How does clutch size vary among pelecaniform birds?

Clutch size in pelecaniform birds varies significantly based on their environment and resource availability. Generally, species inhabiting unproductive tropical seas lay only a single egg, while those in seasonally productive or nutrient-rich higher latitudes tend to lay larger clutches and raise multiple young.1

Specific variations include:

• Single-egg clutches: Tropic birds, frigate birds, the gannet, and the red-footed booby.1
• One to three eggs: Pelicans.1
• Two to five eggs: Cormorants and anhingas.1
• Two eggs: Typical tropical boobies, such as the masked and brown boobies. In these species, the second egg often serves as an insurance policy against infertility or loss, as the older chick typically forces the younger one out of the nest shortly after hatching.1

Environmental factors, such as the upwelling of nutrient-laden water, play a critical role in determining these reproductive strategies, as birds in more productive areas can support larger broods.1