The green heron (Butorides virescens) is a small heron that breeds in North America and winters south through Central America and the Caribbean to South America. Green herons are one of the smaller heron species, standing around 16-18 inches tall with a wingspan of about 25 inches. They have slate-gray feathers on the head, neck and body with a dark green-black cap and back. Their wings are muted chestnut and they have yellowish legs.
Green herons are found near water throughout most of North America during the breeding season. They nest in a variety of wetland habitats including swamps, marshes, ponds, streams and mangrove forests. Many green herons migrate from the northern parts of their range to more southern areas for the winter. Other herons may be resident in parts of their range depending on food availability.
Where do green herons migrate to?
The majority of green herons that breed in Canada and the northern United States migrate south for the winter. Migration starts in August or September as day lengths get shorter and temperatures begin to drop. They fly at night using stopover sites during the day to rest and feed. The migration south takes around 6-8 weeks to complete.
Southern United States
Many green herons stopover or overwinter along the Atlantic and Gulf coasts of the southern United States. Coastal states such as Florida, Georgia, South Carolina, North Carolina, Alabama, Mississippi, Louisiana and Texas host significant wintering populations. They occupy coastal marshes, swamps, ponds and streams where they can find adequate food throughout the winter months.
Mexico and Central America
The majority of green herons continue migration south into Mexico and Central America. Mexico hosts large numbers of overwintering green herons. They occur along both the Pacific and Gulf coasts, as well as inland at wetlands, lakes and marshes.
Many green herons overwinter in wetlands across Central America. Countries like Belize, Guatemala, Honduras, Nicaragua and Costa Rica provide important wintering habitat. They are found in coastal mangrove forests and interior wetlands.
Caribbean Islands
The Caribbean islands host significant populations of overwintering green herons each year. They occur in wetlands across Cuba, Jamaica, Hispaniola, Puerto Rico, the Bahamas and other islands. Mangrove forests are especially important habitat.
South America
Some green herons migrate as far south as northern South America. They overwinter in countries like Columbia, Venezuela, the Guianas and the Amazon Basin wherever suitable wetland habitat exists. The majority overwinter north of the Amazon in coastal regions. Very few migrate south of the equator.
Spring Migration Back North
Green herons begin migrating back north in late February and March. They follow similar migratory pathways north through Central America and Mexico into the southern United States. Most individuals return to breeding sites by late April or May.
Migrating green herons can cover 40-100 miles or more per night depending on weather conditions. They may pause during the day to rest and feed. Some evidence suggests that individual herons return to the same breeding sites in successive years.
Threats During Migration
Green herons face a number of threats during their spring and fall migration:
- Loss of wetland stopover habitat. Draining or destruction of wetlands removes crucial resting and feeding sites.
- Collision with structures. Herons can collide with buildings, communication towers and other structures during night migrations.
- Disorientation by artificial light. Bright city lights can disorient migrating birds.
- Severe weather. Storms or high winds can lead to exhaustion or blow birds far off course.
- Predation. Migrating green herons are vulnerable to raptors and other predators, especially when crossing open water.
Conservation of wetland stopover habitat helps provide migrating green herons with safe resting and feeding sites during their long journeys. Reducing and reshaping artificial lighting near migration routes also helps reduce disorientation and collisions.
Unusual Migration Destinations
While most green herons follow typical migration routes, occasionally individuals show up well outside their normal ranges. Some of the most unusual migration sightings include:
- Washington – A green heron was spotted on Whidbey Island in October 2012, far north of their normal western migration routes.
- Oregon – Individual vagrant green herons have been observed along the Oregon coast during fall migration.
- British Columbia – Rare sightings of green herons have been documented in the southern coastal region.
- Iceland – A surprising green heron sighting was documented in Iceland in 2016, likely blown far off course by storms.
- United Kingdom – A handful of green herons have wandered across the Atlantic and been spotted in England and Scotland over the years.
- Western Alaska – In 2012, an adult green heron was seen along the lower Kuskokwim River, probably after becoming very lost on migration.
These unusual sightings highlight how even experienced migratory birds can sometimes get blown astray by weather or lose their way. The rarity of these out-of-range observations shows green herons are precise and effective migrants under normal conditions.
Notable Migration Feats
Green herons are champions of migration, capable of impressive feats on their long journeys:
- Roundtrip migrations up to 6,000 miles for individuals nesting in Canada and wintering in South America.
- Nonstop flight segments of up to 1,500 miles over open water in the Gulf of Mexico.
- Consistent speed of 20-30 mph powered by steady wing beats when winds are favorable.
- Ability to compensate for wind drift and stay on course to reach specific wintering sites.
- Navigation by the stars and magnetic fields so birds can migrate at night.
- Endurance to fly for 6-8 hours per night, resting and refueling during the day.
Green herons prepare for migration by nearly doubling their weight in fat reserves to power these unbelievable nonstop flights. Their impressive navigation abilities ensure they reach the same overwintering sites safely year after year.
Migration Distance and Timing by Breeding Region
Green herons that nest further north generally migrate greater distances and leave earlier than populations further south:
Canada
- Migration distance: 4,000+ miles
- Departure time: Mid-August to early September
- Overwinter range: Southern Central America to northern South America
Northern U.S. States
- Migration distance: 2,500-4,000 miles
- Departure time: Mid-September
- Overwinter range: Gulf Coast and southern Florida
Mid-Atlantic and Southern U.S. States
- Migration distance: 500-2,000 miles
- Departure time: Late September to mid-October
- Overwinter range: Gulf Coast to southeastern Mexico
Florida
- Migration distance: Many are year-round residents
- Departure time: N/A
- Overwinter range: Throughout Florida
Notable Spring Migration Sites
Some important spring migration stopover sites for green herons include:
Texas Gulf Coast – Coastal wetlands along the upper Texas coast provide a final rest stop before green herons cross the Gulf of Mexico. Anahuac National Wildlife Refuge is an internationally important site.
Veracruz, Mexico – Coastal wetlands in Veracruz host massive numbers of migrating herons heading north in spring. Particularly high concentrations occur in March and April.
Mississippi River Valley – Bottomland hardwood forests and swamps along the Mississippi River and its tributaries in Louisiana, Mississippi and Arkansas provide migratory stopover habitat.
Cheyenne Bottoms, Kansas – This inland wetland complex is an important stopover site for green herons in the Central Flyway as they head north to breeding grounds. High numbers pass through in April.
San Francisco Bay, California – Coastal marshes and wetlands around San Francisco Bay provide stopover habitat for green herons following the Pacific Flyway north. Peak numbers occur in April and May.
Threats to Migration Stopover Habitat
Loss and degradation of wetland stopover habitat threatens green herons during migration:
Threat | Impact on Green Herons |
---|---|
Draining wetlands for agriculture or development | Removes crucial feeding and resting sites |
Pollution and contamination of wetland habitats | Reduces prey abundance and availability |
Overgrazing by livestock around wetlands | Degrades vegetation and reduces cover |
Channelization of rivers for flood control | Eliminates slow flowing pools and backwaters |
Invasive species replacing native wetland plants | Alters habitat structure and diversity |
Conserving a network of migration stopover habitats helps provide green herons with the resources they need to successfully complete their long spring migrations.
Notable Green Heron Migration Research
Important studies that have furthered understanding of green heron migration include:
Banding Studies (1930s-1950s) – Early bird banding efforts by researchers with the U.S. Fish and Wildlife Service provided initial maps of migration timing, routes and wintering areas. Band recoveries documented migration distances over 1,000 miles for some individuals.
Radio Telemetry Studies (1980s-90s) – Researchers at the University of Georgia tracked green herons using radio transmitters. They documented precise timing, stopover sites, roost locations and habitat use during migration.
Stable Isotope Analysis (2000s) – Scientists have analyzed ratios of different isotopes in green heron feathers to determine the relative breeding and wintering locations. This helps map overall geographic patterns.
Motus Tracking Network (2010s-present) – Automated radio telemetry stations can now track green herons and other birds along the entire Western Hemisphere migration system. This reveals precise migratory pathways and timing.
Genetic Studies (2010s-present) – Analysis of green heron genetic markers helps identify distinct breeding populations and their linkage to wintering areas. This helps target conservation to subpopulations.
Continuing research relies on new techniques like GPS satellite tracking, chemical tracers and genetic sequencing to uncover the detailed mysteries of green heron migration.
Interesting Green Heron Migration Facts
- Green herons show strong site fidelity, often returning to the exact same overwinter sites, and even breeding territories, year after year.
- They migrate alone, not in flocks, following well-established pathways passed down genetically.
- Green herons fly low during migration, usually 100-600 feet altitude, helping them spot landmarks and habitat.
- Strong cold fronts can ground migrating green herons for days until weather conditions improve.
- Green herons fat up before migration, nearly doubling their body weight to add fuel for long flights.
- Most green herons take different routes in spring and fall, shifting slightly west in spring and east in fall.
- Juveniles and adults may migrate at different times and to somewhat different wintering areas.
- A few green herons interrupt migration and spend winter along the Gulf Coast before resuming in spring.
- Favorable tailwinds can boost ground speeds to over 35 mph for migrating green herons.
- Green herons fly non-stop 18-28 hours across the Gulf of Mexico, their longest water crossing.
Conclusion
The green heron is a champion migrant that navigates thousands of miles between its breeding and wintering grounds each year. These small herons follow well-defined migration routes and timelines shaped by the locations of important wetland habitats across North America. Conservation of stopover sites and an understanding of threats faced along their migration routes can help ensure green herons continue their epic journeys for generations to come. Ongoing research advances knowledge of their migration patterns, timing, preferred habitats and amazing navigational abilities. Green herons serve as an indicator for wetland health across a continent and their migrations link ecosystems from the far north to the tropics.