The wood thrush is a medium-sized North American thrush. It is a migratory bird, breeding in deciduous and mixed forests across eastern North America and wintering in Central America. Wood thrush have declined in numbers in recent decades, primarily due to loss of habitat and increased predation. Understanding where they go in the winter and what threats they face there is important for conservation efforts.
The wood thrush (Hylocichla mustelina) is a medium-sized thrush found in eastern North America. They have brown upperparts and whitish underparts with dark brown spots. Males and females look similar. Their song is a flute-like ee-oh-lay which gives them their name. Wood thrush are neotropical migrants, meaning they breed in North America in the summer and migrate to Central America for the winter.
Wood thrush breed in deciduous and mixed forests across eastern North America. Their breeding range extends from Manitoba to Nova Scotia in the north down to eastern Texas and northern Florida. They prefer mature forest habitat with a closed canopy and tall trees, a shrubby understory, and moist soil. This habitat provides food, nest sites, and cover from predators.
After breeding and raising young in northern forests during summer, wood thrush migrate to Central America in the fall. Migration occurs at night and they fly non-stop over the Gulf of Mexico to their wintering grounds. Their winter range includes southern Mexico, Guatemala, Belize, Honduras and Nicaragua. Most winter in the lowlands, but some range up to 2500 m elevation in the mountains.
Where Specifically do Wood Thrush Go in Winter?
Research using tagged birds reveals the specific locations and habitats used by wood thrush during the winter months:
- Southern Mexico – states of Veracruz, Oaxaca and Chiapas in particular
- Guatemala – lowlands and foothills on Pacific and Caribbean side
- Belize – widespread across the country
- Honduras – mainly on Caribbean slope and coastal plains
- Nicaragua – lowlands on Caribbean side
Their core wintering range centers around the Caribbean lowlands of southeast Mexico, Guatemala, and Belize. However, populations show connectivity to specific breeding populations in North America. For example, wood thrush from the Midwest and Ontario mainly winter in Mexico and Guatemala. While thrush from eastern Canada and New England go farther south to Honduras and Nicaragua.
Habitats Used in Winter
On their wintering grounds, wood thrush occupy a variety of forest habitats including:
- Tropical evergreen forests
- Semi-deciduous forests
- Shade coffee plantations
- Mixed forests with pine or oak
- Riparian forests along rivers and streams
They tend to prefer shady forests with closed canopies and tall trees. This provides protection, higher humidity, and cooler conditions compared to open habitats. Food availability, predation risk, and competition with other birds also determines which winter habitats thrush occupy.
Some key winter habitats include:
- Sierra de los Tuxtlas – volcanic mountains in Veracruz, Mexico with tropical rainforest
- Lacandon Forest – rainforest region in Chiapas, Mexico and Peten, Guatemala
- Maya Mountains – hilly area with broadleaf forest in Belize
- Moskitia Region – rainforests and wetlands in eastern Honduras and Nicaragua
Maintaining healthy tropical forests across their winter range is crucial for wood thrush survival and successful migration back to their breeding grounds.
Winter Diet
Wood thrush are omnivores, eating a combination of insects and fruit. Animal prey provides protein while fruit provides carbohydrates and fats to fuel migration and survival. Their winter diet consists of:
- Insects – beetles, ants, cicadas, caterpillars, crickets
- Spiders
- Millipedes
- Centipedes
- Snails
- Fruit – wild grapes, blackberries, blueberries, mulberries, cherries, elderberries
- Berries from trees – Cecropia, Miconia, Ficus
They forage primarily in the understory on the ground or in low vegetation. But will also forage in the mid-canopy for fruit. Competition for food resources during winter can be high.
Threats Faced in Winter
Wood thrush face a number of natural threats on their wintering grounds that can impact survival. These include:
- Predators – snakes, mammals, raptors, other birds
- Food limitation – due to habitat loss, competition, weather effects on insects
- Severe weather – storms, cold snaps which reduce insect prey
- Disease – blood parasites transmitted by insects
Habitat loss in their winter range is also a major long-term threat. Deforestation rates in Central America are high, reducing forest cover available to thrush. Climate change may also exacerbate drier conditions and food shortages in the future.
Mortality Rates in Winter
Higher mortality rates occur on the wintering grounds compared to the breeding grounds. Annual survival rates for wood thrush average around 50-60% over the full annual cycle.
However, a study of banded wood thrush from Vermont estimated:
- Winter survival rate: 75%
- Summer survival rate: 85%
Higher mortality in winter is likely driven by food shortages, predators, disease, and severe weather. Maintaining quality winter habitats is key to improving survival. Birds in poorer condition after surviving winter are also less likely to migrate successfully back to breeding areas.
Spring Migration Back to Breeding Grounds
In early spring, wood thrush begin their northward migration back to North American breeding habitats. They time migration to coincide with emerging insects and leaf-out on their breeding grounds which provides food for breeding and rearing nestlings.
Spring migration occurs over a broader time period from mid-March to early June. Migration is rapid and birds fly non-stop over the Gulf of Mexico, typically arriving back on breeding areas by mid-May.
Males tend to arrive first on the breeding grounds to establish territories. Females follow a week or two later. Most birds return to the same breeding site each year through site fidelity. The combination of strong site fidelity to both wintering and breeding areas makes wood thrush vulnerable to habitat changes in both regions.
Conclusion
In summary, wood thrush are neotropical migrants that breed in forests across eastern North America in summer and migrate to Mexico, Central America and the Caribbean for the winter. Their core winter range centers in southeast Mexico, Guatemala and Belize in humid broadleaf forests. Maintaining healthy tropical forests across their winter range is crucial to conserving wood thrush populations. Habitat loss, lack of food, disease, predators and extreme weather are threats wood thrush face in their wintering areas. Understanding where they go and their habitat needs in winter is key for managing this species of conservation concern.
