fed insects for another 8-10 days until fledging, so they grow very quickly. They will continue to trail their parents, begging for another 1-2 weeks after they’ve fledged, which must be very tiresome for the adults.

Black-throated Greens can have very distinctive micro-habitat preferences. As long ago as the 1950s, the famed ecologist Robert MacArthur noted that Black-throated Greens prefer the mid-high outer branches of tall spruces, just below the sunny top of the canopy, a habit that kept them from directly competing with other conifer-loving warblers. This pattern holds in our hemlock-loving birds, and they can be the devil to lure down to view during the nesting season. Even Wayne’s Warblers prefer the high outer margins of bald cypresses, their favored tree in the Carolinas, and Townsend’s Warblers have similar habits in Douglas fir forests in the West. Many times I’ve had to settle for just hearing these birds, even when they were singing all around me.

Conservation Status. Black-throated Greens are one of the few bright spots among warblers, with a fairly stable population. Their wide range and their adaptable breeding and migration habits seem to buffer them against drastic habitat changes. This is not to say that there aren’t problems. Southern birds are having trouble adapting to the loss of hemlocks mentioned earlier, while boreal Canadian birds show wide population fluctuations tied to spruce pests like the spruce budworm. However, birds in southern Canada and the northern US seem to be slowly increasing in numbers and expanding their ranges.

Interestingly, Townsends and Black-throated Gray Warblers also seem to be slowly expanding their ranges, so much of the group is doing well. Only the isolated southern populations—Golden-cheeked Warbler and Wayne’s Warbler—are having problems, mostly to do with habitat fragmentation. This could signal problems for our more isolated Black-throated Green populations in southern Ohio and Indiana if forestry practices or pests ever imperil their hemlock and pine stands.

Here in central and southern Ohio, breeding Black-throated Greens seem to be localized to conifer patches in the unglaciated parts of eastern and southern Ohio. If you want to see them after spring migration, you can pretty much be guaranteed summer Black-throated Greens along the Hemlock Trail in Clear Creek and around Old Man’s Cave. While you can expect them in hemlock areas of the Hocking Hills and Mohican forest, they should also be looked for as breeding birds in the Shawnee and Tar Hollow forests, as well as the Flint Hills. If some of our birds make the jump to pine trees that has occurred in some other parts of their range, they could become even more widespread. In any case, Black-throated Greens appear to be one of the prime beneficiaries of the reforestation of eastern and southern Ohio over the last 80 years. It remains to be seen if they can continue to adapt to human alterations in their habitats. Hopefully, we’ll still have plenty of these fearless little warblers to amaze us for many springs to come.

All photos courtesy of Earl Harrison. You can learn more about the Black-throated Green Warbler at the Audubon Guide site.