Plumage ^ Variation and Identification of the 'Yelkouan Shearwater5 P

British Birds VOLUME 83 NUMBER 8 AUGUST 1990 Plumage ^ variation and identification of the 'Yelkouan Shearwater5 P. Yesou, A. M. Paterson, E. J. Mackrill and W. R. P. Bourne he classification of the European breeding shearwaters related to the TManx Shearwater Puffinus puffinus, and the identification of the Mediterranean forms yelkouan and mauretankus, colloquially known as the 'Levantine Shearwater' and the 'Balearic Shearwater', respectively, have remained a problem since the original discovery that shearwaters from the western Mediterranean belonging to a distinct race {mauretankus) regularly move north up the west coast of Europe during the summer to moult (Witherby 1921; Mayaud 1931). We have already suggested that, in view of their differences from puffinus in structure, coastal distribution, and early breeding season followed by a northward moult-migration, the Mediterranean forms should be treated as a distinct species, the 'Yelkouan Shearwater' P. yelkouan, with mauretankus included as a race (Bourne et al. 1988; see also additional notes in appendix to present paper). We now propose to discuss their identification. [Brit Birds 83:299-319, August 1990] 299 300 Identification ofYelkouan Shearwater The situation is complicated by the fact that when Mediterranean shearwaters were first collected in British seas in the last century they were all referred to as the eastern Mediterranean formyelkouan, which was the only one known at that time, although all specimens that could be traced were eventually identified as mauretanicus (Witherby et al. 1940). Following the increase in seawatching in the second half of the twentieth century, mauretanicus was found to be a regular summer visitor off southwest England from mid May to December, occurring less commonly farther north and east throughout British waters, with a marked peak in the northeast in September (Nicholson 1952; Ash & Rooke 1954; Parrack 1966; Hope Jones & Tasker 1982). More recently, there have also been further claims for the occurrence ofyelkouan (Cade 1983; Curtis et al. 1985; McCartney 1986), which it has already been suggested may be due to the variation and overlap in appearance of the two forms (Rees 1986; Dubois 1986; Yesou 1986). Further complications arise from the potential occurrence in north Europe of members of a similar group of smaller shearwaters with proportionately short wings which are characteristic of warmer waters and which also tend to wander north in the late summer (Bourne 1966, 1967; Post 1967; Curtis et al. 1985; Lee 1988). While these are usually treated as races of two species, the black-backed Little Shearwater P. assimilis and the brown-backed Audubon's Shearwater P. Iherminieri, they intergrade more than puffinus andyelfcouan, so that in this case there may be better reason for uniting them (Vaurie 1965; Bourne 1986). Although their regular occurrence in British waters has been disputed on the grounds that they must be (otherwise unknown) dwarf puffinus (Harris 1965; Perrins et al. 1965; Bourne 1966), small numbers of Little Shearwaters have recently been proved to occur where these 'runts' were reported off southwest Wales (James & Alexander 1984; James 1986). Curtis et al. (1985) and Bourne (1988) have also discussed the recent rejection of poorly authenticated old records of Audubon's Shearwater as well. Identification of Yelkouan Shearwater The following details on identification are based on our experience of mauretanicus off Britain (WRPB, EJM), western France (PY), southern Spain (WRPB, AMP, EJM), Mallorca (WRPB, AMP, PY), Menorca and Morocco (EJM); of yelkouan off southern France and Corsica (PY) and in the eastern Mediterranean (WRPB); of Audubon's off the West Indies (WRPB, AMP), eastern North America and Arabia; and of all the Atlantic forms of Little Shearwater, namely baroli off Madeira and the Canaries, bqydi off West Africa and the Cape Verde Islands, and elegans in the South Atlantic (WRPB). Size, proportions, flight and fizz Individual seabirds show much greater variation in behaviour and appearance with the weather and viewing conditions than is apparent between many closely related species. Thus, it is often difficult to be sure that postulated differences between species really exist without both seeing the species together and establishing the presence of underlying Identification of Yelkouan Shearwater 301 Fig. 1. Yelkouan Shearwaters Puffinus yelkouan. Top row: nominate race, worn plumage at right. Centre row: race mauretanicus, worn/faded plumage at left and right. Bottom row: race mauretanicus, showing some variation in underparts (A M. Paterson) variations in their structure and markings. The measurements and proportions of the North Atlantic representatives of both Manx and Little Shearwaters have already been summarised by Mayaud (1932), Bourne (1962, 1966, 1986) and Bourne et al. (1988). In general, although yelkouan is on average close in size to puffinus while mauretanicus is considerably larger, there is much overlap in their measurements. This difference in size (which varies according to the square of their length) must be associated with an even greater difference 302 Identification ofYelkouan Shearwater 175. Yelkouan Shearwater Puffinus yetkouan of the race mauretanicus, Menorca, Spain, May 1985 (Ed MackriU). Some juveniles can look darker than this individual in weight (which varies according to the cube), which tends to mask the fact that, whereas yelkouan and mauretanicus are similar in their proportions, puffinus has a larger head and longer wing and tail in proportion to the body (Mayaud 1932), but a shorter tarsus and bill and longer tail in proportion to the wing (Bourne et al. 1988). This means, among other things, that, while the Mediterranean forms have shorter tails, their feet project beyond them in flight and conceal the difference. The other Atlantic forms of shearwater are all much smaller, with proportionately short wings, and show a continuous variation in appear ance and proportions with an overlap in size from large, long-tailed Iherminieri of the West Indies, through intermediate boydi of the Cape Verde Islands to small baroli of the central northern archipelagoes in one direction, and large, short-tailed elegans of the South Atlantic in another (Bourne 1986). In consequence, while the appearance of all these shearwaters may vary in similar ways with changes in the weather, under comparable conditions there are differences in their build and flight of the type usual among seabirds (Jouventin & Mougin 1981) and discussed by Mayaud (1932), Bourne (1966) and Curtis et al. (1985). Thus, while puffinus usually appears a graceful, long-tailed species when soaring over the waves in strong winds in the open Atlantic, it can also look quite stout and clumsy when flying low over the water in calmer weather, while feeding over fish shoals and, especially, when in faded plumage and migrating south across the tropics (WRPB). Conversely, both the short-tailed forms (yelkouan and mauretani cus) of the Mediterranean seem very tubby with a bustling flight when flushed off the water in calm weather, fluttering away low over the surface with a series of rapid wingbeats followed by a short glide on stiff, down- held wings without soaring, when mauretanicus may appear comparatively ponderous and lag behind in mixed flocks (J. C. Finlayson in litt., though Identification ofYelkouan Shearwater 303 176. Yelkouan Shearwater Puffinus yelkouan of the race mauretanicus (colloquially called 'Balearic Shearwater'), Menorca, Spain, June 1984 (EdMackritt). Note faded upperwings V. Bretagnolle disagreed verbally). When the wind starts to rise, they may both also lift, glide and arc up to 5-10 m above the water like puffinus. These trends become more marked in the small forms with short wings and fluttering flight found in lower latitudes. There, Iherminieri appears a little smaller than puffinus, with a long tail used for manoeuvring in the air in calm weather over drifting weed and fish shoals, and assimilis looks much smaller and very compact, with its short tail adapted for more active flight in stronger winds. Plumage The Manx and Little groups of shearwaters show a type of countershaded plumage pattern found rather widely among aquatic birds, and often postulated to have a cryptic function since the birds appear dark against the water surface when seen by predators from above, but white against 177. Yelkouan Shearwaters Puffinus yelkouan of the nominate race (colloquially called 'Levantine Shearwater'), Corsica, France, May 1982 (Nicolas Crispini) 304 Identification qfYelkouan Shearwater 178. Yelkouan Shearwater Puffinus yelkouan, Menorca, Spain, June 1984 (Ed Mackrilt): probably a very pale individual of the race mauretamcus (from place and date), but cannot be distinguished from nominate race on plumage 179. Yelkouan Shearwater Puffinus yelkouan of the race mauretanicus, Menorca, Spain, May 1985 [Ed Mackrilt). Note capped and collared effect Identification ofYelkouan Shearwater 305 the sea surface or sky when seen from below. The main form of variation in such plumages, which may assist in control of heat loss through radiation, is in the shade of colour and amount of fading of the upperparts and the extent to which dark pigment extends onto the underparts. This tends to vary, and is difficult to interpret even in the hand without considering many specimens; these are already available (Bourne et al. 1988), but scattered so widely that few people appear to have seen many of them. At sea, further allowances have to be made for changes in the birds' appearance during the year owing to fading and wear of the plumage, the subsequent progression of the moult, and the effect of the light under different viewing conditions. In general, Murphy (1927, 1952) showed that in conformity with Gloger's Rule the forms occurring in cooler climates, such as puffinus and assimilis, tend to have stable black pigment above (which loses its gloss and becomes browner, but does not fade much with age); thus, they maintain throughout the year a fairly uniform black-and-white appearance, enhanced by a comparatively sharp division between the dark upperparts and white underparts.

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