How do frigate birds hunt?

Frigate birds are a family of seabirds called Fregatidae that are found across tropical and subtropical oceans. They are large, black or black-and-white birds with long, narrow wings, deeply forked tails and hooked beaks. Frigate birds are well known for their aerial maneuverability and their ability to snatch food from the ocean surface and from other birds in flight.

Frigate birds do not swim and cannot walk well, and cannot take off from a flat surface. Having the largest wingspan-to-body-weight ratio of any bird, frigate birds are essentially aerial maritime animals, able to stay aloft for more than a week. Their long wings enable them to soar dynamically on thermals and oscillating wind gradients high over the ocean for hours without expending energy.

How do frigate birds hunt?

Frigate birds have a remarkable and unique hunting style that relies on speed, maneuverability and stealing food from other animals rather than catching their own prey. They employ a range of hunting techniques to optimize their ability to snatch food with their beak in an ecological niche reliant on fast reaction times. Here are some of the main strategies frigate birds use when hunting:

Plunge Diving

The most common hunting technique for frigate birds is plunge diving from high flight into the ocean to snatch prey at or near the surface. Using their excellent eyesight, frigate birds scan the water for fish, squid, krill and other prey. When a target is spotted, the frigate bird folds its wings back and plummets sharply down at steep angles, piercing the water surface like an arrow to snatch the prey in its beak. Frigate birds have streamlined plumage to aid in diving. The impact is cushioned by a special technique wherein the bird extends its gular pouch upon hitting the water.

Surface Seizing

Frigate birds also snatch food from the ocean surface while in flight. While cruising slowly along above the water, a frigate bird will dip its beak into the water to grab prey as it flies. This takes greater precision, as the bird cannot use gravity and momentum to its advantage. However, frigate birds are adept at grabbing fish and other prey from just below the surface without landing or diving. Their sharp eyesight and fast reflexes allow them to successfully seize food in this manner.

Kleptoparasitism

The most distinctive hunting behavior of frigate birds is kleptoparasitism or food stealing, whether in flight or on land. Using their speed and large wingspan, frigate birds chase other birds and even mammal marine animals in order to force them to regurgitate any food they have caught, allowing the frigate bird to steal the prey item before it hits the water. Frigate birds may also directly steal fish and squid carried in the beak of other birds in mid-air battles. Up to 95% of a frigate bird’s diet can be obtained through kleptoparasitism.

Unique adaptations

Frigate birds have evolved a range of specialized adaptations that enable their particular hunting techniques and way of life:

– Excellent aerodynamics – Frigate birds have the lowest wing loading (ratio of body weight to wing area) of any bird, allowing them to stay aloft more easily. Their large wings provide substantial lift.

– Streamlined feathers – Their feathers are lightweight, flexible and well adapted to getting wet, aiding plunge diving.

– Sharp eyesight – Their visual acuity is four to five times greater than humans, adapted for spotting prey from high altitudes.

– Fast reflexes – Frigate birds can rapidly initiate deep plunge dives and are nimble flyers, essential for successful food stealing.

– Large hooked beak – The large beak enables frigate birds to securely grab prey and is used as a weapon in kleptoparasitism.

– Powerful neck muscles – Their neck anatomy allows them to reach down and forward to grasp food while in flight.

– Gular skin pouch – The expandable throat pouch acts as a shock absorber upon hitting the water when plunge diving.

Hunting range and habitat

Frigate birds are found widely across the world’s tropical seas. They largely hunt over open waters far from land, but may also be seen hunting in shallow coastal waters and around islands. Their hunting habitat includes:

– Tropical oceans – Frigate birds hunt over warm ocean waters between latitudes of 30°N and 30°S, where sea temperatures are at least 20°C.

– Productive currents – Hunting is more successful where cold and warm ocean currents converge and nutrients rise, attracting fish.

– Inshore waters – Shallow waters around oceanic islands and over reefs provide hunting opportunities closer to breeding colonies.

– Surface waters – Frigate birds are surface hunters, hunting in the upper layers of the ocean rather than diving deep.

– Low precipitation areas – Their feathers are not waterproofed, so they avoid hunting in stormy, high precipitation areas.

Hunting seasons

Frigate birds may hunt year-round. However, their hunting frequency and range varies over breeding and non-breeding seasons:

Breeding season

– Frigate birds have a more limited hunting range during breeding season when incubating eggs and feeding chicks. They may make short foraging trips close to colonies.

– Hunting is focused on feeding young. Food stolen from other seabirds is brought back to chicks.

– One parent may hunt while the other incubates the egg. Parents take turns foraging.

Non-breeding season

– Non-breeding frigate birds have much wider hunting ranges and may be entirely pelagic, living constantly on the wing over open oceans.

– With no central nest site, non-breeders wander more widely searching for patchy food availability.

– Males and females disperse across broad areas away from breeding colonies to hunt.

– Hunting frequency increases as birds are focused entirely on self-feeding.

Prey

Frigate birds are opportunistic hunters and eat a variety of prey snatched from the ocean. Their key prey items include:

In addition to live prey, frigate birds engage in kleptoparasitism to steal already caught food like fish, squid and eggs from other seabirds. They may also eat jellyfish, zooplankton, crustaceans, carrion and even garbage. Parent frigate birds will regurgitate food to feed young chicks.

Hunting behavior

Frigate birds exhibit fascinating complex behaviors and adaptations associated with their hunting strategies:

Fast flight

– Cruise using efficient soaring and gliding between hunts, then unfold wings for powered fast flight to chase prey. Can reach speeds over 95 km/h.

High altitude

– Hunt from altitudes up to 2500 m, scanning for prey and monitoring other birds.

Plunge angles

– Adjust their plunge dive angle and trajectory depending on wind speed, height and moving prey.

Piracy

– Chase other birds for long distances and harass them to force them to release their food catch.

Ballast

– May soak their feathers before diving to give added mass for impact with the water.

Wet feathers

– Spread wings out after diving to help dry and maintain feather integrity.

Cooperative hunting

– May hunt in groups to improve success in stealing prey from other birds.

Conclusion

In summary, frigate birds are highly specialized hunters that employ skillful flight maneuverability, keen senses and swift reflexes to snatch prey without landing on the water. Their remarkable adaptations such as aerodynamic design, sharp vision, hooked bills and throat pouches allow them to hunt effectively through plunge diving and kleptoparasitism. Although they range widely over warm seas, frigate birds must successfully find food each day to survive given their lack of waterproofing, inability to land on water and limited capacity to carry much food at once. Their elegant and acrobatic hunting techniques are intrinsically linked to their evolutionary niche as deft oceanic aerialists.