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Some adaptive features of plumage types K. E. L. Simmons Plates 69-j 6

I. INTRODUCTION The plumage patterns of are very conservative, being mostly simple and restricted in type. Sexual dimorphism is rare and any bright coloration restricted to the unfeathered parts in all but a handful of . Such trends are found even in large species which breed in noisy, conspicuous colonies and have no serious predators. The majority of seabirds, by which I mean members of the families listed in table 1, may be assigned to one of three very broad categories of plumage type, according to the relative amount of 'dark' (usually black, brown or dusky grey) and 'light' (white or pale grey). Type-i consists of species which are wholly or mainly dark with restricted light areas (usually white) and the frontal aspect more or less dark. Type-2 consists of species which are usually dark above and white below, with the dark and light areas more or less equally distributed but the frontal aspect dark or only partly pale. Type-} consists of species which are wholly or mainly white or light grey, or a combination of the two, with restricted dark areas and the frontal aspect all or largely white. Of course, such a classification is an over-simplification, but it is probably adequate for present purposes. One may also, for con• venience, talk of dark, intermediate and light seabirds. Examples are given in fig. 1 and on plates 69-76. Allocation of plumage types in each of seabirds is made in table 1. Dark type-i forms predominate among storm-petrels (especially Oceanodroma and Oceanites), (Phalacrocorax), (Fregata) and skuas (Stercorarius); and are found quite frequently among shearwaters (e.g. Puffinus) and gadfly-petrels (e.g. Bu/weria); but are in a minority among albatrosses (e.g. Phoebetria), other petrels (dark morphs in Macronectes and Fulmarus), pelicans (Pekcanus), (Su/a), gulls (one species of Larus), (e.g. Cblidonias) and auks (e.g. Lunda). Most intermediate type-2 plumages are found among the penguins (all species of Spheniscidae), shearwaters (especially Puffinus), gadfly-petrels (especially Pterodroma), diving-petrels (all species of Pelecanoididae), skimmers (all species of Rynchopidae) and auks (e.g. Uria); but are less frequent among storm-petrels (mainly Pelagodroma and Fregettd), cormorants, frigatebirds (females of two species) and gulls (again, one species of Larus). Light type-3 plumages predominate among albatrosses (most Diomedea), tropicbirds (all species of Phaethontidae), pelicans, (all three species, which I think are best separated

465 Fig. i. Examples of three main seabird plumage types described on page 465. Top, type-i (dark): Phalacrocorax carbo. Centre, type-} (light): Kittiwake RJsta tridactyla. Bottom, type-2 (intermediate): Manx Shearwater Puffinus puffinus {sketches: Robert Gillmor) Seabird plumage types 467 Table 1. Seabird families with preliminary allocation of main adult plumage types In the absence of an up-to-date list of the world's seabirds, this table is based for convenience on Alexander (1955) and, as that author had a narrower species concept for certain groups than is now current, the 'forms' totalled include both species and some . Also included but only where plumage variation within the species necessitates allocation to more than one plumage type, are polymorphic forms (in Puffinus, Pterodroma, Sula and Stercorarius), sexually dimorphic forms (in Fregata), and seasonally dimorphic forms (in Cblidonias, Ceppbus and Plautus). The three types are denned on page 465. One example is given here of each type found in any family and the three in bold lettering (again, one of each type) are illustrated in fig. 1 Total 'forms' in each type Seabird families 1 2 3 Examples (plumage type in brackets)

SPHENisciBAE (penguins) 17 Adelie Penguin Pygoscelis adeliae (2) DiOMEDEiDAB (albatrosses) 4 9 Sooty Albatross Phoebetria fusca (1) Wandering Albatross Diomedea exulans (3) PROCELLARIIDAE (shearwaters 22 35 n Bulwer's Petrel Bui&eria bulwerii (1) and petrels) Manx Shearwater Puffinus puffinus (2) Dove Prion Pacbyptila desolata (3) HYDEOBATIDAE (storm-petrels) *3 7 1 Wilson's Petrel Oceanites oceanicus (1) Frigate Petrel Pelagodroma marina (2) Fork-tailed Petrel Oceanodroma furcata (3) PELECANOiDiDAE (diving-petrels) 4 Common Diving-petrel Pelecanoides urinatrix (2) FHAETHONTIDAE (ttOpicbirds) 3 Red-billed Tropicbird Pbaethon aetbereus (3) PELECANIDAE (pelicans) 2 6 Brown Pelican Pskcanus occidentalis (1) White Pelican Pekcams onocrotalus (3) (gannets and boobies) 2 8 Brown Sula hucogaster (1) Sula (Morus) bassana (3) MUIACROCORACIDAE (cormorants) 20 8 Cormorant Pbalacrocorax earbo (1) Pied Cormorant Phalacrocorax varius (2) FREGATIDAE (frigatebirds) 5 2 Christmas Island Fregata andrewsi (2) STERCORARIIDAE (skuas) 4 3 Great Skua Stercorarius skua (1) Light-phase Arctic Skua Stercorarius parasiticus (2) IARIDAE: LARINAE (gulls) 1 1 40 Dusky Gull Larus fuliginosus (1) Hemprich's Gull Larus bemprichii (2) Kittiwake Rissa tridactyla (3) : STERNINAE (tems) 7 38 Black Cblidonias niger in summer (1) Common Tern Sterna birtmdo (3) MNCHOPIDAE (skimmers) 3 Black Skimmer Ryncbops nigra (2) ALCIDAE (auks) 9 15 2 Tufted Puffin htmda cirrbata (1) Puffin Fralercula arctica (2) Black Guillemot Ceppbus grylh in winter (3)

TOTALS 89 95 118 468 Seabirdplumage types from the boobies in the Morus), boobies, gulls and terns (e.g. Sterna); but are less common among petrels generally (chiefly Fulmarus and Pachyptila), storm-petrels (one species of Oceanodroma) and auks (two Cepphus in winter). In all , survival depends to a large extent on the adequacy and accessibility of the food supply and the ability of the adults and independent immatures to exploit it. This is particularly true of those species which obtain their food from the sea: hence it is vital for seabirds—and especially the many tropical species which live in more impoverished waters—to be as efficient as possible in catching their food, so there must be strong selection pressure favouring any adapta• tions which increase that efficiency. It is to this selection pressure above all else that we must look for an explanation of the plumage colour of seabirds. Although many species are dark (type-i) or inter• mediate (type-2), most attention in the literature has been directed to speculation about the function of the more or less white plumage of certain type-3 birds. In particular, it has been disputed whether white plumage is a social adaptation for conspicuousness in the feeding situation, enabling congregations to form quickly at shoals of prey fish which are unevenly distributed at sea and need to be exploited quickly once located (Armstrong 1944, 1946), or whether it is a cryptic adaptation that facilitates a close approach by plunge-diving birds to their underwater prey (Thayer and Thayer 1909, Craik 1944, Tinbergen 1953). Of course, these two hypotheses are not mutually exclusive and both may apply to some degree in individual cases. The problem of the white coloration of seabirds was reviewed and investigated experimentally by Phillips (1962): although often referred to in the literature, this thesis has never been published and is unknown to the majority of ornithologists. Brief summaries appeared in Tinber• gen (1963, 1964). My own interest in the adaptive significance of seabird plumage types arose from my continuing study of the Sula leucogaster at , where observations were made from February 1962 to February 1964, in April 1966, and in December 1971 and January 1972 (see Simmons 1967a, 1970). I reviewed Phillips's work critically in my own thesis on the Brown Booby (Simmons 1967b) and generally discussed adaptive features of seabird plumage types, but that paper, too, is as yet unpublished and is even less well known than Phillips's. As the question of white coloration in seabirds has been raised again recently by Armstrong (1971), Murton (1971a, b) and Cowan (1972), this seems an opportune time for me to present my own views for more general appraisement. The present review is deliberately restricted and speculative in scope, but I hope it contains the seeds of the truth about certain aspects of the adaptive significance of seabird plumage types. My main aim has been to take the discussion further than Phillips did and, Seabirdplumage types 469 particularly, to produce a background survey against which the coloration of the Brown Booby and other Sulidae may be assessed,

2. PHILLIPS ON SEABIRD COLORATION Phillips (1962) recognised three types of white coloration in seabirds: (1) 'swimmer', in which white is confined to parts below the waterline, the rest of the plumage being dark; (2) 'plunge-diver', in which the white extends also to the head, breast and underwing, in fact the ventral and frontal aspect of the plunging , often including the leading edge of the wing; and (3) 'all-white', in which the bird is either totally white or, more usually, white with only dark rectrices or remiges, or both. The swimmer type of plumage is, according to Phillips, found chiefly in seabirds that pursue fish underwater and also spend much time swimming on the surface (e.g. auks and penguins); and the plunge-diver type in aerial seabirds, particularly those that drop into the water from a considerable height (e.g. many gulls and terns). He found the all-white type less easy to classify and thought it might be a special example of either of the other two, or else adapted to some entirely different (but unspecified) situation not connected with food- getting, as it is commonest among large species that are relatively immune from predation (e.g. albatrosses and pelicans). In a special study of the plunge-diver type, Phillips supported the theory of what he termed 'aggressive camouflage', revising Thayer and Thayer's (1909) argument that birds with white plumage are provided with 'concealment against the sky above, from the eyes of aquatic below them'. He devoted the larger part of his thesis to demonstrating experimentally that some fish, at least, were indeed more readily alarmed by—and started to escape sooner from—a black model than a white one, the latter being less conspicuous against the sky when viewed from below the surface. On the other hand, he suggested that the swimmer pattern possibly renders the bird less conspicuous to prey species when it is moving underwater. Such plumage, however, is probably of even more value to the bird as protective camouflage against underwater predators when it is swim• ming on the surface, those species that spend little time on the surface being dark below (e.g. cormorants). In Phillips's view, therefore, the white coloration of seabirds— particularly that of many plunge-divers—functions mainly as 'aggres• sive camouflage' enabling them to approach prey more easily than if their frontal and ventral aspects were dark. The dark dorsal plumage of certain plunge-divers and other seabirds may possibly, on the other hand, function as a shutter-mechanism to reduce exposure of the skin to ultra-violet light. Wholly or largely dark plumage received no special consideration from Phillips, but he implied that the species 47° Seabird plumage types concerned (such as cormorants, which pursue their prey underwater, and certain tropical terns, which feed from the air without plunging) have no need for the plunge-diver pattern. Also in this category are the dark immature forms of gulls in which protective camouflage is adaptive at the stage when they feed mainly in rocky coastal areas, rather than by plunge-diving at sea like the adults of their species. Phillips admitted, however, to certain exceptions to the rule that plunge-diving birds are largely white: for example, the more or less wholly dark Brown Pelican Pelecanus occidentalis and the immature forms of gannets. Here, he suggested, strong counter-selection for dark ventral plumage (the nature of which he largely did not examine) has asserted itself where the need for white aggressive camouflage is reduced, as in cold current areas where food is superabundant. But nowhere did Phillips consider that dark plumage itself may be an adaptation to the feeding situation. Finally, Phillips discussed whether white plumage functions in social feeding (as suggested by Armstrong). This hypothesis received neither his unqualified support nor his rejection. In the first place, he found it difficult to decide whether white is really more conspicuous than black over the sea as, in his opinion, both can be equally so in different conditions. He believed it most unlikely that white plumage has been selected directly in the social feeding situation as this would carry the implication of group selection. Generally, he argued, it is of no advan• tage to the individual or species which locates the prey (the 'first- finder') to draw other, potentially competing birds to the food source. Further, in cases where there may be no disadvantage to the first- finder in other seabirds joining it (for example, where food is super• abundant, as in cold current areas) or actually positive advantage in their so doing (for instance, when food can be more effectively exploited by group activity, as widi tropical species such as the noddy terns spp), the species concerned tend to be largely dark, not white. Phillips did, however, outline a possible mechanism whereby the white plumage evolved primarily in the individual as aggressive camouflage has become secondarily adapted as an 'obligatory' food- showing signal. This implies that it is of advantage for the 'summoned bird' to join the first-finder—which is almost certainly the case because fish shoals are overdispersed in the environment—selection therefore favouring response to the signal, both from individuals that are conspecific and those that are not. Thus this response would be main• tained even though food-showing was a disadvantage to the first-finder (individual or species).

3. COMMENT Phillips's stimulating findings have received published support from some that have read his thesis (e.g. Tinbergen 1964, Nelson 1965, Seabird plumage types 471 Ashmole 1971, Cowan 1972) and deserve much wider currency, which is one reason why I have summarised them rather fully. Yet, in my opinion, a certain caution is needed in applying them unreservedly, especially in the Sulidae, the family to which the Brown Booby belongs. First, the term 'aggressive camouflage' is inappropriate, for it is now widely accepted that hunting birds are not motivated by aggression in the usual sense (see, for example, Carthy and Ebling 1964); the qualifi• cation 'aggressive' is best reserved for social interactions, especially between conspecifi.es. Therefore, I put forward 'hunting camouflage' as an alternative term, which also seems preferable to 'anticryptk* as used by, for example, Cott (1964). Next, it may be regretted that Phillips did not review all the plumage types found in seabirds, for this omission limits the value of his findings on white coloration considered largely alone. Moreover, his terms 'swimmer' and 'plunge- diver', though useful in individual cases, are 'loaded' in implying function rather than being neutrally descriptive. Also, they are poten• tially confusing in that they refer both to a plumage type and to a form of behaviour which do not always coincide: thus, for example, the wholly white underparts of plunge-diving seabirds may also have a swimmer function, and some species that plunge-dive lack the plunge-diver plumage, while others apparently have it yet never actually plunge-dive. Phillips's conclusion that white plumage in seabirds is an adaptation to the feeding situation should, in my opinion, be extended to cover all the main plumage types, including dark. While other selection pressures must also come to bear in many cases (see section 9 in the second part of this paper), I think that these are of only secondary importance in most seabird species and that adaptations for feeding (in the widest sense) are primary. I suggest that such adaptations take three main forms: in some species, light plumage gives 'social conspicuousness'; in others, dark plumage (especially) bestows 'social inconspicuousness'; in both cases, the plumage may also function as hunting camouflage, but, I believe, chiefly in critical feeding conditions. These ideas will be developed in more detail in following sections. In individual species, of course, selection pressure for one or more of these adaptations may be operating at the same time or, in a few atypical seabirds, the factors may be of a quite different nature. In landbirds, it is often the pressure of predation that is important and this produces cryptic plumage; in most seabirds, however, this seems to be at best of minor importance, though the vulnerable young of certain species are subject to heavy predation and are cryptically coloured. Then there is also the special case of food piracy (clepto-parasitism). Seabirds, with their relatively simple en• vironment, are particularly suitable for the investigation of plumage colour; yet, while any conclusions here may well have wider signifi- 472 Seabirdplumage types cance, generalisations should be made with caution because the factors influencing plumage colour in other groups, with their much more complex environments, may be largely or entirely different. On the other hand, we should not be inhibited from speculating on the adaptive significance of seabird plumage types because the coloration of other birds convergent with seabirds in some aspects of their feeding biology differs markedly.

4. PLUMAGE ADAPTATION FOR SOCIAL CONSPICUOUSNESS Contrary to Phillips, I think it likely that social conspicuousness through plumage characters has been directly selected in many species of seabirds because it is of advantage, both to the first-finder and to the summoned birds, to congregate at shoals offish in to co-operate in exploiting them efficiently. Such social conspicuousness seems cer• tainly to be achieved through plumage that is light rather than dark, though contrasting light and dark plumage may well be the most effective of all. Aspects of colour contrast in the feeding behaviour of certain gulls and terns were discussed by Feare (1967). Although Gillham (1963) was of the opinion that light-coloured seabirds, which spend a lot of time in the air, are more difficult to see than dark ones, especially at a distance and when low down, it is my experience that predominantly light species are much more conspicuous than dark ones at sea in most conditions, particularly in the tropics, at least when viewed from an elevated position such as the deck of a ship or a cliff top. For this reason, Gannets Sula (Morus) bassana and gulls were chosen for aeroplane observations on homing by Griffin and Hock (1949) because 'white birds are easier to see against almost all kinds of terrain than those of any other colour' (Griffin 1965). Exactly how co-operation in feeding is achieved has still to be fully demonstrated for the majority of social feeding seabirds, but, at least on the crudest level, the 'swamping' of the prey by many predators simultaneously and the prevention both of co-ordinated escape move• ments and of the formation of dense, anti-predator 'packs'* are likely to be involved. Thus, it is easier for an individual seabird to secure certain prey as a member of a congregation than when hunting alone.

*Some observations of mine from Ascension are relevant here. In 1962, pelagic fish driven inshore by predatory fish, or coming in to spawn, often grouped into dense, conspicuous, circular shoals or 'packs'; at times, these extended for several metres in diameter on or near the surface and were of unknown depth. A dark, seething throng of fish was formed with the individuals so close to one another that they were pressed tight in a solid mass, splashing and bubbling on the surface. These shoals were primarily defensive against the underwater attacks of large predatory fish, to which they presented an impenetrable barrier; additionally, they served as a defence against the aerial attacks of seabirds. On all but one occasion, these fish packs had not a single seabird—not a booby or even an Ascension Frigate- bird Fregata aquila—in attendance. Seabirdplumage types 473 Of course, besides responding to conspicuous plumage clues, sum• moned birds joining others at a food source respond also to relevant behaviour, such as purposeful flying in a fixed direction and actual hunting, through the process of local enhancement (see also Rand 1954). Additionally, hunting birds of some species have what seem to be special 'food-calls' (Fringse/a/. 1955). A few more elaborate methods of co-operative feeding have been described. Thus, for example, American White Pelicans Pelecanus erythrorhynchos form swimming flocks and drive fish into shallow water (Hall 1925), while Double- crested Cormorants Phalacrocorax auritus congregate at times in very large, closely packed flocks over fish shoals, forming long, narrow, curved lines and diving in unison (Bartholomew 1942). In both cases, the success of individuals appears to be much greater than when fishing alone. Communal feeding over dense shoals of small fish has recently been commented on in the Shag P. aristotelis (King 1972), though this species, like many other cormorants, is mainly a solitary feeder.

5. PLUMAGE ADAPTATION FOR SOCIAL INCONSPICUOUSNESS At the other extreme from light seabirds are those dark ones either totally unrelieved by light markings or with these very restricted in distribution and inconspicuous at a distance. While the plumage of many such birds may have been evolved for hunting camouflage outside the plunge-diving situation (see below), it is possible that selection for social inconspicuousness in one or both of two other functional situations may also have occurred. First, dark plumage might render its wearer less conspicuous at sea to food pirates, such as frigatebirds and skuas. Then, those species that seek their food in impoverished areas or that exercise skilled, individual techniques in food-getting may also have developed dark plumage so that the first-finder does not attract other birds, of its own or other species, which would compete or interfere with it. Ashmole and Ashmole (1967) suggested that the dark plumage of such tropical seabirds as the noddy terns might function in this latter way. They also pointed out that, while these dark birds may be conspicuous against the sky when viewed from below, they are inconspicuous against the sea when seen from above, particularly in tropical waters which are normally dark blue. Insufficient is known about the feeding biology of most dark sea- birds, particularly certain shearwaters and petrels, for me to attempt any general assignment of species to this category. Moreover, at the present state of knowledge, it would be difficult in the majority of cases to distinguish between dark seabirds that are socially incon• spicuous and those which have dark plumage for hunting camouflage— a much better defined category, as we shall see in the next section. 474 Seabird plumage types 6. PLUMAGE ADAPTATION FOR HUNTING CAMOUFLAGE I certainly accept that many seabirds have evolved .plumage with a white frontal aspect for hunting camouflage in the plunge-diving situation, as demonstrated by Phillips (1962). The white plumage of some species may have this as its primary function, while in others it may be an adaptation for social conspicuousness to a greater or lesser extent, as suggested earlier, I believe, however, that what might be called the classic type of plunge-diver plumage (as defined by Phillips) may find its optimum effect only when food-getting is relatively difficult. This may well be the case, for example, in gulls feeding on dispersed fish near the surface, or in shallow water, and plunging more or less vertically from low heights with only partial submersion. Here, the white hunting camouflage may be particularly important in increasing the chances of success in critical situations. On the other hand, such camouflage is probably of little or no use when birds are feeding on dense shoals offish—especially, as is evident from Phillips's account, when they plunge either from great heights or from low elevations at oblique angles to the surface (see section 8 in the second part of this paper). Other seabirds may well have evolved dark plumage as hunting camouflage in quite different conditions than face those with white plumage. Thus, some of the most aerial seabirds that feed on insects (such as marsh terns Chlidonias spp) or on fish and other marine animals above and at the surface of the water (frigatebirds and noddy terns), or are food pirates (skuas and frigatebirds), are largely or wholly dark. The exact manner in which such an adaptation might operate has yet to be demonstrated; it is likely, however, that the dark plumage of certain species has been positively selected as hunting camouflage, not in the plunge-diving situation but in air-to-air and air-to-surface encounters between predator (or food parasite) and prey (or victim). Such plumage would not, therefore, arise merely because the selection pressure for white had been relaxed, thus allowing other pressures outside the feeding situation to operate, as suggested by Phillips. Cormorants may similarly have evolved dark plumage for conceal• ment against prey when hunting inshore on the sea bottom or in muddy water. Conversely, other seabirds that swim underwater in pursuit of their prey, such as most auks, are typically countershaded and this type of intermediate plumage may function, at least partially, as hunting camouflage in clear water or out in the open ocean.

7. PLUMAGE TYPES AND FEEDING BEHAVIOUR AT ASCENSION The above ideas can be tested by relating them in more detail to a seabird fauna. I have chosen that of Ascension as it is the one I know well and as it is small enough to treat briefly. Ascension lies in the tropical South Atlantic (approximately 8°S, i4°W) and the best general PLATE 69. Lesser Black-backed Gull Lar/is fiiscns and, below, Razorbill Alca lorda, Pembrokeshire, June (p/jo/os: David and Ka/ie Urry): examples of seabirds of the light and intermediate plumage types which have white and black frontal aspects respectively (compare especially the leading edges of the wings) (pages 465-479) PLATE 70. White Pelicans Pelecamts onocrotaliis, Bulgaria, May, and, immediately below, Brown Pelicans P. occidenlalis, Galapagos, April {photos: Eric Hoiking). The light plumages of most pelican species may be correlated with social feeding, but the Brown Pelican is a plunge-diver and as such its dark plumage is atypical PLATE 71. Frigate Petrel Pe/agodroma marina, West {photo: John W'arham); below, Cory's Shearwater Caloneclris diomedea, Great Salvage Island, July {photo: F. Ro/ix): two examples of the intermediate plumages of many Procellariiformes, but note that most storm-petrels, unlike the Frigate, are of the dark type (plate 75a) PLATE JZ. Great Frigntebird Fregata minor (left) and Red-billed Tropicbird Phae- tJjon aetberais, Galapagos, April (photos: Eric HosJk/ng); below, Shag Phalacro- corax aris/o/e/is, Northumberland, June (photo: David and Katie Urry). The dark plumages of the frigatebird and the Shag, which are typical of aerial and under• water feeders respectively, contrast with that of the tropicbird, a plunge-diver PLATE 75. Arctic Tern S/erna paradisaea, Northumberland, July (left), and Black Tern Chlidanias niger, Norfolk, May; below, Black-headed Gull Larits ridibimdns, Northumberland, July {photos: David and Katie Urty). Examples of a dark and two light scabirds which all show interesting changes, particularly loss of darkness in the frontal aspect, between the summer plumages seen here and the winter ones PLATE 74. Light-phase Fulmar Fulmar/is glacialis, Northumberland, July {photo: David and Katie Urry); below, Great Skua Stercorarins skua, Shetland, June (left), and light-phase Arctic Skua S. parasiticus, Orkney, June {photos: Eric Hosking). The two skuas have the mainly dark plumage of aerial food-pirates; polymorphism in the Arctic Skua and the Fulmar will be discussed in part 2 in the next issue PLATE 75. Group of Wilson's Petrels Oceaniles oceanicus, South Georgia (photo: Nia// Rankin); immediately below, Common Terns Sterna biritndo, Cornwall, July (p/joto: J. B. and S. Bottom/ey). Most storm-petrels are dark like Wilson's and they hunt typically by aerial-dipping, contact-dipping and pattering (page 478) PLATE 76. Above, Brown Booby Sitla lencu- gas/er, Galapagos, April (pbo/o: Eric Hosking); left, Black-browed Albatross Diomedea mtlano- pbris, Bass Rock(pbo/o: G. I ". Adkiri); below, Gannet S. bassana, East Lothian, July {pboto: David and Ka/ie Urry). Most albatrosses are light, though with a dark leading edge to the wings; plumages of boobies and gannets will be discussed in detail in part 2 in the next issue Seabirdplumage types 475 Table 2. Plumage types and feeding methods of seabirds at Ascension Island The species are illustrated in fig. 2; Audubon's Shearwater is often considered conspecific with the^Little Shearwater P. assimilis. Abbreviations: AP = aerial-pursuit, AD = aerial-dipping, CD = contact-dipping, P = pattering, H = hovering, S = stepping (all 'feeding while flying'); and PS = plunging-to-surface, PD = plunge-diving, SD = surface-diving, SF = surface-feeding (all 'feeding with cessation of flight') (see below and page 478 for further details). Brackets indicate that the method is apparently uncommon, at least at Ascension, and a question mark that it needs confirmation Feeding with Species (plumage type in brackets) Feeding while flying cessation of flight

Audubon's Shearwater Puffinus Iberminieri (2) ?PS ?PD SD SF Madeiran Petrel Oceanodroma castro (1) AD CD P Red-billed Tropicbird Phaethon aethereus (3) ?AP ?AD H PD Yellow-billed Tropicbird Phaethon Upturns (3) PAP PAD H PD Brown Booby Sula leucogaster (1) AP (CD) (H) PD PSD (SF) Sula dactylatra (3) PD Red-footed Booby Sula sula (1, 3) AP PD Fregata aquila (1) AP AD CD H Sooty Tern Sterna fuscata (3) AD CD PS Anous stolidus (1) AD CD (P) H S PS (SF) Anous tenuirostris (1) AD CD (P) H S PS Fairy Tern Gygis alba (}) AD

account of the island and its birds has been given by Stonehouse (i960, 1962). The indigenous seabirds are listed in table 2: of the twelve species, only the frigatebird is endemic; the Red-footed Booby Sula sula now has a remnant population (Ashmole 1963, Simmons 1968), while the status of Audubon's Shearwater Puffinus Iberminieri (now often considered conspecific with the Little Shearwater P. assimilis) is still uncertain. Most are either pelecaniform (two tropicbirds, three boobies and one frigatebird) or larid (four terns), with just a single shearwater and a single storm-petrel. Their plumage types and hunting methods are also summarised in table 2 and illustrated in fig. 2. At Ascension, as widely elsewhere in tropical waters, most of the local seabirds are dependent for food largely on schools of predatory fish and cetaceans which flush otherwise inaccessible prey fish to the surface layer. Ten kinds of hunting methods are considered, the classification being modified and extended from Ashmole and Ashmole (1967), on which I have depended particularly for descriptions of feeding in the terns (see Ashmole 1971 for a more recent classification). In six methods the bird remains airborne throughout ('feeding while flying'): aerial- pursuit, aerial-dipping, contact-dipping, pattering, hovering and stepping. In the other four the bird ceases to fly for a while ('feeding with cessation of flight'): plunging-to-surface, plunge-diving, surface- diving and surface-feeding. In aerial-pursuit the bird flies down its prey (or victim); this method of feeding is characteristic particularly Fig. 2. Twelve seabirds of Ascension, "with examples of feeding methods shown as insets (classified in brackets). Top left, Audubon's Shearwater T?uffinm (assimilis) Ihtrminieri (surface-feeding); right, Madeiran Petrel Oceanodroma castro (pattering and aerial-dipping). Centre left, Sooty Tern Sterna fmeata (hovering); right, Fairy Tern Gygis alba (aerial- dipping). Bottom left, Brown Noddy Anous stolidm (stepping and contact-dipping); right, Black Noddy A. temtirostris (plunging-to-surface). These two pages of drawings not to scale Top left, Brown Booby Sula leucogaster (aerial-pursuit of flying-fish); right, Masked Booby S. daetylatra (steep plunge-diving). Centre left, light-phase Red-footed Booby S. sula (dark phase, start of plunge-dive); right, Ascension Frigatebird Fregata aquila (hovering and dipping). Bottom left, Red-billed Tropicbird Pbaethon aethereus (plunge-diving); right, Yellow-billed Tropicbird P. Upturns (hovering before plunge-diving). The feeding methods of these species are discussed in some detail on pages 475-479 {sketches: Robert Gillmor) 478 Seabirdplumage types of the frigatebird. In dipping the bird first descends to the surface and then secures its prey either just above the water without actually touching it {aerial-dipping), or at or just below the surface by wetting the bill only {contact-dipping); one or both of these methods is charac• teristic of the frigatebird (in getting its own food and in snatching it from other species), of the Madeiran Petrel Oceanodroma castro and of the terns. In pattering the bird uses its feet almost continually to keep above the surface while flying along low; in hovering it rapidly beats its wings so that it becomes more or less stationary over the water; in stepping it lowers its feet at intervals, sometimes to push itself clear of the surface. Pattering on its own is particularly characteristic of the Madeiran Petrel; occasional pattering with hovering and frequent stepping is a trait of the two noddy terns; and the frigatebird often hovers close over its victims in preparation for snatching food. In plunging-to-surface the bird splashes into the water from flight, usually to submerge only partially, and takes off quickly; such behaviour again is characteristic of the two noddies and is also shown by the Sooty Tern Sterna fuscata. In plunge-diving the bird descends quickly from the air, usually from a height, and submerges totally; in surface- diving it disappears below the surface while swimming. At Ascension, plunge-diving is highly characteristic only of the tropicbirds and the boobies (not the frigatebird, which shuns the water and soon gets waterlogged); none of the common Ascension seabirds habitually surface-dives, though this has been recorded for Audubon's Shearwater elsewhere. Finally, in surface-feeding the bird takes its food from or near the surface while swimming there; this form of hunting was seen occasionally in the Brown Booby at Ascension and has been recorded elsewhere for Audubon's Shearwater and the Brown Noddy Anous stolidus. As a general comment, except in the case of some larger species, these tropical seabirds largely avoid sustained contact with the sea while hunting and, indeed, mostly at other times too, especially the Sooty Tern (see below). To a great extent this seems to be linked with the dangers the sea presents in the form of marine predators and offers a partial answer to the query of Halle (1971). If we consider plumage types and feeding methods of Ascension seabirds, some interesting correlations emerge. First, the majority of the dark species (Madeiran Petrel, Ascension Frigatebird, Brown Noddy and Black Noddy A. tenuirostris) obtain their food in the air or just at the surface of the sea; here, selection for hunting camouflage or social inconspicuousness, or both, is probably involved. Another species that feeds similarly, the Sooty Tern, is much more of a pu2zle, however, and shows just how complex the factors determining plumage type can be: this species is also dark above, but has a light frontal aspect and underparts. The latter feature may be linked functionally with its habit of sometimes feeding at night, by moonlight, as well as Seabird plumage types 479 by day. Thus, the dark upperparts may provide a measure of social inconspicuousness during the day, when it is most needed as a protec• tion from the diurnally active frigatebirds, and the light underparts social conspicuousness at night, again when it is most needed because of the poor light (and when its effect is reinforced by the characteristic• ally loud calls of this species). In addition, the Sooty Tern is entirely aerial while at sea, never voluntarily settling on the water (its plumage soon gets waterlogged) and spending long periods far away from land. It could, therefore, be in greater need of light underparts than other smaller seabirds, to camouflage it from large predatory fish when it flies close to the surface. Finally, its white forehead, leading wing edge and underparts combine to give it a typical plunge-diver pattern in the sense of Phillips, and this may well be correlated with its habit of plunging-to-surface, giving hunting camouflage in critical condi• tions when quick success is essential. The all-white Fairy Tern Gjgis alba is also an exception to the tendency for seabirds that feed at or above the surface of the sea to be dark. Ashmole and Ashmole (1967) suggested, however, that the 'pure white plumage of G. alba, and its extraordinary translucent wings and tail, appear to be very efficient in rendering it inconspicuous against the sky in dim light', and they associate this with its habit of crepuscular feeding on undisturbed prey. The plumages and feeding behaviour of the Ascension boobies will be considered in more detail in the next section, along with those of other Sulidae. In the remaining Ascension seabirds, the intermediate plumage of Audubon's Shearwater probably has to do with its habit of settling on the water to feed and also diving below from the surface (Harris 1969), while the light plumage of the two tropicbirds at least correlates with their habit of plunge-diving. It must be admitted, however, that while their largely white plumage appears ideally suited for social conspicuousness, there seem in fact to be no records of tropicbirds actually congregating in large numbers at food sources. Thus, King (1970) found that, in nearly 450 sightings of tropicbirds in the Pacific Trade Wind Zone, only 4.6% involved 'flocks' (defined in this case 'as a group of five or more birds acting as a unit'). Examina• tion of his raw data reveals, however, that while most sightings were of solitary birds, about 10% involved two. As other authors (e.g. Murphy 1936, Palmer 1962, King 1967) have mentioned the occurrence of tropicbirds in 'pairs' at sea, it is possible that they do tend to associate thus for feeding purposes and that social contact is facilitated by their brilliant white plumage (as well as by their loud calls).

(to be concluded)