Variation in Galapagos Shearwater

light to dark underwinged varieties

Variation noted in Galapagos Shearwaters
During a June, 2011 trip to the Galapagos Islands our group led by Jesse Fagan and Peter Freire made some interesting observations in regards to the variation in underwing patterns in the Galapagos Shearwater.

This variation which is primarily seen in the amount of dark pigmentation on the underwing has been noted previously and is illustrated in Albatrosses, Petrels & Shearwaters of the World,2007, by Onley & Scofield. Peter And Jesse both alerted me to an extremely dark-winged variety that had been noted a few years earlier off Isabella Island by Alvaro Jaramillo and Peter during a similar tour run for Field Guides. We observed this form and photographed it as we passed through the area. To my knowledge this variety has not been shown in any publication.

Galapagos Shearwater- topside, off Genovesa

Galapagos Shearwater,
Puffinus subalaris, has until recently been considered a subspecies of Audubon’s Shearwater. Recent work has suggested that genetically this species is more closely related to Christmas Island Shearwater and there has been near universal acceptance that Galapagos Shearwater is a distinct taxon. It breeds on many islets as well as large islands of the Galapagos on sheer ocean side cliffs. Potentially other breeding sites exist inland.

 Onley and Scofield show the variation in underwing pattern of Galapago's Shearwater in Plate 37 of Albatross,Shearwaters & Petrels of the World. These illustrations show considerable variation in the amount of dark pigmentation in the axillary region and a dark bar crossing from the posterior axillary region running at a 45 degree angle towards the mid carpal region. These illustrations are rather different than the variation we observed and photographed.

off Genovesa
off Genovesa
off Genovesa
off Isabella
off Genovesa
off Genovesa
off Baltra
off Genovesa
Variation in underwing pattern in Galapagos Shearwaters

First off we observed a seemingly random variation in underwing darkness among all the birds we observed at sea and along the breeding cliffs of Genovesa, Champion and Espanola Islands. In these birds the variation was considerable with the mid axillary region and the anterior portion of the underwing generally being the least marked even in the darkest birds 9although a narrow dark leading edge was often observed). This is illustrated above in the photographs.

Next we noted a distinct form which was seen only in the mid portion of Elizabeth Bay. They were present in good numbers in small to mid-sized (5-50 birds) relatively homogenous flocks. We encountered this form for an hour or so as we cruised through the region. These birds had a uniformly dark underwing with a small wedge of white in the mid to anterior axillary region running up to the leading edge of the wing. We did not see this particular extreme dark under-winged variation at any other location throughout the trip although a very few of the darker birds in other areas approach this level of underwing darkness but were not as uniform appearing.

Galapagos Shearwater “uniform dark under-winged form” all taken within an hour period in mid Elizabeth Bay off Isabella

Off course we found it most interesting that this form seemed to be present only in one distinct very small geographic area rather than being randomly distributed as the other variations appeared to be. If these are perhaps birds that breed in a separate location and/or habitat this may be a staging area for their congregation in the afternoon. One can only speculate in regards to the possibility of this “uniformly dark-under-winged form” representing a distinct taxa. As usual more study is clearly needed.

Kirk Zufelt
World Pelagic Birding Headquarters
Sault Ste. Marie, ON