Egrets are a group of medium to large birds in the family Ardeidae that includes species like the Great Egret, Snowy Egret, Little Egret, and Cattle Egret. Several egret species have become established in Hawaii, leading some to question if they should be considered invasive species on the islands.
What are egrets?
Egrets are long-legged wading birds that typically inhabit wetlands and marshy areas. They have slender bodies, long necks, and pointed bills that allow them to catch fish, frogs, insects, and other small prey. Some of the most common egret species include:
- Great Egret – A large, white egret with yellow bill and black legs, about 1 meter tall
- Snowy Egret – A medium-sized, white egret with black legs and yellow feet, about 60 cm tall
- Little Egret – A small, white egret about 55 cm tall with black legs and feet
- Cattle Egret – A small, stocky egret with yellow bill and legs, about 45 cm tall
Egrets are very social birds and often feed and nest in groups called colonies. They build platform nests out of sticks in trees and bushes near water. Most species are white or have white plumage, though some develop decorative plumes during breeding season.
How did egrets get to Hawaii?
Multiple egret species were introduced to Hawaii intentionally and accidentally throughout the 1900s including:
- Cattle Egret – Introduced from Africa in 1959 to help control insect pests on farms
- Great Egret – Likely arrived naturally by 1926 after expanding across the Pacific
- Snowy Egret – Introduced accidentally in the 1950s
- Little Egret – First observed in 1963 and likely arrived accidentally like the Snowy
These species established breeding populations on most major Hawaiian islands. Scientists think their spread was aided by the cleared agricultural lands and irrigation ditches that provided ideal wetland habitat.
Are egrets considered invasive in Hawaii?
Most conservation organizations do not consider egrets to be invasive species in Hawaii. The Hawaii Department of Land and Natural Resources includes Cattle Egrets on their list of introduced birds but not the other species.
Invasive species are defined as non-native organisms that cause environmental or economic harm. Egrets do not appear to be significantly disrupting Hawaii’s island ecosystems or causing major damage. However, there is some debate if they compete with native waterbirds.
Arguments for egrets being invasive
- They did not naturally occur in Hawaii before human introduction
- Large egret populations could compete with native birds for food and nesting areas
- Egrets may spread diseases and parasites to native birds
- Some landowners consider them a nuisance
Arguments against egrets being invasive
- Most egret species were accidental introductions, not purposefully released
- Their ecological impacts appear minimal compared to highly damaging species
- They fill an empty niche and utilize human-created habitats
- Their predation helps control populations of non-native fish and frogs
While not officially invasive, conservationists do recommend monitoring egret populations and their effects on native wildlife.
How many egrets live in Hawaii?
Estimates for egret populations in Hawaii include:
- Cattle Egret – 2,000 to 4,000 breeding pairs
- Great Egret – 100 to 300 breeding pairs
- Snowy Egret – 50 to 200 breeding pairs
- Little Egret – Less than 100 breeding pairs
Breeding populations seem stable or slightly increasing over the past 20 years. Christmas Bird Count data indicates Cattle Egrets are the most abundant, followed by Great Egrets and far fewer Snowy and Little Egrets.
Where are egrets found in Hawaii?
Egrets utilize a variety of wetland habitats across the major Hawaiian islands including:
- Flooded agricultural fields
- Irrigation ditches
- Ponds
- Lakes
- River mouths
- Coastal wetlands
- Mudflats
They nest in trees near foraging habitat, with largest colonies found in non-native mangroves and eucalyptus groves. Cattle Egrets tend to inhabit drier areas than other species.
Island distribution
All four egret species occur on the main Hawaiian islands of Kauai, Oahu, Maui, Molokai, and Hawaii. Their distribution includes:
- Kauai – Small populations of Cattle, Great, and Snowy Egrets
- Oahu – Large populations of Cattle and Great Egrets
- Maui – Moderate numbers of Cattle and Snowy Egrets
- Hawaii – Small populations of Cattle, Great, and Snowy Egrets
Snowy Egrets and Little Egrets also inhabit the northwest Hawaiian islands. Egrets are scarce or absent on small or dry islands.
What impacts do egrets have on Hawaii’s environment?
The environmental impacts of egrets in Hawaii are not fully understood but seem minimal based on current research:
- No evidence they displace native waterbird species
- Predation on non-native fish and bullfrogs may benefit native species
- Don’t cause major damage to natural habitats
- Utilize man-made wetlands and modified habitats
However, some potential concerns include:
- Localized competition with native birds for food and nest sites
- Egret feces can degrade water quality in wetlands
- Predation of eggs, chicks, and adults of endangered waterbirds
Overall egret impacts are considered neutral or slightly negative. Further monitoring is needed of their interactions with native birds like the endangered Hawaiian stilt.
Are egrets beneficial to Hawaii?
Egrets may provide some beneficial services in Hawaii’s human-altered environments:
- Prey on invasive fish like mosquitofish and bullfrogs
- Eat destructive crop pests like armyworm caterpillars and crickets
- Help disperse native wetland plants
- Convert aquatic nutrients into guano which may fertilize plants
- Provide aesthetic value for birdwatchers and tourists
Farmers intentionally introduced Cattle Egrets to help control insect pests. All egret species likely help suppress populations of non-native prey species to some degree.
How can egret populations be managed?
Since egrets are well-established in Hawaii, complete removal is not feasible. Potential management options include:
- Protect habitat of endangered waterbirds from egret encroachment
- Remove egret nests near sensitive native bird colonies
- Discourage egret nesting in inappropriate areas via disturbance or barriers
- Alter wetland hydrology to reduce quality of egret foraging and nesting habitat
- Selective culling of problem birds
- Continue monitoring egret populations and interactions with native species
More research on egret impacts is needed to determine if and where management actions may be warranted. Any efforts would require permits and authorization from wildlife agencies.
Conclusion
Egrets have successfully colonized the Hawaiian islands since being introduced last century, but are not considered highly invasive. While they may compete locally with native birds, their overall environmental impacts seem relatively minor. Active management of egret populations is not currently needed but continued monitoring of their population trends and ecological effects is recommended.