Monday, August 7, 2017

Audubon's Shearwater in Nova Scotia

Field Encounter
The two best birds seen during the recent pelagic trip out of Pubnico were two Long-tailed Jaegers and an Audubon's Shearwater, but the latter was only confidently identified two days later. A small black and white shearwater was seen at 8:09 am on August 4, 2017, in 26-27 degree Celsius, 2 km deep, waters south of Browns Bank (Fig. 2). It flew off as we approached and landed nearby; another approach allowed more photos, and then the bird was off for good. In all, the encouter lasted about 3 minutes and 23 seconds.

Figure 1. Audubon's Shearwater south of Browns Bank, Nova Scotia, 4 August 2017. Photo by Alix d'Entremont.

Some of the birders looked through their binoculars while others (including me) madly snapped away at it with their cameras. We knew full well that Audubon's was a possibility, in fact that species was one of the main reasons for making the trip. There was a short discussion about the bird once it disapeared. Manx and Audubon's Shearwaters are quite similar and some noted the appearance of dark undertail coverts while others mentioned that they had seen more extensive white on the face than on the typical Manx. The images of the bird on the LCD screens on the back of cameras seemed to show that the bird had white undertail coverts. Other birds were to be seen, so the group decided to leave it as a probable Manx.

Figure 2. This sea surface temperature map is modified and rectified from a map from the Rutgers Coastal Observation Lab, shows the location of the Audubon's Shearwater sighting as a yellow "X". The map is from 5 am local time, and is the map whose time stamp is closest to our observation at 8:09 am. I assume that the blue area n.w. of the sighting is an error since the water temperature stayed warm as we made our way through the area.

A more detailed look at the bird and a conclusion as to its identification was done on August 6, 2017, by Mark Dennis upon seeing photos that Richard Stern had posted to Facebook. The word was spread that we had in fact photographed one of the megas that we had hoped to see.

Audubon's is slightly smaller than Manx and has short and broad wings, and a longer tail (Howell 2012). The relative tail length can be seen in Figs. 3 and 4. Figure 4 shows how the tail feathers project farther past the undertail coverts in Audubon's. Howell (2012) states that Manx's upperparts are slaty-blackish relative to the browner upperparts shown by Audubons; this is visible in Figure 3.

Figure 3. Audubon's Shearwater south of Browns Bank (L) and Manx Shearwater on German Bank (R). This comparison shows how the tail tip in Audubon's falls farther past the primary tips than in Manx. Photos by Alix d'Entremont.

The wing shape of the bird in question is difficult to appreciate due to its current stage of wing moult, but the wings do appear less pointed and relatively shorter (Fig. 4). Howell (2012) describes how the undertail coverts of Manx are white while those of Audubon's are "solidly blackish (less common) to extensive white basally (most frequent)". It appears that our recent Audubon's has mostly white undertail coverts except for at least one dusky covert on the right side (Fig. 4). Figure 4 shows how the underwing in Manx often looks whiter overall whereas the underwing of Audubon's had broader dark magins (Howell 2012).

Figure 4. Audubon's Shearwater south of Browns Bank (L) and Manx Shearwater south of Browns Bank (R). Photos by Alix d'Entremont.

Manx is said to be darker-faced with a whitish hook cutting around the auriculars, relative to the whiter faced Audubon's lacking the white hook (Fig 5).  (Howell 2012)

Figure 5. Audubon's Shearwater south of Browns Bank (L) and Manx Shearwater south of Browns Bank (R). Photos by Alix d'Entremont.

Boyd's Shearwater is extremely similar to Audubon's, but has a smaller bill and blueish gray legs (Howell 2012). Our recent bird shows pinish legs and what appears to be too large of a bill for the much rarer Boyd's.

Range and Occurrences in Nova Scotia
The subspecies of Audubon's Shearwater believed to range up to Nova Scotia is lherminieri which breeds in the Bahamas and from Puerto Rico to Tobago. It favours warm waters over the continental shelf slope, often found on weed lines at ocean fronts in the Gulf Stream. It ranges north over the Gulf Stream waters from Florida to North Carolina. It is uncommon to rare north to southern New England (mainly Jul-Aug) and is casual to Nova Scotia. (Howell 2012)

Our first report was of an observation at Western Bank on 7 October 1979 by an experienced birder with few published details. The first fully confirmed and captured by camera was one photographed south of Browns Bank on 17 August 2012 by Tom Johnson. None had been photographed by birders on non-goverment birding trips until our August 4, 2017 record. The list below from eBird, Nova Scotia Birds and McLaren (2012) contains a total of 42 reports, but some of them, especially the early ones, are by a single observer and/or have no photos.

7 Oct 1979 - Western Bank (few published details)
27 Aug 1980 - 30 km s.w. of Yarmouth (few published details)
9 Aug 1992 - 8 km west of Brier Island (details by experienced observer)
4 October 2003 - 175 km s.s.w. of Sable Island (details by experienced observer)
30 Sep 2009 - 240 km s.w. of Halifax (2 ind., details by experienced observer)
17 Aug 2012 - south of Browns Bank (eBird) **FIRST PHOTOGRAPHED**
6 Aug 2013 - 200-156 nm south of CSI (9 ind., Michael Force)
9 Oct 2014 - Seal Island (eBird)
1 Jul 2015 - south of Browns Bank (eBird, no details)
16 June 2015 - 78 nm south of CSI (photographed)
26 Aug 2015 - Bon Portage Island (3 ind., eBird)
30 Sep 2015 - 42.59, -59.00 by ECSAS
1 Oct 2015 - 40.8441, -60.28 by ECSAS (3 ind.)
31 May 2016 - Scotian Slope at 42.75, -61.66 by ECSAS
Summer 2016 - UNKNOWN by ECSAS (3 ind.)
28 Jul 2016 - US cetacean and seabird surveys, 180 nm south of CSI
14-21 Aug 2016 - south of Georges and Browns Banks (11 ind., NOAA surveys)

It is apparent that Audubon's Shearwaters occur May-Oct in warm waters south of Nova Scotia. It is observed infrequently because of the difficulties involved in visiting its extreme offshore habitat.

Howell, S.N.G. 2012. Petrels, albatrosses and storm-petrels of North America. Princeton University Press, New Jersey.

McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada

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