Storm-petrels - Breeding

Breeding

Storm petrels nest colonially, for the most part on islands; although a few species breed on the mainland, particularly Antarctica. Nesting sites are attended nocturnally in order to avoid predators; although the Wedge-rumped Storm Petrels nesting in the Galapagos Islands are the exception to this rule and attend their nesting sites during the day. Storm petrels display high levels of philopatry, returning to their natal colonies to breed. In one instance a Band-rumped Storm Petrel was caught as an adult 2 m from its natal burrow. Storm petrels nest either in burrows dug into soil or sand, or in small crevices in rocks and scree. Competition for nesting sites is intense in colonies where storm petrels compete with other burrowing petrels, with shearwaters having been recorded killing storm petrels in order to occupy their burrows. Colonies can be extremely large and dense; 840,000 pairs of White-faced Storm Petrel nest on South East Island in the Chatham Islands in burrow densities of between 1.18 – 0.47 burrows/m²; densities as high as 8 pairs/m² for Band-rumped Storm Petrels in the Galapagos and colonies 3.6 million strong for Leach's Storm Petrel have been recorded.

Storm petrels are monogamous and form long-term pair bonds that last a number of years. Studies of paternity using DNA fingerprinting have shown that, unlike many other monogamous birds, infidelity (extra-pair matings) is very rare. As with the other Procellariiformes, a single egg is laid by a pair in a breeding season; if the egg fails, then usually no attempt is made to relay (although it happens rarely). Both sexes incubate in shifts of up to six days. The egg hatches after 40 or 50 days; the young is brooded continuously for another 7 days or so before being left alone in the nest during the day and fed by regurgitation at night. Meals fed to the chick weigh around 10–20% of the parent's body weight, and consist of both prey items and stomach oil. Stomach oil is an energy rich (its calorific value is around 9600 calories per gram) oil created by partly digested prey in a part of the foregut known as the proventriculus. By partly converting prey items into stomach oil storm petrels can maximise the amount of energy chicks receive during feed, an advantage for small seabirds that can only make a single visit to the chick during a 24 hour period (at night). The average age at which chicks fledge depends on the species, taking between 50 or 70 days. The time taken to hatch and raise the young is long for the bird's size but is typical of seabirds, which in general are K-selected, living much longer, delay breeding for longer, and invest more effort into fewer young. Storm petrels have been recorded living as long as 30 years.