The boat-tailed grackle (Quiscalus major) is a large, conspicuous blackbird found in coastal areas of the southeastern United States. In recent decades, boat-tailed grackle populations have expanded northward and westward across the eastern US, leading some to question whether they may be becoming invasive. Here we review what is known about the ecology and population trends of boat-tailed grackles to evaluate whether they should be considered an invasive species.
What are boat-tailed grackles?
Boat-tailed grackles are a medium-to-large species of icterid blackbird distinguished by their keel-shaped tails and glossy black feathers. Males are larger than females and have longer tails. Their name comes from their fan-shaped, boat-like tail which aids them in swimming.
Boat-tailed grackles are native to coastal marshes and mangroves of the southeastern United States from North Carolina to Texas. They also occur in the Caribbean. However, in the past 50+ years their range has expanded north to New England and west to Oklahoma.
What do boat-tailed grackles eat?
Boat-tailed grackles are omnivorous, feeding on a wide variety of plant and animal matter. Their diet includes:
- Seeds and grains
- Aquatic invertebrates like crabs, fish, mollusks
- Frogs, snakes, eggs
- Insects
- Fruit
- Carrion
- Human food waste
They often forage in wetlands, along shorelines, and in grasslands. Boat-tailed grackles will also readily visit bird feeders, farms, and urban areas for food. They have a strong preference for protein-rich foods like fish and invertebrates during breeding season.
Are boat-tailed grackle populations increasing?
Over the past century, boat-tailed grackles have dramatically expanded their breeding range along the Atlantic coast and inland:
Year | Breeding Range |
---|---|
1900 | Southeast coasts of NC, SC, GA, FL |
1970s | North to Chesapeake Bay area |
Today | North to Maine and west to Oklahoma |
Their populations have also increased within their original southern range. For example, in South Carolina the estimated number of boat-tailed grackle nests increased exponentially from 1978 to 2008.
Several factors may be contributing to the grackle’s range expansion and population growth:
- An increase in suitable habitat as rice plantation agriculture expanded in the 1800s-1900s.
- The creation of reservoirs, farm ponds, and irrigation ditches in drier inland regions.
- An abundance of food waste and fish discards provided by humans.
- A decrease in control efforts and persecution by humans.
- The adaptability and opportunism of boat-tailed grackles.
Their flexible diet, ability to nest colonially, and tolerance of disturbance by humans seems to allow boat-tailed grackles to thrive across an increasingly wide range of habitats.
What effect are boat-tailed grackles having?
The impacts of expanding boat-tailed grackle populations are complex. In some cases, they appear to displace other blackbird species through competition and aggression at nesting colonies. However, clear environmental harm has been difficult to demonstrate. Known impacts include:
Competition with other colonial waterbirds
- Grackles may outcompete other blackbirds for nest sites.
- They have been observed displacing tricolored herons from nesting colonies.
- However, their overall effects on other bird populations remain unclear.
Predation on other bird species
- Grackles will eat the eggs and young of other bird species, though the frequency is unknown.
- This nest predation may contribute to population declines of species like Least Terns in some areas.
Damage to rice and fruit crops
- Boat-tailed grackles will feed on rice, sunflower, corn, and fruit crops.
- However, quantitative data on actual agricultural damage is limited.
- No major economic impacts have been documented, but local effects may occur.
Risks to aircraft
- Like other blackbird species, boat-tailed grackle flocks can pose hazards at airports.
- Their tendency to form large roosts increases strike risk to aircraft.
Overall, while boat-tailed grackles may have some localized impacts, there is little evidence that they are causing significant environmental or economic damage on a broad scale. More research is needed to clarify their competitive interactions with other species and quantify their predation rates on other birds.
Should boat-tailed grackles be considered invasive?
The term “invasive species” is notoriously ambiguous, but generally refers to non-native organisms that spread in ways that cause harm. To evaluate whether boat-tailed grackles should be considered invasive, we can apply some key criteria:
Range expansion
- Boat-tailed grackles have clearly expanded their range dramatically over the past century.
- However, this expansion has occurred within their native continent, not to new regions.
Environmental impacts
- Negative effects on other species and ecosystems have not been clearly demonstrated.
- More research is needed to quantify their competitive and predatory interactions with other birds.
Economic impacts
- There is little evidence that grackles cause substantial damage to agriculture overall.
- Further study is needed on quantifying local crop losses.
Origin
- Boat-tailed grackles are native to North America, so do not meet definitions of an exotic invasive species.
Based on these criteria, there is not sufficient evidence to consider boat-tailed grackles an invasive species at this time. Their range expansion alone does not warrant that designation without demonstrated environmental or economic harm. More research should examine the degree of competitive exclusion versus coexistence with other colonial waterbirds within their expanding range. Careful study is also needed to quantify if boat-tailed grackles cause substantial losses to rice and other crops at a large scale. For now, “native opportunist” seems a more appropriate term than “invasive species” for the boat-tailed grackle.
Conclusion
In summary:
- Boat-tailed grackles have greatly expanded their range over the past century, but are native to North America.
- Their effects on other bird species remain unclear and require more study.
- There is little quantitative evidence that they cause major economic impacts to agriculture.
- Given the lack of demonstrated environmental or economic harm, boat-tailed grackles do not meet the criteria to be considered an invasive species.
- More research is needed on competitive interactions with other waterbirds and predation rates on other species.
- Impacts to crops should also be quantified to assess if boat-tailed grackles negatively impact local agriculture.
- “Native opportunist” is a more appropriate term than “invasive” for boat-tailed grackles at this time.
Careful monitoring and more study of areas where boat-tailed grackles have expanded in range will help clarify their long-term impacts. Management actions may be warranted in the future if they are shown to detrimentally exclude native species dependent on coastal nesting colonies or cause substantial damage to rice and fruit agriculture.