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= Galápagos petrel =

The Galápagos petrel (Pterodroma phaeopygia) is one of the six endemic seabirds of the Galápagos. Its scientific name derives from Ancient Greek: Pterodroma originates from pteron and dromos, which translate to "wing" and "runner" respectively; phaeopygia comes from phaios and pugios, meaning "dusky" and "rump". Members of Pterodroma genus are also called the gadfly petrels because their erratic twisting and turning flight resembles that of gadflies.

Known by locals as pata pegada or "web-footed one", Galápagos petrels are highly adapted to the ocean and spend most of their time at sea eating invertebrates and fish. At sea, they are indistinguishable from the Hawaiian petrels: consequently, they were considered the same species for a while. When breeding season arises, they return to their large colonies of burrowed nests located on the highlands of some Galápagos islands.

The populations of Galápagos petrels have declined substantially over the last three generations (60 years) due to anthropogenic disturbances. Introduced predation (i.e. feral cats, dogs, pigs, and rats) and agricultural expansion have decreased the species' overall breeding success, survivability, and quality of habitats and nesting colonies. Although many conservation efforts are underway, its population trend is currently still declining with an estimate of 6,000-15,000 mature individuals remaining .The species is now classified under “critically endangered” in the IUCN Red List of Threatened Species.

Description
The data describing the Galápagos petrel are sparse because most were collected when it was still considered the conspecific of the Hawaiian petrel, which was assessed to a greater extent. However, they are quite similar.

The Galápagos and Hawaiian petrels are large, long-winged gadfly petrels. Their upperparts are almost uniformly gray, darkening gradually towards the tip of the wings and tail. A narrow white line on each side of their rump is often present. The species are also characterized by a distinct black hood that extends below their head and to the sides of the neck. In contrast, their face and underparts are white, but the underwings have black margins and a diagonal bar that extends across the coverts to near the base of the wings. While all Hawaiian petrels have entirely white foreheads, many Galápagos petrels have black freckles. Moreover, their legs and feet are pink, and the webbings are black.

Galápagos petrels do not exhibit sexual dimorphism and juveniles are indistinguishable from the adults. Although adult petrels sometimes acquire a brown tinge due to abrasion, their feathers remain relatively similar throughout the year. Since these subtropical birds spend most of their time at sea, the details of molt are not well known, but it seems like most feathers are replaced during the non-breeding season. Furthermore, like all gadfly petrels, Galápagos petrels have short, stout, hooked bills with sharp edges that are used to seize, manipulate and cut up small prey. One of the most remarkable features in Procellariforms, however, is that all have external tubular nostrils, suggesting that they have developed an ability to smell, detect and locate their food. They can even find their nest burrow within a colony in complete darkness.

Despite all the similarities, there are also some subtle differences between the Galápagos and Hawaiian petrels. Galápagos petrels appear to have longer and narrower bills than Hawaiian petrels. In addition, they are lighter (i.e. average for the Galápagos petrel is 420 g, whereas the Hawaiian petrel is 434 g) but have longer wings and tarsi. Altogether, their total wing area and weight-to-wing-area ratio are also slightly lower than that of the Hawaiian form, and therefore, Galápagos petrels have lower flight speeds, but more agility compared to the Hawaiian seabird.

Taxonomy
The Galápagos petrel is one of the 35 species of gadfly petrels within the Procellariidae family and Procellariiformes order, in which all members are collectively referred to as "petrels" .The order is one of the most primitive bird orders.

Taxonomically, its genus and family are extremely complex and are often subject to revision. Indeed, the Procellaridae family displays a very slow rate of speciation compared to other bird groups and consequently, classification in this group is more complicated. Plus, it appears to have differentiated from the other Procellariiformes over 40-50 million years ago in the Southern Hemisphere, although they are now widely distributed around the world. A DNA study revealed that the closest relatives to the petrels are the divers, penguins, and frigatebirds. However, many members of Procellariidae are still poorly known, as it is the case with the Galápagos petrel.

The species was first described as Estrelata phaeopygia by Salvin in 1876. In 1918, he renamed the Galápagos petrel because he thought it was conspecific of the Hawaiian petrel (together, they were also known as the Dark-rumped petrel); it became known as Pterodroma phaeopygia phaeopygia . However, their genetic variation, their geographical separation and their morphological and behavioral differences provided enough evidence to split them into two unique species. The Galápagos petrels were finally elevated to a species taxon in 1997 as Pterodroma phaeopygia. Plus, recent studies found that some genetic, morphometric and behavioural dissimilarities among individual island populations of the Galápagos petrel also exist. It appears that the Isabela population recently derived from the Santiago population, and the San Cristobal population is a combination of all the other populations.

Habitat and distribution
The Galápagos petrel is endemic to the Galápagos Islands. They are mainly distributed in the Galápagos Marine Reserve area, but studies have revealed that individuals frequently leave the reserve and disperse northeast towards South America and up to 2000 km southwards between January and February. Like other gadfly petrels, these subtropical seabirds are highly pelagic and are maladapted to the terrestrial habitats. Therefore, they are usually found far from the land and will only return to their nest to breed.

The nesting colonies are distributed among five islands: Santiago, Floreana, Isabela, Santa Cruz, and San Cristobal, which are all less than 170 kilometers apart. Moreover, multiple colonies can occur within one island; each can be as large as 200 meters by 300 meters and are separated by a distance that varies from 300 meters to 5 kilometers. Nowadays, the Galápagos petrels commonly nest offshore islets because their habitats have been destroyed or displaced by anthropogenic disturbances. Their nests typically occur in highlands that are at least 180 meters above the sea .Most can be found in thickly vegetated sites where the soil is soft enough to burrow deep cavities but some individuals also choose to nest in volcanic crevices. These ground nests make the species more vulnerable to introduced predators.

Behavior
In general, all Procellarids are very mobile and can travel long distances for several months. The Galápagos petrels are no exception, but it is still unclear what behavior they exhibit when it is not the breeding season. However, large aggregates of petrels around shoals or other rich food sources have been observed, although they are generally solitary feeders.

All Procellariids are adapted to the oceans but not so much to the lands. As a result, they only frequent landmasses for a prolonged period during the breeding season. Galápagos petrels are highly philopatric, meaning that every year they return to their natal colonies or the same nesting sites with the same partners. Also, these petrels are gregarious and form large discrete breeding colonies on islands. They are often strategically located to be easily accessible from the sea since it is where they spend most of their time.

Many petrels have developed nocturnal habits, including the Galápagos form. Indeed, they forage in the evenings and return to feed their offspring during the day. They often fly just above the water to feed on their prey without diving into the water. Since they have a sense of smell, they can easily detect food and their own nest in complete darkness. Furthermore, courtship also occurs in the night and the petrels are often seen towering into the sky and flying erratically as a display to attract a female.

Diet
Galápagos petrels are adapted to make efficient use of food resources which are typically distant from the colonies, patchily distributed and have low in nutritive value. The petrels predominantly take mesopelagic prey such as squids, small fish, crustaceans and other invertebrates. These are presumably taken at night because they are generally driven to the surface by tunas and porpoises, or they migrate vertically to feed at the surface. Plus, it appears that some of their prey (about 82% in number and 75% by mass) are bioluminescent, which may help the petrels detect them at night. They are also caught and eaten on wings by the Galápagos petrel.

During the nesting season, the parents take their turn to feed the chick by regurgitating their meal. A study of the chicks' stomach content revealed that they are fed semi-digested seafood mixed with a relatively large amount of high-caloric stomach oils produced in the proventriculus of the parent. These oils are the main source of energy and water for the chicks. Interestingly, the feeding rates are highest during the month that follows hatching but decreases as the chicks prepare to fledge. It is hypothesized that much energy is required in young chicks to develop their thermoregulatory ability just after hatching and their mass-specific metabolic rate is also the highest.

Vocalization
Most gadfly petrels tend to remain silent at sea but will become very vocal at night and when they are near their breeding colonies. During communal courtship, they sometimes emit various shrill cries. Typically, gadfly petrels will make squeaky calls when flying over burrows. From inside their burrows, they often produce different crooning or growling notes and various moaning cries. Studies have reported the Galápagos petrels calling from after sunset until dawn on all islands. This occurs when they circle or fly out of their colonies and during high-speed chases. Their calls reach a maximum just before sunrise when the number of birds in the air increases. However, one study also noted that they were silent for most of the night, and only began aerial calling a few hours before sunrise (6 AM).

The same study established that Galápagos petrels mainly use three flight calls. There is a short 3-5 syllable call that is possibly used for identification; a long call of 6-20 syllables that may be used in aggressive or defensive situations; and a single-syllable call that is seemingly used for extreme stress situations. The short and long calls are sexually dimorphic: calls in males are sweet and pleasant but are coarser and grating in females. Interestingly, individuals of different islands also have different dialects that distinguish them. In comparison to the Hawaiian petrel, their vocalizations are quite different. The main flight call recorded sounded like “kee-kee-kee-(c)ooo” with the last note drawn out.

Reproduction
Pterodroma species reach sexual maturity at age 5-6 and nest once every year from then on. The breeding season occurs at different times for different Galápagos petrel populations. In Santa Cruz and Santiago, the season of the petrel starts from March to January; in Isabela, from the end of April to January; in Floreana, from October to August; and in San Cristobal, from May to October. Although some seasons overlap, there is little to no interbreeding between populations. These populations have likely diverged from each other because of their rigid cycle and high fidelity to their mate and nesting site.

Newly matured individuals either dig or take up abandoned rabbit burrows while others will prepare for nesting by enlarging their burrows and gathering dry vegetation. When finding their life-long partner, male gadfly petrels have different displays to attract females. They exhibit elaborate courtship flights that include aerial chasing, high-speed swoops, and towering, which are accompanied by loud calls. Breeding pairs associate in their burrows and engage in mutual preening and series of calls. After copulation, they return to the sea to feed abundantly in prepare for the gestation period and the subsequent long incubation period. During this time, which lasts approximately two weeks, each of the pair will pay short visits to the nest so that others don't think it's deserted.

Each Galápagos petrel pair lay one egg that is white and ovate. In a study, the eggs of the Galápagos petrels weighed about 17 or 20% of the female body weight, but on average eggs weigh 20-24% of that in all gadfly petrels. The incubation period is about 50 days, and the male and female take turns to incubate the egg for an average of 12 days. While the incubating parents lose an average of 10-15 g per day, the others are foraging at sea with no evidence that they return to fed their mates. Although the eggs are rarely left unattended, both parents will sometimes leave the nest to forage at sea when food is scarce. As an adaptation to this, the eggs are resistant to the cold. However, the introduced black rat (Rattus rattus) prey on the eggs.

Upon hatching, chicks are covered by an off-white down on the front and sides of the throat, as well as the center of the belly. Parents take turns feeding them semi-digested foods and stomach oils and the chicks quickly become fat, exceeding the weight of their parents. The Galápagos petrel chicks have adapted to the scarcity of food as they are resistant to starvation. The second down appears after around 10 days and the feathers, after about 5 weeks. Once the chicks are fully feathered and look like molted adults, parents stop their visits. Overall, their nesting success is low.