Thursday, November 6, 2014

Cere position on Sharp-shinned & Cooper's

On September 23, 2014 with the sighting of a Cooper's Hawk in Argyle, Yarmouth County, I could finally say that I've seen all three North American accipiters - the Sharp-shinned Hawk, the Cooper's Hawk and the Northern Goshawk.
Juvenile Sharp-shinned Hawk - Cape Forchu - Sept. 24, 2014
In Nova Scotia the Sharp-shinned Hawk is our most numerous accipiter and it breeds throughout the province. The Cooper's does not regularly breed in Nova Scotia and thus is quite rare (a handful of sightings per year). Northern Goshawks breed throughout the province as well but in smaller numbers than the Sharpie. [All the Birds of Nova Scotia, Ian McLaren]
Differentiation between these three species can be challenging. They all have similar shapes - short, rounded wings with long and narrow tails. Goshawks are largest followed by Cooper's and then Sharp-shinned. Juvenile birds in all three species are brown above and streaked below. The adult Northern Goshawk is quite unique compared to an adult of the two smaller accipiters. [Hawks in Flight, Dunne et al.]
Cooper's Hawk - Argyle - Sept. 23, 2014
Many reference books discourage observers from relying too much on size differences for ID purposed in the field. Female Sharp-shinned Hawks approach the size of a male Cooper's [Hawks in Flight, Dunne et al.]. Below are dimensions as given by the website All About Birds.
Length (in)
Wingspan (in)
9.4 - 13.4
16.9 - 22.0
14.6 - 15.4
24.4 - 35.4
20.9 - 25.2
40.6 - 46.1
For accipiters it is always best to use as many field marks as possible to come to a conclusion in its ID. Birds with contradictory field marks should be ID'd using all of its characteristics as a whole placing more weight on the stronger field marks. For example, Sharp-shinned Hawks can show a rounded tail when in flight. [Hawks in Flight, Dunne et al.]
Northern Goshawk - Brier Island - Aug. 30, 2014 
There are many great texts that provide wonderful field marks to help to differentiate these three confusing birds so I won't repeat them all here.
There is one field mark that I haven't yet found any direct mention of in any of my reference materials. This field mark was passed on to me by a very accomplished Nova Scotia birder named Fulton Lavender. The yellow coloured part at the base of the maxilla of a raptor is named the cere, as shown in the image below.
Sharp-shinned Hawk showing the yellow "cere"
According to this field mark, the top of the cere on a Sharp-shinned hawk is lower than that of a Cooper's Hawk. I've done some searching through images of these two birds on Flickr and it does look very consistent. The following is the same Sharpie as above with lines showing the height of the cere with the lines adjusted for the head position of the bird (looking up). The line on the top of the head is used to get an idea for the positioning of the head (looking up or down).
Sharp-shinned Hawk cere position in relation to the eye (mid-eye)
Sharp-shinned Hawk cere position in relation to eye (mid-eye) - photo by Sheila Briand
The top of the cere in the above Sharpies seems to pass through the middle of the eye. Below are photos of two Cooper's Hawks - one photographed by myself and the other by my uncle Ellis d'Entremont.
Cooper's Hawk position of cere compared to the eye (above eye)
Cooper's Hawk position of the cere compared to the eye (above eye) - Photo by Ellis d'Entremont
The newest version of The Sibley Guide to Birds also shows this field mark. The beak of the Cooper's seems pushed up as compared to the Sharp-shinned.
The Sibley Guide to Birds, David Sibley
Determining the position of the cere relative to the eye is somewhat subjective. The angle of the head must first be estimated. This process introduces a risk of error right from the start. A more relaxed bird may provide more accurate assessment of head angle due to flattened head feathers.

Like all other field marks used to differentiate between all three accipiters, the relative position of the cere should be used as one of the many supporting field marks. Like a Sharpie showing a rounded tail in flight, a Sharp-shinned can also have a tall cere or a Cooper's can have a low cere. These contradictions would be exceptions, but they do occur.

All the Birds of Nova Scotia by Ian McLaren makes reference to comparing the relative size of the bill (height) versus the eye diameter to help differentiate between Sharpies and Cooper's. The relatively larger bill of a Cooper's may contribute to the appearance of a taller cere which does contribute greatly to the Cooper's head shape. Dr. McLaren prefers to use the relative size of the bill compared to the eye (easily measured on a digital image). With these confusing accipiters we often need as many field marks as we can get and the cere height may be useful as another of the many characteristics that contribute to the overall look of these birds.

I'd love to hear if anyone else has heard of or is using this field mark.

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