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.