Some adaptive features of seabird plumage types K. E. L. Simmons Plates 69-j 6 I. INTRODUCTION The plumage patterns of seabirds 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 species. 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 family of seabirds is made in table 1. Dark type-i forms predominate among storm-petrels (especially Oceanodroma and Oceanites), cormorants (Phalacrocorax), frigatebirds (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), boobies (Su/a), gulls (one species of Larus), terns (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, gannets (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): Cormorant 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 subspecies. 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) SULIDAE (gannets and boobies) 2 8 Brown Booby Sula hucogaster (1) Gannet Sula (Morus) bassana (3) MUIACROCORACIDAE (cormorants) 20 8 Cormorant Pbalacrocorax earbo (1) Pied Cormorant Phalacrocorax varius (2) FREGATIDAE (frigatebirds) 5 2 <? Magnificent Frigatebird Fregata magnificent (1) $ Christmas Island Frigatebird 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) LARIDAE: STERNINAE (tems) 7 38 Black Tern 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 genus 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 birds, 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 Brown Booby Sula leucogaster at Ascension Island, 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 bird, 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 animals below them'.