Florida’s waterbirds: Expert divers with bold hunting techniques

Florida’s wetlands are home to two striking and often confused waterbirds – the anhinga and the double-crested cormorant. Both are expert divers that rely on their feet to propel themselves underwater. They are commonly seen perched on branches or docks with their wings spread wide to dry.

However, despite their similar appearances and behaviors, new research from the University of Miami reveals that these birds are quite different in their physiology, hunting techniques, and metabolism.

A research team led by Jeff White, a recent biology doctoral graduate, spent months studying these species to understand their unique adaptations.

The findings, which build on earlier work by ornithologist Oscar “Bud” Owre, offer fresh insights into these fascinating birds.

How anhingas and cormorants hunt

To the casual observer, anhingas and cormorants might seem interchangeable.

Both birds dive beneath the water’s surface in search of fish and then emerge to dry their fully soaked feathers in the sun. However, their approaches to foraging and movement in the water set them apart.

Cormorants are aggressive hunters. They chase their prey with strong, rapid movements, often diving deep and pursuing larger fish. Their ability to consume more oxygen allows them to sustain this high-energy hunting style.

“Cormorants have a really large muscle in their leg and are able to really prioritize storage of oxygen in their leg muscles,” explained White, who is affiliated to the University of Miami. This muscle adaptation gives them an edge in deep-water environments, including coastaland oceanic waters.

In contrast, anhingas are stealthy ambush predators. Rather than chasing fish, they wait patiently in shallow water, camouflaged among vegetation, until their prey swims close enough for a lightning-fast strike.

This sit-and-wait strategy is common among ambush predators like herons and alligators, but it is unusual among diving birds. By minimizing movement, anhingas conserve energy and use less oxygen than cormorants.

How do they stay underwater?

Both species dive for about two minutes at a time, but their bodies handle oxygen storage differently. Unlike ducks and other diving birds, anhingas and cormorants fully wet their feathers.

This reduces buoyancy and allows them to stay underwater more easily. However, their methods of oxygen management vary.

“They both must hold their breath to dive, so we observed how each of these species convert their oxygen stores into energy in the swimming and flying muscles,” noted McCracken, a co-author of the study.

Oxygen use of anhingas and cormorants

Cormorants store most of their oxygen in their leg muscles, which power their aggressive swimming style. These large, oxygen-rich muscles allow them to generate bursts of speed and maneuver swiftly to catch prey. This trait aligns them more with deep-diving seabirds like penguins.

Anhingas, on the other hand, rely on neutral buoyancy, meaning they neither sink nor float. This allows them to remain motionless under the water without needing to swim to stay in place.

“They are the only aquatic bird I know of that can sit underwater without having to swim to stay in place,” said White. This passive diving adaptation is unique among aquatic birds and helps anhingas hunt with minimal energy expenditure.

Additionally, anhingas have a low metabolism, which means they require less oxygen to generate energy. While cormorants prioritize storing oxygen in their muscles, anhingas make efficient use of the oxygen in their blood.

Flight differences: Soaring vs. powerful wingbeats

Beyond their diving behaviors, the two species differ in flight mechanics.

Cormorants are strong fliers. Their powerful wing muscles allow them to fly quickly and with sustained flapping, making them more agile in the air. This strength is necessary since they often travel between freshwater and saltwater environments in search of food.

Anhingas, by contrast, are less suited for continuous flapping. Instead, they soar and glide, using thermals (rising air currents) to stay in the air with minimal effort. This flight style is similar to hawks and vultures, rather than the rapid, flapping flight of cormorants.

Significance of the research

While this study sheds light on anhingas and cormorants, its implications reach far beyond ornithology. Understanding how these birds manage oxygen could offer insights into human physiology, exercise science, cardiovascular health, and metabolism.

“There might be benefits to understanding how other species can generate energy without oxygen,” White noted. Some medical conditions and athletic performance studies could benefit from learning how these birds use oxygen efficiently.

Additionally, this research helps scientists better understand metabolism. Different species process nutrients and oxygen in distinct ways, and the anhinga’s ability to function with minimal oxygen use is a fascinating case study.

Future research on anhingas and cormorants

Despite this latest research, anhingas remain one of the least studied aquatic birds. Jeff White hopes to continue his work to uncover more about their unique adaptations. He realized that scientists know surprisingly little about these birds, especially compared to other diving species.

Future studies could explore how anhingas achieve neutral buoyancy, allowing them to stay underwater without sinking or floating. Researchers also want to examine their metabolic rates to understand how they survive on lower oxygen levels compared to other diving birds.

Additionally, studying their evolutionary adaptations could reveal how they developed their distinct physiological traits over time.

Anhingas’ unique energy conservation and oxygen use strategies could also help scientists learn more about metabolism in other animals, including humans. Further research will help unravel the mysteries of this remarkable waterbird.

New perspective on Florida’s waterbirds

For those who frequent Florida’s lakes, swamps, and coastal waters, the anhinga and the double-crested cormorant are familiar sights. Yet, beneath their seemingly similar exteriors, these birds have evolved very different strategies for survival.

Cormorants are the powerful, fast-moving predators, excelling in deep-water hunting and flight. Anhingas are masters of stealth and efficiency, conserving energy and using their neutral buoyancy to ambush prey.

Next time you see one of these birds perched with wings outstretched, take a closer look. The mystery of their adaptations is still unfolding, and there’s much more to learn about these remarkable waterbirds.

The study is published in the Journal of Comparative Physiology B.

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