Monday, November 24, 2014

Hybrid Herring x Great Black-backed Gull

Gull hybridization occurs rarely, but it is most frequent in large white-headed gulls. Hybrids are highly variable and can show almost any combination of parental characters. The resultant hybrid offspring often look intermediate between the parent species. This combination of features can produce a hybrid gull that closely resembles another species entirely.
In Newfoundland, there are Herring x Lesser Black-backed Gull hybrids that resemble the sought after Yellow-legged Gull. Bruce Mactavish likens differentiating between a real Yellow-legged Gull and its hybrid imposter like splitting an atom in his blog.
On November 22, 2014 I found a rather odd gull at Dennis Point Wharf in Pubnico, Yarmouth County. It was large, perhaps about the size of a Herring Gull or larger. Its mantle was similar in shade to a nearby Lesser Black-backed Gull. Its legs were lead-pink and it had a fairly large bill.
Experienced larophiles on the North American Gulls Facebook Group have labelled it as a probable hybrid Herring x Great Black-backed Gull. I use the word "probable" because one can never be sure of a hybrid's exact lineage without having direct information on the parents (which would be available if a mixed pair's chick was banded). The speculation on the parent species of a hybrid are simply educated guesses.
HERG x GBBG - Dennis Point Wharf - Nov. 22, 2014.
HERG x GBBG - Dennis Point Wharf - Nov. 22, 2014.
The following is a list of features of this hybrid and their probable origins.
GBBG-like Features
  • faintly streaked nape and head
  • heavy bill with extreme gonydeal angle
HERG-like Features
  • White mirror on P9 and P10
  • Black on P5-P10
Intermediate Features
  • Mantle shade (half way between HERG and GBBG)
Unique Features
  • Lead pink legs 
The upper wing tip pattern shown below more closely resembles an average HERG. The hybrid has a large white mirror on P10 and a small one on P9 like a HERG. The black in the primaries goes from P5 to P10. A GBBG in comparison typically has fewer primaries with black (P6-P10), P10 has a completely white tip and P9 has a white mirror. See the wing tip comparison image below which shows these features.
Wing tip comparison
This hybrid gull is superficially similar to a Slaty-backed Gull. A few missing features are the bright pink legs, broad white tertial crescent, broad white secondary tips and dark streak through the eye. A Western Gull would show much less white on the wing tips and an even whiter head and neck.
A similar gull also believed to be a HERG x GBBG was found in Glace Bay in the spring of 2002. The initial thoughts of it possibly being a Slaty-backed Gull soon vanished after the gull was thoroughly analyzed. The lack of deep pink kegs and extensive window in the outer primaries ruled out Slaty-backed and it was then presumed to be a hybrid. [NS Birds, Vol. 44, No. 3]

HERG x GBBG at Glace Bay - March 7, 2002 - Ian McLaren Photo
Howell, S.N.G. and J. Dunn. 2007. Gulls of the Americas. Houghton Mifflin Company, New York, N.Y.

Wednesday, November 19, 2014

My First Red-shouldered Hawk

On November 16, 2014 I observed my first Red-shouldered Hawk (RSHA) on Cape Sable Island, Shelburne County. This buteo was flying Northeast over the road near the intersection of the Centerville South Side Road and the Daniel's Head Road.
Red-shouldered Hawk - Cape Sable Island - November 16, 2014.

From the mid-1960's until the end of 2010 there have been 106 reports of this bird in Nova Scotia. Many of the 65 fall sightings have occurred during large hawk movements over Brier Island. [All the Birds of Nova Scotia, Ian McLaren]
There are 5 subspecies of RSHA but three of them are essentially indistinguishable from one another and they occupy most of the eastern states. Southern Florida and California birds are separable from the eastern subspecies group. [Hawks in Flight, Dunne et al.]
All About Birds
This hawk most closely resembles the expected eastern subspecies group. A juvenile California RSHA would have thick black bars on the tail and dark rufous underwing coverts. The Florida subspecies would not show such an evenly patterned underside and would have smaller light bands and wider dark bands on the tail. [The Sibley Guide to the Birds 2nd Ed, Sibley]
Juvenile Red-shouldered Hawks are somewhat similar to juvenile Broad-winged Hawks (BWHA). There are a few field marks that allow an observer to easily differentiate between both species given clean views. 
Both have quite different wing tip shapes. The BWHA shows pointed wings reminiscent of a candle-flame. The wings of a RSHA are described as looking like a rectangular plank. BWHAs show narrow dark tail bands in comparison to the RSHAs evenly wide light and dark tail bands. The pale crescent windows produced by the translucent outer primary bases are only found on RSHAs. [Hawks in Flight, Dunne et al.]
The Sibley Guide to the Birds 2nd Ed, Sibley (added annotations)
The annotated photo of my Nov. 16th RSHA below shows all of the field marks that differentiate it from a BWHA.
Red-shouldered Hawk with annotations

Wednesday, November 12, 2014

My First Swainson's Hawk

Ervin Olsen and I were driving on the Melbourne Road in Pinkney's Point on November 8, 2014 when we spotted an interesting hawk flying above the road. My initial impression was of an Osprey. It had the characteristic "M" shape that an Osprey often shows. I stopped the car and took a few photos as it flew by. By then I had noticed that this bird was in fact not an Osprey and I couldn't make it match with any raptor that I knew.
It was only once I put it on the computer and lightened the image to see the details that I knew I had something great. I searched through Sibley and landed on Swainson's Hawk. I sent the image to David Currie and was pleased to hear that he agreed with the ID.
Swainson's Hawk - Pinkney's Point - November 8, 2014

The Swainson's Hawk is a rare buteo for Nova Scotia. It breeds in western North America and is rare further east. All the Birds of Nova Scotia by Ian McLaren which is current until the end of 2010 describes only 11 sightings of this hawk in Nova Scotia. 
The following are the diagnostic characteristics that are discernible from the photos that were taken on November 8 accompanied by an annotated photo afterwards.
1. Light patch at the base of the tail
2. Body and underwing linings are a light buffy colour
3. Patchy chest markings (differing from an adult's solid chest bib)
4. Flight feathers darker than underwing lining.
5. Pale outer primaries
Annotated Photo
This bird, as all other Swainson's that have been found in Nova Scotia has not yet reached adult plumage. See below for a photo of a light morph juvenile Swainson's Hawk from Hawks in Flight by Dunne et al showing an Osprey-like "M" shape.
There have been two other fall 2014 sightings of Swainson's Hawks in Nova Scotia. Both were of light morph juvenile birds at Brier Island. The first sighting was on September 25th by Rick Whitman and the second on October 31st by Richard Stern, Rick Whitman and Bernard Forsythe. We may be able to better compare my November bird with the October bird since flight shots of both were obtained.
All photos of the Oct. bird were taken by Richard Stern. The comparison below seems to show that my Nov. bird has lighter markings on the underside than the Oct. bird.
My Nov. bird (L) vs. the Oct. bird (R)
The next composite shows that the Nov. bird has much cleaner flanks than the Oct. bird.
My Nov. bird (L) vs. the Oct. bird (R)
My distant photos certainly don't allow for accurate colour and contrast comparisons, but we can
accurately compare moult between the two birds. My Nov. bird shows more moult.

Swainson's Hawks take 2 years to reach adult plumage. This gives us the possibility of distinguishing between 3 ages in the field - juvenile, sub-adult and adult. [Nemesis Bird]

The Oct. bird looks like a bird born this year (juvenile) because of the lack of a dark trailing edge to the wing and tail and its very fresh plumage. Juvenile plumage is retained on most birds for their first year of life [Nemesis Bird]. For these reasons I believe that the Oct. bird is a freshly plumaged juvenile born in 2014.

We can clearly see that the Nov. bird is moulting because of the missing feathers. The trailing edges of the wing and tail appear much less neat as opposed to crisp as would be seen in a juvenile. During fall migration, Swainson's Hawks showing symmetric wing and tail moult, while exibiting juvenile body plumage, are sub-adults [Hawks at a Distance, Ligouri]. Since my Nov. bird does show symmetric wing moult, I believe that my sighing on Nov. 8th is of a sub-adult. A flight feather where the primaries meet the secondaries (it looks like P1 to me) seems to be missing from both wings on my Nov. bird. The Nov. bird also looks to have dropped P7 in both wings.
This kind of primary dropping is attributed to stepwise moult which involves simultaneous waves of primary moult. Birds with stepwise moult have one or more of the following characteristics: long distance migrants, spends time in open habitats, are relatively long-winged and large mass [Molt in North American Birds, Howell]. Ninety percent of after hatch year (AHY) / after second year (ASY) birds show this stepwise moult in the primaries and secondaries [Identification Guide to North American Birds, Pyle]. This moult pattern is also known as staffelmauser moult (in Pyle) or serial moult (this paper).
Nov. 8th bird - left wing and right wing moult
Molt in North American Birds by Howell mentions that Swainson's Hawks suspend their wing moult during migration so it is unlikely that the Oct. bird with all its flight feathers would start moulting before reaching its wintering grounds. The time period of one week between sightings isn't enough to produce the feather wear and moult differences between the Oct. and Nov. birds.

In my opinion it seems clear that there have been at least 2 Swainson's Hawks in Nova Scotia in 2014. There is a continuum of plumages from light to dark in Swainson's Hawks [Hawks at a Distance, Ligouri]. I believe that the Oct. bird has slightly more and darker markings than my Nov. bird. A more convincing argument is that the Oct. bird is a juvenile and my Nov. bird is sub-adult.

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.