The wood stork ( Mycteria americana) is the only stork species native to North America and is a large wading bird found in subtropical and tropical regions of the Americas. Although the wood stork may not resemble the typical European white stork, it is indeed a true stork, belonging to the family Ciconiidae, which includes 17 species distributed across temperate and tropical zones.
Distribution and Habitat
Wood storks are commonly found nesting from coastal Mexico and northern Argentina, across interior South America, and in the Caribbean islands such as Cuba and Hispaniola. Their range extends north into the southeastern United States, including Florida, Georgia, and South Carolina. These storks thrive in various wetland habitats with fluctuating water levels, which are crucial for initiating breeding. They typically nest in trees standing in or surrounded by water, favoring Taxodium trees in freshwater habitats and mangrove forests in estuarine environments.
General Feeding Habits
Wood storks are opportunistic feeders, consuming a variety of prey such as fish, frogs, crayfish, large insects, and occasionally small alligators and mice. However, their diet primarily consists of fish, especially those ranging from 1 to 6 inches in size. This preference for larger fish means that storks often feed on fish that have grown for more than one season.
Detailed Dietary Components
Studies of wood stork diets have revealed a diverse range of food items. While fish constitute the bulk of their nutrition, they also consume aquatic invertebrates, aquatic insects, snakes, baby alligators, small turtles, frogs, rodents, and even some seeds and plant material. During the dry season, their diet mainly comprises fish, supplemented by insects, whereas, in the wet season, fish constitute about half of their diet, with crabs making up approximately 30%, and insects and frogs accounting for the remainder.
Notably, wood storks tend to eat larger fish more often than smaller fish, even when the latter are more abundant. An adult wood stork requires an estimated 520 grams (1.15 lb) of food per day to sustain itself.
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Foraging Strategies
Wood storks employ specialized tactile strategies to capture their food, a method known as "grope feeding" or tacto-location. They wade through shallow water, moving their partially-opened beaks back and forth beneath the surface. When their bill touches a fish or other prey, it snaps shut with remarkable speed. Upon catching a fish, the stork raises its head and swallows the prey.
They forage in flocks when not breeding but often forage alone or in small groups during the breeding season. In the dry season, they typically walk slowly with their bill submerged in water, groping for food. During the wet season, this method is used about 40% of the time, while foot stirring, where the stork pumps its foot up and down before each step, is used about 35% of the time.
Wood storks also exhibit other interesting foraging behaviors:
- They monitor the activities of other wading birds to locate potential food sources.
- In areas with algae and aquatic plants, they plant their open beak and use one leg to shake the aquatic plants, startling and dislodging hidden prey.
- They may spread a wing rapidly over the water to create a shadow, which startles and dislodges prey into moving past the open beak.
Dependence on Environmental Conditions
The wood stork's foraging success is highly dependent on environmental conditions, particularly shallow water and a high density of fish. The water depth in which they forage averages about 17 centimeters (6.7 inches) during the dry season and about 10 centimeters (3.9 inches) during the wet season. Falling water levels are especially beneficial because they concentrate fish and other prey in remaining pools.
Feeding the Young
Both parents participate in feeding their chicks by regurgitating food onto the nest floor. The chicks are primarily fed fish that range from 2 to 25 centimeters (0.79 to 9.84 inches) in length, with the size of the fish increasing as the chicks grow older. The amount of food provided to the chicks increases from hatching to about 22 days, after which food intake levels off until about 45 days, when consumption starts to decrease.
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Threats to the Wood Stork Population
Several factors can result in the death of wood storks:
- Breeding-related mortality: Only about 1.3 nestlings survive to become independent from an average clutch size of 3 eggs per nest. Predators, nest collapse during storms, and insufficient food supplies leading to starvation are common causes of egg and nestling loss.
- Mortality of adult storks: Predators like alligators and bobcats, pesticide poisoning, collisions with power lines, and being struck by automobiles are factors that cause adult deaths.
- Habitat Loss: Habitat alteration has been implicated as the main threat to wood stork populations in the United States.
- Human Interference: Hunting and egg-collecting by humans has been implicated as a factor in the decline of South American wood storks.
Conservation Status and Efforts
Globally, the wood stork is classified as being of least concern by the International Union for Conservation of Nature due to its large range. However, in the United States, the species is considered to be threatened.
Observing Wood Storks
Most wood stork colonies are located on private lands or in remote swamp areas, making them difficult to access. Coastal colonies are more accessible by boat traffic. If you wish to observe these nesting birds, it is recommended to use binoculars and maintain a minimum distance of 100 yards from the colony. Disturbing storks can cause them to flush from their nests, exposing eggs or young nestlings to overheating or cooling, as well as to predators.
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