Tuesday, December 4, 2012

The soaring DC cormorant Identification issue

May 2011. It is only 4pm, but we (Andy Dettling, Laurent Fournier, Jacco Gelderloos) have been up for about 14 hours, and biked and hiked for about 75 miles, smashing our own previous Washtenaw county (Michigan) bigby (=carbon free) record of 114 species. We enter Ann Arbor's Nichols Arboretum where dozen of people are sunbathing, enjoying the warm spring afternoon.

Suddenly, one of us notice something soaring in the blue sky. Black, long neck, long pointy wings, fanned out tail, soaring without a wingbeat and rising in a thermal next to a Turkey Vulture.

We look in silence at the bird for a few seconds, and finally, one of us states the obvious. Anhinga. Mega, mega bird for Michigan (this would have been the third state record for Michigan). Pictures were taken and later posted in local and Florida lists. Initially at least, the consensus was unanimous. The behaviour was fitting Anhiga extremely well.

Later, though,it appeared that something was not quite right about the ID. The fanned out tail, in particular, was a bit too short and proportions were not quite right.

For comparison purposes, here is an Anhiga's picture taken taken from the 10,000 birds gallery (photo : Corey Finger)

Caleb Putman, local lister and former member of the Michigan Rare Bird Commitee posted this comment on my flickr picture :

"I think it's a Cormorant. When they soar they often look very different from when they are flapping, and the shape begins to approach Anhinga. The tail looks proportionately longer, especially. Here is a very similar photo I took at Port Huron : www.flickr.com/photos/27846187@N07/5430008150 /

Later, local birding listserv owner Bruce Bowman compared the pictures with some Anhinga pictures found in field guides :

"My calculations are below.  First, I determined the expected W/L ratios for Anhinga and Double-crested Cormorant using values from the Sibley field guide. Next I used The GIMP image editor to measure W and L in pixels for the four posted photos, one at Grove Street and the three at Flickr.  These values and the associated W/L ratios are in the table below".

Anhinga DC Cormorant Our bird
W 45" 52" 53.9 px
L 35" 33" 31.5 px
W/L 1.29 1.58" 1.72

Suddenly, it became pretty clear that our initial identification was wrong and that we, in fact, witnessed a soaring DC cormorant. Our bird was simply too short.

Interestingly, 10,000 birds beat writter Jochen experienced a similar identification crisis the same location a few years earlier :

I later decided to dig a little deeper to explore the various forms of DC cormorant flight. Fact is, soaring DC Cormorants are not very well documented in the commonly available field guides.

In most birding field guides (Sibley, National Geographic are the ones I owned and checked), there is no mention of any potential confusion between soaring DC Cormorants and Anhingas. In the Cornell Lab Website, for instance, it is simply mentioned that "In flight, they often travel in V-shaped flocks that shift and reform as the birds alternate bursts of choppy flapping with short glides".

However, in the online version of Birds of North America, there is a brief mention of the Double Crested Cormorant capacity of soaring when the conditions are right :

"Taking flight from perch, initially loses altitude; takeoff from level land requires several hops combined with vigorous flapping. On water, makes similar 2-footed thrusts, synchronized with wing-beats; most other waterbirds paddle their feet alternately for takeoff. If no wind, and after fishing, takeoff may require up to 10 m (Lewis 1929). When over water, usually flies close to surface; over land, flies much higher. During long foraging flights or on migration, flocks may travel in shallow Vs or echelons. Soaring infrequent, except slope-soaring along cliffs, but occasionally soars in thermals"

Furthermore, the Pete Dunne's Essential  Field Guide Companion (Houghton Mifflin, 2006) says that they sometimes soar.  From the D-c Cormorant species account, p. 122: "On days with good thermal production, Double-cresteds > frequently soar, with wings at a  right angle to the body, neck stretched, and tail fanned.  The shape suggests a bulky Anhinga."

As a conclusion, we believe that this soaring behavior of DC Cormorant should be more widely publicized. This bird took a long time for even the top birders in the state to analyse and come to an identification. We were lucky enough to have gotten a photo of the bird. While it is much easier to see that the tail was too short in the photo, real time observation did not give that impression. The fanned tail appeared to be long and widely fanned. Without a photo to scrutinize we believe the group would have settled and written the bird up as an Anhinga. While it may be rare to see a DC Cormorant soaring it is crucial for the birding public to know about this possibility so that an accurate identification can be made in the field without the aid of a photo


corey said...

So maybe they CAN lead to confusion. Darned soaring cormorants.

Les Fous du Cap said...

Merci pour votre passage sur "Les Fous du Cap". Le secteur de la plage de l'ancien hoverport à Calais est un endroit où une poignée de Bécasseaux violets passe une partie de l'hiver.
Pour les voir, il faudra donc venir à cette période ;-)
Céline & Philippe