Bird of the Month: Wedge-rumped Storm-Petrel

The Wedge-rumped Storm-Petrel (Oceanodroma tethys) is a relatively small member of the Hidrobatidae family (the Northern Storm-Petrels) that breeds on the Galapagos islands (nominate tethys) and on islets off the central coast of Peru (ssp. kelsalli).  This is the commonest storm-petrel species in Panamanian waters, present year-round, but commoner from May to November.

Wedge-rumped Storm-Petrel (Panama Gulf)

It is sooty brown overall (sooty black when fresh), with darker flight feathers and tail and pale upperwing panel and conspicuous white uppertail coverts.  There are slight differences among the two different subspecies, with kelsalli being smaller, shorter winged, shorter tailed, with smaller white "rump", slighter bill and with relatively long tail projection and more forked tail than nominate tethys.

Wedge-rumped Storm-Petrel (Panama Gulf)

Like other storm-petrels species, it have a steep forehead due to the large olfactory bulbs that facilitates a keen sense of smell, which is very important to locate food and for social interaction at its colonies.  Also like many other storm-petrels species, it can be seen pattering its feet on the waves while fluttering over the water.  This behavior is why it is call "Paíño Danzarín" in spanish (meaning "Dancing Storm-Petrel").

Wedge-rumped Storm-Petrel (off western Azuero Peninsula)

They are usually seen as small and dark little birds flying swiftly over the water among the waves, but they can be attracted by chumming to a boat, where you can have nice and prolonged views of these feathered marvels.

Wedge-rumped Storm-Petrel (Panama Gulf)

About specific ID to subspecies, more studies are needed to separate both subspecies in the field.  Presumably, all the photos in this post pertain to the kelsalli subspecies.  Notice the short arm, long tail projection beyond the white uppertail coverts, the forked tails and the small size noticed at sea.  We can not be 100% about the IDs, since both subspecies have been recorded in Panama and all these noted differences depend on wear, shape, angle of sighting, light and so on; however, some references indicate that the form found closer to shore is kelsalli, while nominate tethys seems to be more pelagic in general (more than 100 km from shore).  For these, and many other reasons, is why we chose the Wedge-rumped Storm-Petrel as our Bird of the Month!

Wedge-rumped Storm-Petrel (off western Azuero Peninsula)

Literature consulted:

1. Ridgely R, Gwynne J. A Guide to the Birds of Panama. Princeton University Press 1989.

2. Angehr G, Dean R. The Birds of Panama. A Field Guide. Zona Tropical 2010.

3. Howell SNG. Petrels, Albatrosses and Storm-Petrels of North America.  Princeton University Press 2012.

4. Carboneras, C., Jutglar, F. & Kirwan, G.M. (2014). Wedge-rumped Storm-petrel (Hydrobates tethys). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.) (2014). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/52591 on 1 September 2015).