How many eggs do procellariiform birds usually lay in a breeding season?

Procellariiform birds, which include albatrosses, shearwaters, and petrels, typically lay only a single egg during a breeding season.1

Because these birds have a slow rate of reproduction and a long fledgling period, they do not produce large clutches. In the case of the largest albatrosses, the nesting period is so protracted that they cannot rear more than one young every other year. To compensate for this low reproductive output, these birds are generally long-lived, often reaching several decades of age.1