The comparative energetics of petrels and penguins

•Especially petrels are adapted at life at low food intake, some 80% of the maximum.•Allocation to reproduction in petrels and penguins is typical for birds.•Petrels have large eggs, long incubation, low reproduction, high age at puberty.•Penguins have small eggs, crop milk feeding, and offer lipid-rich food to offspring.•Differences between petrels vs penguins relate to feeding in surface vs deeper waters. Petrels and penguins form the clade Austrodyptornithes. The Add-my-Pet collection of referenced data and Dynamic Energy Budget (DEB) parameters presently has 64 members of this clade. Adaptations to their wandering life style could now be studied; I here try to characterise this clade from an energy perspective. It turned out that petrels are in several respects extreme among birds, showing adaptations to life at low mean food levels. Penguins show the same tendency, but less extreme. Almost all petrel species show a weight-reduction well prior to fledging till the adult weight. This permanent weight reduction suggests that the mean adult feeding level is about 80% of the maximum one. This pattern is well-captured by the standard DEB model, in which (energy) reserve plays an important role. They also have relatively large eggs, and long incubation times and low reproduction rates. I argue that large eggs minimize the age at fledging, given that a particular maturity threshold needs to be reached for fledging. Austrodyptornithes have a high age at puberty, which, in combination with low reproduction rates, causes very low maximum population growth rates. The energy allocation to reproduction for petrels is typical among birds, so the low reproduction rates are the results of the large eggs. Another adaptation to low mean food levels is the large stomach/gut capacity, which appeared to be coupled to a high reserve capacity. This points to be an adaptation to a patchy occurrence of their food, probably in combination with spells of poor weather conditions for feeding. Petrels have a high supply stress and altriciality coefficient, compared to birds, while penguins are a very small altriciality coefficient. This reflects a very high maturity level at puberty for both taxa, in combination with high maturity level at birth for petrels and a typical one for penguins. They have a relatively long life span, compared to other birds, but not extreme. The relative egg size of penguins is typical for birds, to smaller than typical. Penguins combine small eggs with l