Why do tropical pelecaniform species often lay fewer eggs than species in productive waters?

Tropical pelecaniform species typically lay fewer eggs than those in productive, higher-latitude waters because they inhabit environments where food is generally scarce.12

In these unproductive tropical seas, the limited availability of food resources restricts the number of young a pair can successfully raise. Consequently, many tropical species, such as tropic birds and frigate birds, lay only a single egg. In contrast, species living in areas where surface waters are enriched by nutrient-laden upwellings or in seasonally productive temperate zones can support larger clutches and raise multiple young simultaneously.1

For some tropical ground-nesting species that lay two eggs, such as the masked and brown boobies, the second egg often serves as an insurance policy against infertility, loss, or breakage of the first, rather than an attempt to raise two chicks. Furthermore, because food is scarce in tropical areas, young birds often require an extended period of parental feeding after fledging to develop the hunting skills necessary for survival, which further limits the reproductive capacity of the parents.1