The life cycle of all Procellariiformes is similar. They lay only one egg each breeding season, and the egg is unusually large. The tiny storm petrel produces an egg that is up to 29% of the female’s body weight, the highest ratio of any bird species.
Between copulation and egg-laying, most petrels spend from a few days to a month at sea. The female needs to obtain the nutrients required to produce such a demanding egg, and the male needs to put on weight in preparation for the many days he will spend incubating the egg while the female recovers. She will then return to relieve her mate. The species with the longest pre-laying exodus is New Zealand’s grey-faced petrel, which spends 50–80 days at sea between courtship and egg-laying.
Most of the smaller petrels breed in burrows, but a few nest in crevices. Cape petrels (Daption capense) nest on cliff ledges and some giant petrels and albatrosses breed in the open.
The burrow-breeding petrels come ashore only after dark and depart for sea before sunrise. They dig their own burrows or will occupy existing ones. The burrows are renovated or extended from year to year, and some sooty shearwater and Chatham Island tāiko (Pterodroma magentae) burrows are over 2 metres long. If both birds survive, pair bonds are usually retained from one year to the next, and pairs most often occupy the same burrow in successive years.
Procellariiformes have long incubation periods: 40 or more days for the tiny storm petrels, 51–53 days for the medium-sized shearwaters and 69–79 days for the albatrosses. Both parents take turns incubating. Those species that feed furthest from land, such as the mottled petrel (Pterodroma inexpectata), have incubation spells that may last 10–14 days, whereas the diving petrels (Pelecanoides spp.) that feed inshore have shorter incubation spells and both adults tend to visit the nest each night.
Chick rearing is an equally drawn-out affair. Their unusual growth pattern and feeding regimens highlight the challenges petrels face in exploiting scattered food sources far from their breeding colonies. The nesting period is 55 days for the small diving petrels, almost 100 days for the sooty shearwater and 278 days for the giant wandering albatross (Diomedea exulans).
Once hatched, the chick is guarded by one of its parents. Some burrow-nesting species watch over the chick for a few days, while surface-nesting birds such as albatrosses and the Cape petrel protect the chick for several weeks. After that, the parents return only to feed the chick.
In all species, both parents feed the chick. The hatchling is fed once every few days in most species, and the interval between meals gets longer as the chick gets older. Species that forage near the colony may feed their chick at regular intervals throughout development, but for species that feed far offshore it can be a week or more between visits. To compensate, each meal can be large. There are records of sooty shearwater and mottled petrel chicks doubling in weight overnight. Such gargantuan meals are no doubt the result of both parents returning on the same night.
This feast and famine lifestyle lasts throughout the birds’ lives. Food for most species is widely dispersed, and birds often have to travel hundreds of kilometres between feeds. Petrels (except diving petrels) have a unique ability to convert their bulky, heavy food into lightweight, energy-rich oil, which is produced in the proventricular gland – a part of their stomach. The oil is a far more concentrated energy source than body fat, which is the way most animals store energy. This allows petrels to feed far from their colony even while provisioning their chicks.
Chicks grow quickly, and they eventually weigh from 15% to 50% more than their parents, depending on the species. In many species the parents stop feeding their chicks some time before fledging, and the accumulated fat is used to complete feather development and growth. By the time chicks fledge, their weight is similar to that of their parents. With some migratory species, the parents depart before the chick has completed its development. Once the chick goes to sea it must learn to feed by itself, and find its own way to non-breeding grounds that may be more than 10,000 kilometres away.
Age at breeding
The life of a petrel is complex, and it takes years to develop the foraging skills necessary to breed successfully. Shearwaters don’t breed until they are about seven years old, and albatrosses start breeding between 8 and 12 years of age, depending on the species. Diving petrels are the youngest breeders. Most breed at two years old, and a few when only a year old. Petrels live longer than most birds, with even the small species living several decades.