2014 was a banner year in Nova Scotia for Cory's Shearwater (COSH) sightings. The first and last reports of Cory's were from Bon Portage (Aug. 15 - Oct. 21). The one day high count from Bon Portage was 18 on Oct. 18. During a NSBS near shore (~ 23 km) pelagic trip out of Sambro on Sept. 16, a crew of experienced birders observed an unprecedented 57 Cory's Shearwaters. See the links below for more information and photos from this historic day.
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This northern hemisphere breeder is a rare visitor to NS during summer and fall. In All the Birds of Nova Scotia, Ian McLaren describes how COSH is almost annual in warmer waters off our Atlantic coast. The many sightings of COSH in 2014 (137 from eBird) combined with the single day high count of 57 leads one to wonder what factors lead to this exceptional year.
Cory's Shearwater - Sept. 16, 2014 - Bruce Stevens
There were only ten provincial sightings of COSH in 2013 and twelve in 2012. 2011 had a high count of 19 during the Sept. 25 NSBS pelagic trip out of Sambro. In the Winter 2012 edition of NS Birds, Eric Mills explained that the unusually warm waters on the Scotian Shelf contributed to the high number of Cory's in 2011 (NS Birds Winter 2012). COSHs feed mainly on fish and squid in warm open ocean waters.
The Coastal Ocean Observation Laboratory of Rutgers University in New Jersey produces, and keeps an archive of sea surface temperature (SST) models of the Gulf Stream. The most striking trend found in the Sept. 2014 data is that the temperature of the majority of the Scotian Shelf is constant throughout the month (16 - 20 degrees Celcius). Previous years show much more variability in temperature, from 10 to 23 deg. Celcius.
After passing Cape Hatteras, the Gulf Stream turns towards the east and some of its meanderings break free and keep heading northward. These warm cores are typically 100 to 200 km across and can be up to 1.5 km deep. The usual alternating warm and cold waters on the Scotian Shelf can be attributed to this warm core mechanism.
The model below shows a cool Scotian Shelf in Sept. 2013 that never occurred in Sept. 2014.
SST for Sept. 27, 2013 from Rutgers Coastal Ocean Observation Laboratory
The following model depicts what the Scotian Shelf was like throughout all of Sept. 2014. The models do not show any days with substantial amounts of cold waters over the shelf (less than 16 deg. C.).
SST for Sept. 17, 2014 from Rutgers Coastal Ocean Observation Laboratory
We can hypothesise that continuous warm water conditions on the Scotian Shelf may have allowed warm water prey to make its way close to Nova Scotia. The variability of the temperature in previous years may not have been consistently warm enough to entice the Cory's Shearwaters's prey.
The sparse sampling of the abundance of Cory's Shearwater off of Nova Scotia's southern coast does not allow for a robust analysis of trends. There are probably a multitude of factors that influence the abundance of COSH that occur near NS but we can assume that food location and thus sea temperature is high on the list.
McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada.
Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.
Thurston, H. 2011. The Atlantic Coast: a natural history. Greystone Books, Vancouver, BC, Canada.