Green Heron Breeding Surveys

Herons and egrets are among the largest and most spectacular of the worlds water birds.  There are 15 different species recorded from Bermuda.  Most are migrants and are mainly found in wetland and coastal habitats.  Before humans arrived there were established breeding colonies belonging to many different species.  These were quickly destroyed due to human hunting and predation from introduced mammals in the years following human habitation.

The green heron, Butorides virescens,  is one of the smaller and more colourful species of heron in Bermuda.  This heron is quite stocky with relatively short legs.  The overall body colouration is dark.  This heron, like so many of the other species, has continued to visit Bermuda as a migrant, often staying throughout the winter months.  However, there had been an increasing tendency for them to stay in small numbers throughout the year. 

Herons and egrets are colonial breeders with birds of several species often nesting in close proximity to each other. It has been speculated that the presence of nesting yellow-crowned night herons, Nycticorax violacea (a species deliberately introduced in the 1970s), encouraged the green herons to stay and breed on Bermuda.

In 2002 green herons were first observed nesting around two isolated saltwater ponds in Hamilton Parish.  Both ponds are surrounded by a nearly continuous belt of red mangrove trees.  These two ponds are the largest in Bermuda and are important habitats for a number of rare and endangered species, many of which are not found anywhere else in Bermuda. Mangrove Lake is the largest pond in Bermuda, measuring over 30 acres in area, and is fringed by an extensive band of red mangrove trees – up to 50 feet thick in places.  Mangrove Lake is also home to the endemic Bermuda killifish and the rare diamondback terrapin. Trott's Pond is Bermuda’s second largest pond, measuring 10 acres in area, and is located immediately to the east of Mangrove Lake.  Over 90% of its shoreline is bordered by a thick fringe of red mangrove trees and, like it’s neighbor Mangrove Lake, Trott’s Pond is home to a large population of Bermuda killifish.  It is also the only other brackish water pond in Bermuda to contain diamondback terrapins.

Nesting Surveys

In the summer of 2002 4 adults and 4 juveniles were seen in Mangrove Lake, which was the first direct proof of successful breeding in Bermuda. These observations indicated that there was a resident breeding population of at least 2 pairs of green herons in the Mangrove Lake/Trott’s Pond area. By 2003 the number of adults had increased to 20, with 12 nests in the area.

In the summer of 2002 4 adults and 4 juveniles were seen in Mangrove Lake, which was the first direct proof of successful breeding in Bermuda. These observations indicated that there was a resident breeding population of at least 2 pairs of green herons in the Mangrove Lake/Trott’s Pond area. By 2003 the number of adults had increased to 20, with 12 nests in the area.

A more comprehensive survey was performed in the summer of 2004.  Nesting herons are very sensitive to disturbance, so a small camouflaged kayak was used to discreetly check for signs of nesting activity.  The survey revealed that a total of 10 nests, with 22 chicks and juveniles, existed in the mangrove trees around both ponds.

By 2005 the numbers had increased to 19 nests containing 25 chicks and juveniles.  As in 2004, there were more nests in Trott’s Pond than in Mangrove Lake.  Surveys in 2006 and 2007 revealed that green herons had also begun to expand their nesting range to other mangrove communities across Bermuda. Adults in full breeding plumage were observed tending nests in Bartram’s Pond, Walsingham Pond, and in the mangroves at the Foot of the Lane.

Threats to Bermuda's Green Heron Population

Green heron nests are built very close to the surface of the water and are vulnerable to wave action.  Therefore hurricanes and other severe weather events are considered to be the most serious threats, especially to eggs and chicks.  Other threats include cats (which have been observed killing and eating inexperienced juveniles), and rats (which are capable of climbing into nests and preying on unattended chicks and eggs.)

A Unique Opportunity

The monitoring of the green heron population in Bermuda presents a unique opportunity to bird ecologists in that it has been possible to observe and learn how a new species colonizes a remote oceanic island. So far the surveys have revealed that this new heron population is increasing in size.  As a result of this successful breeding activity green herons are becoming a more frequent sight on Bermuda.

Surveys will continue on a monthly basis during the nesting season to monitor breeding success, and as the population expands, frequent checks will be carried out in other potential nesting locations around Bermuda. Additionally, the banding of juveniles has begun so that conservationists may identify individuals in an effort to determine their survival rate and movement patterns as adults.

Download Green Heron Project flier [PDF].

Further Reading