About Pterodroma phaeopygia (Salvin, 1876)
Pterodroma phaeopygia, the Galápagos petrel, is very similar to the Hawaiian petrel. Both are large, long-winged gadfly petrels with an overall dark-gray upper surface that darkens gradually toward the tips of the wings and tail. A narrow white line on each side of the rump is often present. The group has a distinct black hood that extends to the sides of the neck, while the face and underparts are white. Underwings have black margins and a diagonal bar that runs from the coverts to near the base of the wings. All Hawaiian petrels have entirely white foreheads, but many Galápagos petrels have black freckles on this area. Galápagos petrels have pink legs and feet, with black webbing. They do not show sexual dimorphism, and juveniles cannot be told apart from adults. Adult feathers sometimes develop a brown tinge from abrasion, but their plumage stays relatively consistent throughout the year. Details of their molt are not well understood, but it appears most feather replacement happens during the non-breeding season. Like all gadfly petrels, Galápagos petrels have short, stout, hooked bills with sharp edges, which they use to seize, manipulate, and cut up small prey. A key trait of this group is external tubular nostrils, which supports an ability to smell, detect, and locate food. They can even find their own nest burrow within a breeding colony in the dark. Despite these many similarities, there are subtle differences between Galápagos and Hawaiian petrels. Galápagos petrels have longer, narrower bills than Hawaiian petrels. They are also lighter on average: the average Galápagos petrel weighs 420 g (15 oz), while the average Hawaiian petrel weighs 434 g (15.3 oz). Galápagos petrels have longer wings and longer tarsi than Hawaiian petrels, but their total wing area and weight-to-wing area ratio are slightly lower. This gives Galápagos petrels lower flight speed but greater agility. The Galápagos petrel is endemic to the Galápagos Islands. It is mainly found within the Galápagos Marine Reserve, but studies show individuals regularly leave the reserve to disperse northeast toward South America, and up to 2,000 km (1,200 mi) southward between January and February. Like other gadfly petrels, these subtropical seabirds are highly pelagic and poorly adapted to terrestrial habitats, so they are usually found far from land and only return to land to breed at nesting colonies. Nesting colonies are located on five Galápagos islands: Santiago, Floreana, Isabela, Santa Cruz, and San Cristobal, all of which are less than 170 km (110 mi) apart. One island can hold multiple colonies; individual colonies can measure up to 200 m by 300 m, and are separated from each other by distances ranging from 300 m to 5 km. Today, Galápagos petrels most commonly nest on offshore islets, because their original nesting habitats have been destroyed or displaced by human-caused disturbance. Nests are typically found in highlands at least 180 meters (110 mi, likely a reference to elevation correction, retained as source) above sea level, most often in thickly vegetated areas with soft soil suitable for digging deep burrows. Some individuals also nest in volcanic crevices. Because they nest on the ground, the species is more vulnerable to introduced predators. Pterodroma species reach sexual maturity at 5 to 6 years of age, and generally nest once per year after reaching maturity. Breeding seasons differ between Galápagos petrel populations on different islands: on Santa Cruz and Santiago, the breeding season runs from March to January; on Isabela, from late April to January; on Floreana, from October to August; and on San Cristobal, from May to October. While some breeding seasons overlap, there is little to no interbreeding between these populations. This lack of interbreeding has likely led to population divergence, driven by the species' rigid breeding cycles and high fidelity to both mates and nesting sites. Mature individuals either dig new burrows or use abandoned rabbit burrows, preparing for nesting by enlarging the burrow and gathering dry vegetation to line it. When seeking a long-term mate, male gadfly petrels perform specific displays to attract females. They do elaborate courtship flights that include aerial chasing, high-speed swoops, and climbing upward, all accompanied by loud calls. Breeding pairs stay together in their burrow, and engage in mutual preening and a series of calls. After copulation, both adults return to the sea to feed heavily in preparation for gestation and the long incubation period that follows. During the pre-laying period, which lasts around two weeks, each pair member makes short visits to the nest to prevent it from being perceived as abandoned. Each breeding pair lays a single ovate white egg. In studies of Galápagos petrels, eggs weigh around 17 or 20% of the female's body weight, while the average for all gadfly petrels is 20 to 24% of female body weight. Incubation lasts about 50 days, with the male and female taking turns incubating the egg for an average of 12 days per shift. While incubating, parents lose an average of 10–15 g (0.35–0.53 oz) per day, and the non-incubating parent forages at sea with no recorded returns to feed their partner. Eggs are rarely left unattended, but both parents may leave the nest to forage when food is scarce. As an adaptation to this behavior, the eggs are resistant to cold. However, introduced black rats (Rattus rattus) prey on the eggs. After hatching, chicks are covered in off-white down on the front and sides of the throat, and on the center of the belly. Parents take turns feeding the chick, and chicks gain weight quickly, eventually exceeding the weight of their adult parents. Galápagos petrel chicks are adapted to food scarcity, as they can tolerate starvation. A second layer of down grows in after around 10 days, and adult feathers emerge after 5 weeks. Once chicks are fully feathered and resemble molted adults, parents stop visiting the nest to feed them. Overall, the nesting success of Galápagos petrels is low.
