Phaethontidae: Tropicbirds
The tropicbirds are a small, distinctive group of seabirds that occur throughout the tropical oceans. Adult tropicbirds have a unique, elegant appearance: crisp white-and-black plumage, with two greatly elongated streaming tail feathers, and graceful, buoyant flight.
The tropicbirds were long believed to be related to several other seabird families, including cormorants, boobies, and frigatebirds (all members of the order Suliformes), but genetic studies have revealed that their closest living relatives are either the peculiar Kagu and Sunbittern (the order Eurypygiformes), the pigeons and doves, or the sandgrouse.
Identification
Adult tropicbirds can be challenging to identify if seen only from below, as all have entirely white underparts and the bill and tail coloration may be difficult to ascertain against the sky. The adults’s dorsal patterns are diagnostic when seen clearly.
Immature tropicbirds, which lack the streamers and have an unkempt appearance, are easily confused with large terns in their nonbreeding plumages. The tropicbirds can generally be recognized by their heavier body shape and more direct flight. As with adults, the dorsal patterns are diagnostic, but the differences are subtle and may require a close study.
Taxonomy
The internal systematics of the Phaethontidae have traditionally been regarded as stable: a trio of distinctive pantropical species. Recently, however, there is an ongoing reassessment of bird classifications received from prior centuries, especially the status of similar but distinguishable forms, now giving higher priority to distinctions and the appreciation of diversity. In this vein, Howell and Zufelt (2019) propose to subdivide White-tailed and Red-billed Tropicbirds into two species each, expanding the family from three species to five. These cases are not equivalent: one is easy and the other is hard—but both proposals have sufficient basis to be accepted, as they are here. For a detailed discussion of reasons that support the adoption of these proposals, see Frontiers of Taxonomy: Split the Tropics!
References
Boyd, J.H., 2016. Taxonomy in Flux: Mirandornithes & Ardeae I. http://jboyd.net/Taxo/List8.html#phaethontidae. (Posted December 15, 2016. Accessed August 16, 2020.)
Howell, S.N.G., and K. Zufelt. 2019. Oceanic Birds of the World. Princeton University Press.
Le Corre, M., and P. Jouventin. 1999. Geographical variation in the White‐tailed Tropicbird Phaethon lepturus, with the description of a new subspecies endemic to Europa Island, southern Mozambique Channel. Ibis 141:233-239.
Roberson, D. 2000. Bird Families of the World: Tropicbirds, Phaethontidae, http://creagrus.home.montereybay.com/tropicbirds.html. (Posted January 21, 2000. Accessed September 15, 2017.)