The Desertas Petrel is a small seabird in the gadfly petrel genus which breeds on Bugio Island

Bugio Island

Bugio Island is one of the Desertas Islands, a small chain of islands in the archipelago of Madeira, located to the southeast of the island of Madeira....

 in the Desertas

Desertas Islands

The Desertas Islands are a small Portuguese archipelago, located about 25 km to the southeast of Ponta de São Lourenço, the eastern tip of the island of Madeira, the whole chain located roughly between Madeira and the Canary Islands.-Geography:...

 off Madeira

Madeira

Madeira is a Portuguese archipelago that lies between and , just under 400 km north of Tenerife, Canary Islands, in the north Atlantic Ocean and an outermost region of the European Union...

. The gadfly petrels in the genus Pterodroma are seabirds of temperate and tropical oceans. Many are little-known, and their often similar appearance have caused the taxonomy of the group to be rather fluid. Although the systematics on this species has not yet been definitively established, provisionally some authorities have split the Desertas Petrel, separating the Desertas breeding birds from those in the Cape Verde

Cape Verde

The Republic of Cape Verde is an island country, spanning an archipelago of 10 islands located in the central Atlantic Ocean, 570 kilometres off the coast of Western Africa...

 archipelago., while others consider it a subspecies of the Fea's Petrel

Fea's Petrel

Fea's Petrel is a small seabird in the gadfly petrel genus, Pterodroma. It was previously considered to be a subspecies of the Soft-plumaged Petrel, P. mollis, but they are actually not closely related at all. However, P. feae is very closely related to Zino's Petrel and Deserta's Petrel, two...

.

Description

As all three of the species in the Pterodroma feae/madeira/desertae complex were once believed to be subspecies of a single species: Pterodroma mollis. Well-understood is the extreme difficulty in telling them apart. A summary of data from a morphological study sheds light on the issue:

• In overall size feae is closer to deserta and both are bigger than madeira (wing length - f:262, d: 264, m: 248; tarsus - f:34.7, d: 35.2, m: 32.7).
• Bill length once again similar in feae and deserta and both larger than madeira (f:29.0, d: 28.6, m: 26.3).
• Bill depth by far biggest in deserta, lowest in madeira, and intermediate in feae (f:11.9, d: 12.9, m: 10.5).

In terms of ratios:

• (Bill length/wing length) (f:11.1%, d: 10.8%, m: 10.6%) is rather similar in the three taxa.
• (Bill length/bill depth) (f: 2.44, d: 2.22, m: 2.53) is lowest in deserta, and highest in madeira. In feae it is intermediate, but more than twice closer to madeira than to deserta.