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Why are tropic birds sometimes classified in their own order, Phaethontiformes?
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Pelecaniform
European white pelicans (Pelecanus onocrotalus) in flight.
Tropic birds are traditionally grouped within the order Pelecaniformes based on shared morphological and behavioral traits, such as the totipalmate foot (where all four toes are connected by webbing). However, this classification is debated because these unifying characteristics may have arisen through convergent evolution.123
Ornithologists have proposed placing tropic birds in their own order, Phaethontiformes, for several reasons:
- Genetic Evidence: Molecular genetic analyses indicate that tropic birds are only distantly related to other pelecaniform birds.1
- Anatomical Differences: Tropic birds differ from other members of the order in various ways. For instance, the first toe in tropic birds is distinctly more elevated on the tarsometatarsus than in other pelecaniform groups.2
- Evolutionary Origins: Some researchers suggest that the resemblances between tropic birds and certain members of the order Charadriiformes (such as gulls and terns) may indicate that tropic birds arose from a different ancestral stock entirely.2