The black-throated green warbler is a small songbird found throughout the eastern and central United States and Canada during the spring and summer months. These warblers migrate south to Central America, the Caribbean, and northern South America for the winter. Black-throated green warblers are insectivores, meaning they eat insects and other arthropods. Their diet consists of a variety of insects that they glean from leaves and branches in trees and shrubs. Read on to learn more about what these colorful songbirds eat.
Insects
Insects make up the majority of the black-throated green warbler’s diet. They are insect generalists, meaning they eat a wide variety of insect prey rather than specializing on a few select types. Some of the main insects that black-throated green warblers are known to eat include:
- Caterpillars – moth and butterfly larvae are an important food source.
- Beetles
- Flies
- Spiders
- Grasshoppers
- Crickets
- Cicadas
- Aphids
- Ants
- Bees
- Wasps
- Leafhoppers
The warblers glean through foliage searching for insects to eat. They capture resting insects or pick them directly off leaves and branches. Their thin pointed bill allows them to extract insects from crevices in bark. They are very active foragers, quickly moving through branches and leaves probing for prey.
Caterpillars
Caterpillars are a preferred food source for black-throated green warblers. The larvae of moths and butterflies provide a soft-bodied food source rich in protein. Some caterpillars have stinging or toxic defenses to deter predators, but black-throated green warblers are able to consume even hairy and spiky caterpillars. Studies have found caterpillars can make up over 50% of their diet during spring and summer. Availability of caterpillars is an important factor governing habitat suitability and breeding success.
Other Prey
In addition to insects, black-throated green warblers occasionally supplement their diet with other arthropods and small prey:
- Spiders
- Harvestmen
- Centipedes
- Millipedes
- Snails
- Worms
- Eggs and nestlings of other birds
- Pollen and fruit
Spiders are a common supplemental prey. The warblers glean spiders from foliage and bark crevices. On rare occasions, they have been observed consuming eggs and nestlings of other songbirds, likely opportunistically when encountering an unguarded nest. They supplement their diet with pollen and fruit, especially in fall and winter during migration and on their wintering grounds.
Foraging Behavior
Black-throated green warblers forage actively in a few characteristic ways:
- Foliage gleaning – This is the most common foraging method. The warblers glean through leaves of trees, shrubs, and herbaceous plants searching for insects.
- Hover gleaning – They sometimes hover briefly next to foliage to pick out prey items.
- Flycatching – On occasion black-throated green warblers will sally out from a perch to catch flying insects in mid-air.
- Bark gleaning – They probe under bark and in crevices for hiding insects.
- Berry eating – In winter, they eat berries by plucking them directly off bushes.
Their needle-like bill is perfectly adapted for removing insects and spiders from cracks and crevices in bark. Their flexibility in foraging strategies allows them to take advantage of a wide range of food resources.
Habitat
Black-throated green warblers forage in a variety of forested habitat types during the breeding season, including:
- Deciduous forests
- Mixed forests
- Pine forests
- Swampy woodlands
- Mangrove forests (winter habitat)
- Pine-oak woodlands (winter habitat)
They prefer foraging in the upper canopy of mature forests. Trees like oak, maple, hickory, and pine provide excellent foraging opportunities. During migration, they also forage in orchards and backyard trees. On their tropical wintering grounds, they forage in mangroves, pine-oak forests, and shade coffee plantations. Access to productive foraging habitat is key for black-throated green warblers. Loss of mature forests can impact their ability to find sufficient food.
Breeding and Nesting
The availability of insect prey is especially important during the breeding season when black-throated green warblers are raising young. After arriving on the breeding grounds in spring, males establish and defend breeding territories with adequate foraging resources. Females build an open cup nest out of twigs, bark, lichens, and spider webs. Nests are usually located in the fork of a tree branch around 10-40 feet high.
Females lay 3-5 eggs which hatch after 11-14 days. Nestlings are fed insects and spiders by both parents. The constant effort of finding food for their young keeps the parents busy from dawn to dusk. Nestlings fledge from the nest about 9-12 days after hatching. Newly fledged young continue to be fed by the parents for several weeks as they learn to forage on their own. Abundant insect prey is critical for successful breeding to raise young.
Diet of Nestlings
Female black-throated green warblers provision their nestlings with a wide variety of soft-bodied insects and spiders including:
- Caterpillars
- Grasshoppers
- Crickets
- Spiders
- Cicadas
- Flies
- Beetles
- Treehoppers
- Aphids
- Lacewings
The most common insects fed to nestlings are lepidopteran larvae, specifically caterpillars, which provide an excellent source of protein for growing nestlings. Spiders are also an important supplement. The parents deliver food to nestlings by either passing it bill to bill or by the nestlings reaching up and taking it directly from the parent’s bill. Nestlings can consume up to 67 percent of their body mass in insects each day!
Migration and Winter Diet
On their wintering grounds in Central America, the Caribbean, and northern South America, black-throated green warblers occupy a variety of tropical habitats. Their diet shifts during the nonbreeding season to adapt to local food availability:
- More fruit eating (ex. wild berries, bananas)
- Nectar from flowers
- Spiders
- Insects like mosquitos, wasps, ants
In tropical regions, fruit eating provides an important source of carbohydrates. Nectar gives a boost of energy. Small insects continue to be an important protein source during winter. On migration, black-throated green warblers stopover in a variety of woodland habitats, refueling on insects and fruit to replenish energy before moving on. They are flexible in adapting their diet to take advantage of locally abundant food.
Adaptations for Food Acquisition
Black-throated green warblers have several physical and behavioral adaptations that help them find and acquire insect prey:
- Slender pointed bill – Excellent for extracting insects from bark and foliage.
- Enlarged premaxillary bones – Give their bill extra mobility.
- Large eyes – Aid in visually locating prey while foraging.
- Good color vision – Helpful for spotting camouflaged caterpillars.
- Rapid mobility – Quickly moves through branches probing for insects.
- Hovering ability – Allows them to glean prey not perched on surfaces.
Their distinctive black and yellow facial markings may also help flush out insects by creating a distraction or breakup pattern as the birds probe through dense foliage. In all, black-throated green warblers exhibit excellent adaptations for locating and capturing small insects and arthropods.
Conclusion
In summary, black-throated green warblers are insectivores that feast on a diverse array of insect prey as well as spiders and other small invertebrates. Caterpillars are a preferred food source, especially for provisioning young. They forage actively, gleaning foliage and bark for insects. Their adaptations include a slender bill and enlarged premaxilla for extracting hidden insects. Abundant insect prey is critical for their breeding success. During migration and winter, they supplement their diet with fruit and nectar. Loss of mature forest habitat threatens their foraging resources. Protecting productive breeding habitat will help conserve populations of these colorful neo-tropical songbirds.
