This summer in Cornwall has been notable for a number of interesting seabird occurrences. Dave Flumm takes a look at some of the factors at play and possible reasons behind these extraordinary occurrences, with an emphasis on two species in particular, the enigmatic and rarely encountered Great Shearwater and Wilson’s Petrel which we have witnessed in the the South West recently …
In his article in The New York Times July 14th 2017 entitled ‘A mystery of seabirds blown off course and starving’, Joe Trezza describes the mass stranding of “hundreds” of Great Shearwaters washed ashore during June, all “emaciated” and in very poor condition. This mass mortality event which occurred along the stretch of coast from New York City south to Cape May was described as “extraordinary for the region” and those collecting the dead and dying birds concluded they had starved. It was thought that a lack of food in the Caribbean was the main cause rather than the waters adjacent to the beaches around New York. There had been no strong winds associated with their arrival so it seemed logical to assume the problem of food shortage lay further south.
Great Shearwaters breed on islands in the South Atlantic between September – April with the nearest to us in the Tristan da Cunha group, 1700 miles west of Cape Town, South Africa. After breeding, they undertake an annual transequatorial migration which takes them initially to the eastern seaboard of the USA up to Newfoundland and beyond, after which they track east across the North Atlantic to northern UK and Irish waters. Numbers here fluctuate yearly but we normally expect to see them in August/September although a few are recorded in July most years. The ‘Birds in Cornwall’ 2004 annual report tabulates the monthly summaries by year 1958 – 2004 and show only two years (1972 and 1999) with June occurrences. In contrast, this year one was seen from a pelagic off The Mermaid in Mount’s Bay 21st June with it or another off Porthgwarra next day, then one off Lizard Point 23rd June, followed by three off there 24th June…this was the start of a large influx into our waters and by July it was clear there were huge gatherings being seen off the south coast from Lizard to Scilly. Seawatchers on the north coast fared less well and even on strong northerlies, there were few being recorded; this seemed to be a south coast phenomenon.
Although a high count of 550 was recorded on one particular pelagic off Scilly (13th August), it hasn’t been the numbers involved that have been outstanding, so much as the regularity with which they have been seen. I can’t remember a time when Great Shearwaters could be almost guaranteed on a seawatch at Porthgwarra for example on a daily basis; it’s almost as if they have set up residence here!
So what are they doing here and why have they stayed? The answer to the first part of this question may lie with those off the eastern seaboard of the States. Could it be they became caught up in weather systems crossing the Atlantic in June and July and too weak to fight against them, simply got carried here? We have been told by the Met Office that the “bad weather” experienced in the UK this summer has been brought about by the jet stream “lying further south than usual”. It is of course the position of the jet stream that steers and develops our weather systems. Maybe this year the birds took a short cut instead of heading north to Newfoundland!
The second part of this question, ‘Why have they stayed?’ may be easier to answer: food availability. For example, on that Scilly pelagic, 13th August, in addition to the 550 Greats were 1200+ Cory’s, dolphins, 5 Minke Whales and 100 Atlantic Bluefin Tuna – all feeding on a fish food bonanza. All summer we have witnessed thousands of Manx Shearwaters and hundreds of Gannets in what birders have described as “feeding frenzies”. I’ve been seawatching for 51 years but I can’t recall a time like this – with so many rafts of Manx feeding in scattered groups, some hundreds strong for as far as the telescope can see and for days on end. Atlantic Bluefin Tuna are rare (I saw my first ever only last year off Pendeen) but this year I’ve seen them most weeks – and my first Minke and Fin Whales too, three of the latter in one day! Common Dolphins really have lived up to their name this summer, and Harbour Porpoise, Bottle-nosed and Risso’s Dolphins have also been regular.
What are they feeding on? I put this question to Terry George (of the Sennen Cove fishing community and webmaster of the bird news website http://www.sennen-cove.com/birds.htm) who commented as follows: “Certainly all the Pollack and Mackerel are full of small sprat. There are massive feed ‘marks’ on the fish-finders offshore. I see they had some Mackerel at Penberth right in against the cliff (they have never caught Mackerel in so tight) and again the thinking was that they were chasing Sprats…I suspect hundreds of thousands of tons of sprat out there!” This agrees with other fishermen I have asked who have seen “huge numbers of sprat” being chased in Penzance harbour and Lamorna Cove by Mackerel recently. On seawatches, it is clear from the feeding behaviour of Gannets diving into pods of dolphins that there must be huge shoals of fish out there and the surface-feeding antics of the shearwaters and dolphins themselves suggest they are taking small sprat concentrated in/on the surface waters – perhaps pushed there by the predatory Mackerel, dolphins and Tuna.
In relation to the presence and distribution of surface food, the following two observations are of interest. Firstly, on 20th August Reuben Veal watched a Black-tailed Godwit feeding over the sea at Mount’s Bay amongst a flock of Manx Shearwaters: “The godwit was picking something off the water and flying back up again on repeat for at least 3 minutes before I left.” Secondly, the extent of this food bonanza may be more widespread than Cornwall observers appreciate if the following account by Martin Cade on 22nd August at Portland Bill is relevant: “…some very promising-looking gatherings of hundreds of gulls attracted to bait fish shoals.”
So a good amount of feed must help explain why all these birds, dolphins, whales and tuna are here and why they are staying so long.
What of other species, for example Wilson’s Petrel? This has been an unprecedented year for Wilson’s Petrel. No fewer than 90 were seen in one week alone in UK and Irish waters! They have been a constant throughout the same period, with birds not only being seen on pelagic trips out of Scilly, but also out of Falmouth and Penzance this year – even those of us watching from the mainland at Mevagissey, Lizard Point, Mousehole and Porthgwarra have struck lucky. With more than 50 million pairs breeding around the Antarctic they have been described as ‘one of the commonest birds in the world’, and are the most numerous breeding seabird. They do have a northward migration into the North Atlantic each year, but they are still a very rare bird in our waters.
There may be other factors at play here such as ocean temperature, nutrient and plankton concentrations as these are surface-feeders which rely on a different food supply to the above. It is a harder question to answer why we have seen such numbers this year, but perhaps the postulated change in the North Atlantic Drift, (our branch of the Gulf Stream with some suggesting it has slowed, moved north, cooled and may even switch off altogether) and temperature drop in the northern part of the North Atlantic – in contrast to rising ocean temperatures globally – due to ice melt in the arctic, may have something to do with it. Are the record levels of melting polar ice affecting salinity, temperature, nutrient and plankton concentrations and hence food supplies for such species? These are also factors quoted in relation to the presence of Ocean Sunfish which have also been a feature of our summer. It’s complicated!
Whatever is responsible for these changes and abundance of seabirds, whales, dolphins, tuna and the like, there has never been a more exciting time to go seawatching in Cornwall.
Dave Flumm, Sancreed 27th August 2017.
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