The brown pelican (Pelecanus occidentalis) is a large seabird found on the coasts of North and South America. As adults, brown pelicans have few natural predators, but eggs and nestlings are vulnerable to a variety of threats.
Main predators of brown pelican eggs and nestlings
The main predators of brown pelican eggs and nestlings are:
- Raccoons
- Coyotes
- Foxes
- Skunks
- Feral cats
- Snakes
- Gulls
- Crows
- Opossums
- Rodents
These predators often raid pelican nesting colonies to eat eggs and nestlings. Raccoons are perhaps the most significant predator, especially on coastal islands where they have been introduced. One study found raccoons were responsible for up to 98% of brown pelican egg and nestling mortality in some areas.
Other animal predators
In addition to those main predators, brown pelican eggs and nestlings may also face threats from:
- Bald eagles
- Peregrine falcons
- Ravens
- Vultures
- Night herons
- Egrets
- Owls
- Hawks
- Sandhill cranes
- Sharks
- Fish (e.g. garfish)
These predators are more opportunistic and account for a smaller amount of nest predation overall, but can still have local impacts. Bald eagles in particular have been known to prey heavily on eggs and nestlings in some brown pelican colonies.
Humans as predators
Historically, humans greatly impacted brown pelican populations through hunting the adults and collecting their eggs. Brown pelicans were hunted for their feathers and meat, and their eggs were taken for food. These practices led to huge declines in brown pelican numbers in the late 19th and early 20th centuries.
Following legal protections starting in the 1900s, most direct human predation on brown pelicans has ended. However, human activity still indirectly impacts nesting success when development encroaches on nesting islands. Disturbance, habitat loss, light pollution and other factors linked to human presence can make colonies more vulnerable to predators.
Predation on adult pelicans
While predation on eggs and nestlings is common, predation on adult brown pelicans is rarer due to the large size of these birds. Possible predators of adult pelicans include:
- Coyotes
- Foxes
- Bobcats
- Cougars
- Golden eagles
- Bald eagles
- Great horned owls
- Alligators
- Killer whales
- Sharks
These predators may opportunistically attack injured or vulnerable individuals, but healthy adult pelicans are less frequently targeted. Eagles are the main aerial predator known to strike adult brown pelicans.
Predation rates and impacts on populations
The overall impact of predation on brown pelican populations depends on various factors including:
- The number and density of predators in colonies
- Availability of alternative prey for predators
- Isolation of nesting islands from predator access
- Defenses used by pelicans against predators
- Phase of breeding cycle (eggs or nestlings)
- Health and age of adult pelicans
- Habitat quality and disturbances
In areas with many predators and little protection, over 90% of eggs can be taken and nest productivity is very low. Just a few predators can quickly decimate a colony. However, pelican populations can rebound if predicate are controlled and favorable conditions return.
Nesting island isolation and protection
Brown pelicans rely on predator-free nesting islands to achieve high reproductive success. Isolation from land predators is a key protective factor. Some conservation efforts have eliminated predators from nesting islands or installed predator exclusion devices.
Defensive strategies
Brown pelicans have evolved some defensive strategies against predators. Adult pelicans may mob or dive-bomb predators in colonies. Nests are often located on cliff edges or up in trees which provides some protection. Adults also aggressively defend nests from opportunistic gull or crow attacks when present.
Synchronized nesting
Brown pelicans tend to nest in huge seasonal colonies. This synchronized nesting helps swamp predators. The sheer number of nests reduces the per capita impact of predation during critical phases. Pelicans may also mob predators more effectively when dense aggregations are formed.
Predator population impacts
While predation on eggs and nestlings is expected, very high levels of loss can limit pelican population growth. Sustained predation pressure prevents colonies from reaching carrying capacity. Some studies have identified predation as the primary factor limiting pelican reproductive success and populations in certain regions.
However, the overall impact depends on the extent, since pelicans are long-lived and can offset nest losses with improved adult survival and breeding success in other years. Complete reproductive failures are more concerning for population impacts.
Case study: Predation impact in Louisiana
Studies in Louisiana provide examples of high predation rates:
- One study found only 15% of pelican eggs survived to hatching due to heavy predation.
- Raccoons were estimated to take up to 96% of eggs from some colonies.
- Coyotes took up to 91% of eggs on islands where they occurred.
- Colonies protected from predators had very low egg losses.
This demonstrates how substantial predation effects can be for brown pelican reproductive success. Louisiana implemented predator control programs to help boost nesting productivity by reducing raccoon, coyote and other predator populations in key colonies.
Conclusions
In summary, the main predators of brown pelicans are:
- Raccoons – The most impactful nest predator overall
- Coyotes – Also very significant in many regions
- Other mammals like foxes, skunks, feral cats
- Gulls, crows and other opportunistic birds
These predators mainly target eggs and nestlings, but can severely reduce colony reproductive success when unchecked. Isolation of nesting islands and predator control programs help to limit impacts and boost pelican breeding productivity.