Bird O'clock

Audubon’s Shearwater: The Mysterious Seabird of The Open Ocean

Audubon’s Shearwater, scientifically known as Puffinus lherminieri, is a medium-sized seabird that commonly breeds on offshore islands. This elegant bird is an expert at gliding over the ocean waves while searching for fish near the surface of the water.

Here is everything you need to know about Audubon’s Shearwater, including its identification, plumage, and molts.

Identification

Field

Identification: When it comes to field identification, Audubon’s Shearwater has a distinct appearance that sets it apart from other shearwaters. It has a dark grey-brown upper body with a white throat, breast, and belly.

Its bill is black, and its legs and feet are pink. Audubon’s Shearwater also has a forked tail and long, narrow wings that enable it to fly for long distances with minimal effort.

Similar Species: Audubon’s Shearwater can sometimes be confused with other shearwaters that have a similar look to it. Cory’s Shearwater is one such species that can be difficult to distinguish from Audubon’s Shearwater because of their similar coloration.

However, Cory’s Shearwater has a more robust bill and a broader head. Other shearwaters like Great Shearwater, Sooty Shearwater, and Manx Shearwater can be distinguished from Audubon’s Shearwater by their unique plumage and flight characteristics.

Plumages

Audubon’s Shearwater undergoes two plumage changes during its life cycle. Juvenile birds have a brownish-black upper body, with feathers edged in buffy-white coloration.

Underparts are creamy-white with delicate white barring across the upper chest area. The bill and feet are dark-colored.

Over time, the juveniles molt into an adult plumage. Adult birds have a dark grey-brown upper body with a white throat, breast, and belly.

The legs and feet are pink, and the bill is black. During breeding season, adults develop a ruff of elongated pale feathers on their throat and upper breast, which is thought to be used in courtship behavior.

Molts

Audubon’s Shearwater undergoes two molts each year. The pre-basic molt occurs after the breeding season, usually from July to September, where birds replace their feathers.

The second molt is the pre-alternate molt, which occurs in late winter or early spring, usually between January and April. During this molt, birds replace their feathers to prepare for the upcoming breeding season.

Conclusion

Audubon’s Shearwater is a beautiful and fascinating bird that is well-suited for life on the open ocean. Its unique appearance, distinct plumage, and molting behavior make it a unique and interesting bird species to study.

With this guide, you should now be able to identify Audubon’s Shearwater and appreciate its spectacular features.

Systematics History

Audubon’s Shearwater, scientifically known as Puffinus lherminieri, belongs to the family Procellariidae, which comprises a group of seabirds that inhabit the world’s oceans. The species was first described by French ornithologist Louis Jean Pierre Vieillot in 1818, who named it after his friend and famous ornithologist John James Audubon.

Since its original description, the taxonomy of Audubon’s Shearwater has undergone many changes as new data insights became available.

Geographic Variation

Audubon’s Shearwater has a wide distribution range, and different populations exhibit varying levels of genetic differentiation due to geographic isolation and historical events. The species breeds on tropical and subtropical islands in the Caribbean, West Africa, the Indian Ocean, and the Pacific.

It also migrates over open water towards the eastern Pacific Ocean up to the western coast of South America.

Subspecies

Based on geographic variation, several subspecies of Audubon’s Shearwater were once recognized. However, subsequent mitochondrial DNA analyses have indicated that this variation is likely clinal, meaning that subtle genetic differences exist along a gradient without any clear boundary between populations.

Currently, the species is recognized as a single polytypic species without subspecies.

Related Species

Audubon’s Shearwater belongs to the genus Puffinus, which consists of around 20 species that breed in different parts of the world’s oceans. Among shearwaters, Audubon’s is most closely related to the closely-related Wedge-tailed Shearwater (Puffinus pacificus) from the Pacific and the critically endangered Balearic Shearwater (P.

mauretanicus) from the Mediterranean. These three species were once classified as subspecies of a single widespread species known as Manx Shearwater (P.

puffinus).

Historical Changes to Distribution

Historical records suggest that Audubon’s Shearwater was formerly abundant in the eastern Pacific and breeding colonies were present on Guadalupe Island and Santa Barbara Island off California’s coast. However, within a century, the breeding colonies vanished from the California archipelago, and populations in Eastern Pacific ocean subwaters became less.

The reasons behind the decline in the Eastern Pacific are unclear, but several hypotheses have been suggested. The birds could have been affected by invasive species like rats and cats on the islands where they breed.

In addition to this, human activities such as fishing, pollution, oil exploration, and natural events like El Nino could also have had adverse impacts on the bird’s population. Despite this, the bird continues to maintain a significant population globally and currently breeds on dozens of tropical islands in the Caribbean, Africa, the Indian Ocean, and the Pacific.

Recently, the species was listed as a “near threatened” species by the International Union for Conservation of Nature. The global population is estimated to be around two million individuals, but local populations can be vulnerable to habitat destruction, climate change, and hunting in some areas.

The current trend of its population is stable, but it is necessary to monitor the population carefully to prevent it from declining in the future.

Conclusion

Audubon’s Shearwater is an important member of the world’s seabird community, with a wide distribution range spanning the tropical and subtropical oceans. The species has undergone changes in taxonomy and distribution over time, but it continues to survive despite facing several anthropogenic and ecological challenges.

The current conservation status of the bird shows that more efforts are needed to improve its welfare in the face of various threats it encounters. Habitat:

Audubon’s Shearwater is adapted to life on the water and is well-suited to its oceanic environment.

The bird spends most of its life at sea, typically foraging near the surface of the water for small fish and squid. During the breeding season, Audubon’s Shearwater returns to rocky or sandy offshore islands to nest.

The birds are known to nest in burrows, crevices, and under rocks, and they typically select habitats that are relatively sheltered from predators. Movements and Migration:

Audubon’s Shearwater is an expert flyer and has been reported to travel vast distances in search of food and suitable breeding sites.

The bird often travels long distances across the open ocean, but it rarely comes to land except for breeding. The movements of birds outside of the breeding seasons are not well understood, and tracking studies are challenging to conduct due to the subtropical and tropical locations where the birds frequently occur.

In the Caribbean, where the birds breed in proximity, it was found that movements were generally local with a radius of ~30km from the colony. In the eastern Pacific, migration patterns seem to be more complex.

Studies show that the birds that breed on the Galapagos Islands move to the Peruvian waters to feed, whereas other scientists also report birds moving northward towards the Gulf of Tehuantepec and California current. These diverse movements could be associated with El Nini and other climatic events, which alter ocean currents and primary productivity.

Audubon’s Shearwater breeding colonies are typically located on isolated islands, where habitats that are not disturbed by humans may persist, offering a reliable breeding location. Some colonies can be vulnerable to predation by invasive species like rats and cats, which can decimate entire populations.

On the other hand, the offshore islands typically support diverse seabird communities that can offer additional foraging sites and safe roosting places for migrants.

Considering the global distributions of Audubon’s Shearwater, the species faces different threats in different habitats.

Fishing is a significant impediment that the species encounters globally. It reduces the abundance of baitfish that the species depends upon to forage.

Furthermore, overfishing can have significant impacts on the species’ population, especially in breeding areas, where the species may be culled for consumption purposes. Global warming may also reduce the quality of habitat through changes in temperature, atmosphere, and chemistry of the climate; thus, it is important to monitor the population of Audubon’s Shearwater in case of future risk.

Overall, the movements and migration patterns of Audubon’s Shearwater are complex and poorly understood, but they are critical to the species’ well-being. The species’ reproductive success is dependent on locating suitable breeding habitat, and its long-term survival depends on the ability to forage for food.

The offshore islands and shallow continental shelves are critical habitats that cannot be disregarded during conservation efforts. Efforts aimed at preserving the habitats and food sources of this bird species are crucial to secure its long-term survival.

Diet and Foraging

Feeding:

Audubon’s Shearwater is an opportunistic forager and feeds mainly on small fish and squid found near the surface of the water. The birds often feed by surface plunging, where they swoop down and scoop up prey with their bills.

While feeding, Audubon’s Shearwaters are known to feed together in groups, often in association with other seabirds. Diet:

The bird’s diet varies based on the availability of prey, but some of the common types of food consumed by Audubon’s Shearwater include small fish such as anchovies and sardines and squid and crustaceans.

The species also feeds on small planktonic invertebrates depending on the location of the feeding area. Metabolism and Temperature Regulation:

Audubon’s Shearwater has a relatively low metabolic rate, which enables it to fly efficiently over long distances while conserving energy.

To regulate their body temperature, the birds have an oil gland near their tails, which they use to secrete a waxy fluid that helps to waterproof their feathers. By doing so, they maintain their body temperature in cold waters.

Sounds and Vocal

Behavior

Vocalization:

Audubon’s Shearwaters are relatively quiet birds, and they vocalize infrequently outside of the breeding season. Their vocalization in breeding colonies comprises a wide repertoire of harsh, croaking, and grunting calls that make up their courting and territory-defending vocalizations.

The male bird creates a “wheer-wheer-wheer” calling sound, while the female has a slower, lower-pitched call. During the breeding season, the birds vocalize frequently to communicate with their partners and potential mates.

The birds also rely on vocal cues to locate their nests and defend their territories. The calls of Audubon’s Shearwater can be heard from a good distance around the breeding area, where they gather in massive numbers and create a noisy ambiance.

Conclusion:

Audubon’s Shearwater is a significant player in the marine ecosystem, where it feeds primarily on small fish and squid near the surface of the water column. To conserve energy, the species has a relatively low metabolic rate that also helps it fly for long distances out in the open water efficiently.

When it comes to communication, the birds have a wide repertoire of vocalizations comprising grunting, croaking, chirping, and calling sounds that they use to communicate with one another during their breeding season. The vocalizations play an important role in partner selection, territory defense, and parent-offspring communication.

Overall, understanding these important aspects of the species helps us gain insight into the ecology and biology of Audubon’s Shearwater and highlights the importance of fostering conservation practices. By monitoring and protecting the breeding colonies, securing food sources, and ensuring that their habitats and food sources are preserved, we can ensure the continued survival of the species.

Behavior

Locomotion:

Audubon’s Shearwater demonstrates a unique flying behavior, which is characterized by the use of dynamic soaring, a technique that allows the bird to exploit wind gradients. The bird can gain speed through wind gusts and then slow down by climbing in the updrafts to repeat the cycle and maintain high speed flights.

This technique allows the bird to use minimal energy while flying long distances. Self Maintenance:

Audubon’s Shearwater is a relatively self-maintaining bird species and relies on preening to keep its feathers clean and ensure efficient flight.

The bird’s uropygial gland, located near the tail, secretes oil, which is then distributed by the bird during preening to ensure that their feathers remain waterproof and aligned. Agonistic

Behavior:

Audubon’s Shearwaters exhibit agonistic behavior, which is characterized by actions aimed at establishing dominance and maintaining territories.

During the breeding season, these behaviors are exhibited through vocalizations and physical interactions that help the birds establish their respective territories. Mature birds are known to engage in physical battles during the breeding season to assert dominance and ensure reproductive success.

Sexual

Behavior:

During the breeding season, Audubon’s Shearwater typically forms monogamous pairs. The birds participate in a courtship display that involves performing aerial acrobatics and vocalizations before engaging in copulation.

The pair bond established during the breeding season can last for several years, and the birds often return to the same breeding site each year to re-establish their nests.

Breeding

Audubon’s Shearwaters are colonial breeders, and they typically establish their nests on offshore islands that offer suitable habitat and safety from predators. The birds arrive at their breeding sites in January, where they establish their territories and attract their mates through courtship display.

The birds nest in burrows or crevices in rock outcrops, and they create a nest made up of soil and plant material. After the female bird lays her egg, both parents take turns incubating it until it hatches.

Once the egg hatches, the parents take turns feeding the chick with regurgitated food. The chick grows quickly and can fledge within sixty days.

After fledging, young birds leave the nest and start feeding on their own.

Demography and Populations

The global population of Audubon’s Shearwater is believed to be around two million individuals, with no significant decline being reported yet. Despite the general stability in numbers, some populations of the bird are threatened by several factors such as invasive species, predation, habitat destruction, and bycatch in fisheries.

Populations in some locations have seen significant improvements, thanks to conservation practices focusing on removing invasive species from breeding islands, reducing bycatch and preventing habitat destruction. Audubon’s Shearwater is currently considered a species of Least Concern by the International Union for Conservation of Nature (IUCN).

However, it is noted that continued monitoring of the population and measures to address threats to the species are required to maintain the stability of global populations and prevent localized decline. Audubon’s Shearwater is a fascinating seabird that has evolved specialized adaptations to enable its survival in an oceanic environment.

This bird species uniquely uses wind currents to fly efficient long distances, and it feeds primarily on small fish and squid near the surface of the water. Furthermore, they have complex vocalizations used for communication, which is highly crucial during their breeding season.

The bird also exhibits unique agonistic behavior and engages in monogamous pairing during the breeding season, which is crucial to the survival of the species. Audubon’s Shearwater is a significant member of the marine ecosystem, and the understanding of its ecology and biology highlights the importance of implementing conservation measures to secure the continued survival of the species.

Conserving the offshore islands where they breed, reducing the fishing pressure, protecting prey species, and monitoring populations are among the measures needed to conserve the species. While the bird’s global population remains stable, the continued monitoring of the population is necessary to sustain its numbers and ensure its survival for generations to come.

Popular Posts