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BIRDS AT SEA

Bill Bourne 

Watching seabirds can be discouraging. Lots of seabirds can be seen from the shore, but they are mainly familiar inshore species milling around- any strangers are liable to be too far away or to pass too quickly for reliable identification. Trips in small boats are liable to be similar with the addition of sea-sickness. During longer trips on larger boats little may be seen out at sea for much of the time except for terns calling at night in the tropics, until you go below to pack when approaching port, when you are liable to be told on returning that thousands of unrecognisable birds appeared over hundreds of cetaceans and tuna while you were below. Nobody will believe what you see anyway. In order to do better it is necessary to consider what is going on, and this is an attempt to describe some outstanding past investigations and summarise my own contributions.

The sea used to be regarded as a large quantity of undrinkable water mainly notable for unreliable weather. The distribution of birds over it begun to be plotted in the eighteenth and nineteenth centuries using guns, which was then necessary because they were still often new species, and nobody knew what any of them looked like alive. One of the first attempts to identify this was made by Edward Wilson during his trips to the Antarctic a century ago, for which he had to prepare his own field guide, now in the Alexander Library at Oxford; unfortunately he did not return to publish it. Then in the 1920s W.B. Alexander himself prepared what Roger Peterson recognised as the first proper field guide, The Birds of the Ocean, after trips to Australia and round the Southern Ocean, with guidance from Robert Cushman (Bob) Murphy who had made a trip to South Georgia with sealers (Murphy 1947).

One of the first attempts to work out what controls seabird distribution was made by Poul Jesperson (1930) during a Danish marine research expedition in 1920-22. He found that the number of birds seen in ten degree rectangles in the North Atlantic varied with the density of the plankton. Then Vero Wynne-Edwards (1935) recorded the birds seen in five-degree stretches during eight voyages between the UK and Canada in 1933, and showed that they could be divided into inshore, offshore and pelagic communities, with migrants passing by at certain seasons. Neither of these paid much attention to the bird communities of the subtropical boundary currents, but meanwhile Bob Murphy (1936) was relating the information about the seabirds of South America, and especially the Humboldt Current, to the knowledge emerging about oceanography from the Discovery investigations (it is a pity there is still nothing comparable for the northern hemisphere).

I became interested in the seabirds of the Canary Current during a student expedition to the Cape Verde Islands in 1951, which led to the study of the world-wide sea reports of the Royal Naval Bird-watching Society, and comparisons with the similar birds of the Arabian Sea (Bourne 1963). It became clear that there was a need for better organisation of British and Irish observations, which led to the formation of the first Seabird Group in 1966 (Bourne 1983a, 1989). While this was originally devoted to "sea-watching"- that is, watching birds from the shore- it soon emerged following the wreck of the oil tanker Torrey Canyon off Cornwall in 1967 (Bourne et al. 1967), and then a mass mortality of auks in the Irish Sea in 1969 (Bourne 1976a) that there was room for concern about both oil and toxic chemical pollution (Bourne 1976b). Funds were obtained for a national census of breeding seabirds named "Operation Seafarer" by the pioneer James Fisher, who unfortunately did not live to complete it (Cramp et al. 1974), and then a study of birds at sea around Scotland (Bourne 1982a). Full development of the latter required the use of computer technology by the Natural Environment Research Council (Tasker et al. 1987, Webb et al.1990).

The conclusion from all this is that seabird distribution tends to be controlled by water movements, and especially mixing which brings the nutrients that support the growth of plankton at the bottom of the food chain to the surface. In high latitudes this is promoted by bad weather in the winter, which is followed by the growth of plankton in the spring. In the summer here and in lower latitudes the nutrients are soon exhausted and the water becomes stratified with a sterile surface layer. Mixing occurs in the oceanic boundary currents, reinforced by tidal movements along the shore, and where currents diverge or converge out at sea, notably where the Antarctic, Subantarctic and Subtropical water masses characteristic of the main belts of winds meet in the southern hemisphere and along the Equatorial Currents and central Counter-current.

These phenomena are less marked in the closed oceans of the northern hemisphere, where the Corioli's force due to the rotation of the earth causes currents to deviate to the right, leading to a figure-of-eight circulation where the water moves clockwise around the oceans in the lower latitudes and anticlockwise in the higher ones, with upwelling in the intermediate area comparable to the southern subtropical convergence which seems to have received little attention. Where the prevailing wind drifts the surface water offshore there is additional upwelling along the boundary currents, reinforced by turbulence due to the tide, which incidentally combined with the SW prevailing wind may help explain the strong seabird communities off north-east Britain and Ireland.

All this results in the abundant birdlife offshore feeding largely on fish (and not only sand-eels: Bourne 1982a) which we all know about. In north-west European waters there is a wide continental shelf, so that until recently we have been less aware of events further out to sea, which are more obvious where the shelf is narrow, as on both coasts of the Americas. Here there tends to be further upwelling where deep currents hit the continental slope and other irregularities in the sea floor, such as submarine volcanoes or "sea-mounts" (Bourne 1986, 1992b), and birds such as the shearwaters, gadfly petrels tend to feed more on cephalopods, and possibly South Polar Skuas on the goose barnacles on floating objects (Bourne & Curtis 1994). There are no possible breeding-places so far offshore, so the birds feeding out there tend to breed to the south in our winter in the North Atlantic Islands and southern hemisphere, and are only seen exceptionally inshore. 

The situation is further complicated by variations in classification and nomenclature. Basically the Victorians went on splitting species until there were an impossible number. Then in the last century people started sticking them together again as races and superspecies including some rather distinct entities in the process. I first encountered this with the observation that the variation of giant petrels included two fairly distinct entities that bred at different times and moved in different directions (Bourne & Warham 1966). Then it emerged that the same thing was happening in the north with both the "soft-plumaged" petrels (Bourne 1983b) and the "Manx" shearwaters (Bourne et al. 1989). The main thing to remember about such things is that the birds remain the same whatever the fashionable variations in classification and nomenclature.

It may be asked what still needs to be done. Along the coast there is a need for the more methodical analysis of the vast accumulation of results of sea-watching along both sides of the British Isles (Wallace & Bourne 1981, Verrall & Bourne 1982) in relation to the season and weather using computer technology. Out at sea there is a need to get further out, a couple of hundred miles to the edge of the continental shelf, where there are sometimes a lot of interesting birds (Bourne 1986). The big break-through has come with observations from the UK-Spanish ferries which cross the deep water in the Bay of Biscay, where people appear to have become so intoxicated with the cetaceans that the birds have not been properly studied yet.

These observations need to be made methodically, with timed counts (I used to be subject to interruptions, so made them for ten minute periods, but tried to accumulate at least a hour's worth two or three times a day) and precise positions- I have usually been able to use the bridge with charts showing exact positions and depths with the water temperature recorded regularly, so that one knew where one was in relation to shelfbreak fronts and sea-mounts and upwelling, but it is now possible for anyone to obtain a satellite position indicator and find out at least their position for themselves. There are a lot of birds out there somewhere, if you can work out the right places.

References 

Bourne, W.R.P. 1963. A review of oceanic studies of the biology of seabirds. Proc. Int. Orn. Congr. 13: 831-854. (Discusses the work of the Royal Naval Bird-watching Society, and among other things compares the seabird communities of the subtropical areas of upwelling off West Africa and southern Arabia).

Bourne, W.R.P. 1972. Threats to seabirds. I.C.B.P. Bull. 11: 200-218.

Bourne, W.R.P. 1976a. The mass mortality of Common Murres in the Irish Sea in 1969. J. Wildl. Mgmt. 40: 789-792. (Reviews a lot of discussion. Originally attributed to pollution, it now seems more likely it was due to gales breaking up a front where the birds moult).

Bourne, W.R.P. 1976b. Seabirds and pollution. Pp. 403-502 in Johnston, R. (Ed.) Marine Pollution. Academic Press, London.

Bourne, W.R.P. 1980. The habitats, numbers and distribution of northern seabirds. Trans. Linn. Soc. New York 9: 1-14.*

Bourne, W.R.P. 1981. Some factors underlying the distribution of seabirds. Pp. 119-134  in Cooper, J. (Ed.) Proc. Symp. Birds of the Sea and Shore, 1979. African Seabird Group, Cape Town.*

Bourne, W.R.P. 1982a. The distribution of Scottish seabirds vulnerable to oil pollution. Mar. Pollut. Bull. 13: 270-273. (Shows the relation to satellite views of, fronts, and that their diet is not entirely composed of sand-eels).*

Bourne, W.R.P.  1982b. The manner in which wind drift leads to seabird movements along the east coast of Scotland. Ibis 124: 81-88.

Bourne, W.R.P. 1983a. Seabird problems. Pp. 226-231 in Hickling, R. (Ed.) Enjoying Ornithology. Poyser, Calton.

Bourne 1983b. The Soft-plumaged Petrel, the Gon-gon and the Freira, Pterodroma mollis, P. feae and P. madeira. Bull. Brit. Orn. Cl. 103: 52-58.

Bourne, W.R.P. 1986. Late summer seabird distribution off the west coast of Europe. Irish Birds 3: 175-198. (Unfortunately a satellite photo of the shelfbreak front SW of Britain did not reproduce well).

Bourne, W.R.P. 1989. Viewpoint- The Organization of Seabird Research. Mar. Pollut. Bull. 20: 158-163.

Bourne, B. 1990. Bird movements about the North Sea. Pp. 71-82 in Alexander, S.M.D. (Ed.) Birds and the North Sea. 10th Anniversary Publication of the North Sea Bird Club. North Sea Bird Club, Aberdeen. (Also covers radar observations of landbird migration).

Bourne, W.R.P. 1992a. Debatable British and Irish seabirds. Birding World 5: 382-390. (All the best unaccepted marine hoodwinks).

Bourne, W.R.P. 1992b. A concentration of Great Shearwaters and White-bellied Storm-petrels over the RSA Seamount in the South Atlantic east of Gough Island. Sea Swallow 41: 51-53.

Bourne, W.R.P. & Curtis, W.F. 1994. Bonxies, barnacles, and bleached blondes. Brit. Birds 87: 289-298. (Includes the distribution of the South Polar Skua in the northern hemisphere).

Bourne, W.R.P., Mackrill, E.J., Paterson, A.D. & Yesou, P. 1989. The Yelkouan Shearwater Puffinus (puffinus?) yelkouan. Brit. Birds 81: 306-319.

Bourne, W.R.P., Parrack, J.D. & Potts, G.R. 1967. Birds killed in the Torrey Canyon Disaster. Nature 215: 1123-1125. (Analysis of the beached birds).

Bourne, W.R.P. & Warham, J. 1966. Geographical variation in the giant petrels of the genus Macronectes. Ardea 54:45-67.

Cramp, S., Bourne, W.R.P.  & Saunders, D. 1974. The seabirds of Britain and Ireland. Collins, London. (The first complete census).

Jesperson, P. 1930. Ornithological Observations in the North Atlantic Ocean. Oceanogr, Rep. Danish 'Dana' Exp. 1920-1922. 7: 1-36. (First marine census).

Murphy, R.C. 1947. Logbook for Grace: whaling brig Daisy 1912-1913. MacMillan, New York. (How it used to be).

Murphy, R.C. 1936. The Oceanic Birds of South America. 2 vols., American Museum of Natural History, New York. (The most monumental pioneer study).

Tasker, M.L., Webb, A., Hall, A.J., Pienkowski, M.W. & Langslow, D.R. 1987. Seabirds in the North Sea. Nature Conservancy Council, Aberdeen.

Verrall, K. & Bourne, W.R.P. 1982. Seabird movements around western Islay. Scott. Birds 12: 3-10. (What happens on the west coast).

Wallace, D.I.M. & Bourne, W.R.P. 1981. Seabird movements along the east coast of England. Brit. Birds 74: 417-426. (What happens on the east coast- the title should have referred to Britain).

Webb, A., Harrison, N.M., Leaper, G.M., Steele, R.D., Tasker, M.L. & Pienkowski, M.W. 1990. Seabird distribution west of Britain. Nature Conservancy Council, Aberdeen.

Wynne-Edwards, V.C. 1935. On the habits and distribution of birds on the North Atlantic. Proc. Boston Soc. Nat. Hist. 40: 233-346. (Pioneer transects).

(I still have reprints of the obscure publications with asterisks).

W.R.P. Bourne

wrpbourne@yahoo.co.uk