Many bird species lay only one egg per clutch, whilst a further large number lay two eggs to guard against the loss of one but never fledge more than one nestling.

An (incomplete) list of birds that lay only one egg would include:

1. almost all pelagic seabirds (procellariforms, tropicbirds, frigatebirds, many gannets, most auks)
2. many large raptors (vultures, eagles)
3. all flamingos
4. many tropical frugivores (Papuan bowerbirds and bird-of-paradise, all satinbirds, all berrypickers, fruit pigeons, most cotingas, many manikins, some cuckoo-shrikes)
5. many sunbirds (which are nectarivorous)
6. some large parrots (e.g. black cockatoos)
7. some Australo-Papuan insectivorous songbirds of heathland or rainforests (e.g. lyrebirds, the Noisy Scrub Bird, the Chowchilla or Southern Scrub Robin)
8. most swifts that nest in hotter regions
9. many nightbirds (all potoos, most nightjars, larger frogmouths)
10. some bustards (e.g. Eupodotis)

Birds that rear only one chick despite laying two eggs include:

1. some gannets and pelicans
2. the crested penguins (Eudyptes)
3. many cranes
4. a large number of raptors
5. many hornbills and large parrots

The reason why these species lay only one egg or fledge only one chick is either:

• that the collection of food for more young is simply impossible (as with pelagic seabirds that must forage far from small islands) or
• that adult mortality is so low that rearing more young would lead to reduced adult survival through extra labour (as with many birds of aseasonal climates in the tropics, Australia and Southern Africa)

In geological terms, the restraints these species have on fast reproduction are exceptionally typical. It is almost certain that the large clutches (five or more eggs) typical of altricial birds on the extremely young Enriched World soils were unknown before the first ice sheets swept across the northern hemisphere in the late Pliocene. In hot climates, small birds are very long-lived due to the difficulties catching them and the limited, seasonally stable food supply. The small clutches of seabirds and Tropical and Unenriched World landbirds make them very sensitive to disturbance, since population turnover is slow and recovery from losses difficult.

It is important to recognise that species laying only one egg do not necessarily have the highest cost of reproduction. Obligate cooperative breeders such as the Australian mudnesters and Varied Sittella cannot rear a single young without helpers at the nest. These species lay two to four eggs per clutch, but need at least six adults to feed them.

In contrast, with birds that lay only one egg (seabirds especially) remaining on a nest site with parents is very costly to long-term survival because of competition for food.