Download Some adaptive features of seabird plumage types K. E. L. Simmons

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 convenience, 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 infig.1
Seabird families
SPHENisciBAE (penguins)
DiOMEDEiDAB (albatrosses)
PROCELLARIIDAE (shearwaters
Total 'forms'
in each type
1
2
3
17
4
9
22
35
*3
7
n
and petrels)
HYDEOBATIDAE (storm-petrels)
PELECANOiDiDAE (diving-petrels)
1
4
FHAETHONTIDAE (ttOpicbirds)
3
PELECANIDAE (pelicans)
2
6
SULIDAE (gannets and boobies)
2
8
MUIACROCORACIDAE (cormorants)
20
8
FREGATIDAE (frigatebirds)
5
2
STERCORARIIDAE (skuas)
4
3
IARIDAE: LARINAE (gulls)
1
1
LARIDAE: STERNINAE (tems)
7
MNCHOPIDAE (skimmers)
ALCIDAE (auks)
TOTALS
40
38
9
3
15
89
95
2
118
Examples (plumage type in brackets)
Adelie Penguin Pygoscelis adeliae (2)
Sooty Albatross Phoebetria fusca (1)
Wandering Albatross Diomedea exulans (3)
Bulwer's Petrel Bui&eria bulwerii (1)
Manx Shearwater Puffinus puffinus (2)
Dove Prion Pacbyptila desolata (3)
Wilson's Petrel Oceanites oceanicus (1)
Frigate Petrel Pelagodroma marina (2)
Fork-tailed Petrel Oceanodroma furcata (3)
Common Diving-petrel Pelecanoides
urinatrix (2)
Red-billed Tropicbird Pbaethon
aetbereus (3)
Brown Pelican Pskcanus occidentalis (1)
White Pelican Pekcams onocrotalus (3)
Brown Booby Sula hucogaster (1)
Gannet Sula (Morus) bassana (3)
Cormorant Pbalacrocorax earbo (1)
Pied Cormorant Phalacrocorax varius (2)
<? Magnificent Frigatebird Fregata
magnificent (1)
$ Christmas Island Frigatebird Fregata
andrewsi (2)
Great Skua Stercorarius skua (1)
Light-phase Arctic Skua Stercorarius
parasiticus (2)
Dusky Gull Larus fuliginosus (1)
Hemprich's Gull Larus bemprichii (2)
Kittiwake Rissa tridactyla (3)
Black Tern Cblidonias niger in summer (1)
Common Tern Sterna birtmdo (3)
Black Skimmer Ryncbops nigra (2)
Tufted Puffin htmda cirrbata (1)
Puffin Fralercula arctica (2)
Black Guillemot Ceppbus grylh in
winter (3)
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 adaptations 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 intermediate (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 Tinbergen (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 . P H I L L I P S 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 foodgetting, 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'. 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 swimming 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 'aggressive 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 advantage to the individual or species which locates the prey (the 'firstfinder') to draw other, potentially competing birds to the food source.
Further, in cases where there may be no disadvantage to the firstfinder in other seabirds joining it (for example, where food is superabundant, 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 Anous 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' foodshowing 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 maintained 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 qualification '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 'plungediver', though useful in individual cases, are 'loaded' in implying
function rather than being neutrally descriptive. Also, they are potentially 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 criticalfeeding 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 environment, 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 order to co-operate
in exploiting them efficiently. Such social conspicuousness seems certainly 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 movements 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,
thesefishpacks had not a single seabird—not a booby or even an Ascension Frigatebird Fregata aquila—in attendance.
Seabirdplumage types
473
Of course, besides responding to conspicuous plumage clues, summoned 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 Doublecrested 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 seabirds, 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 inconspicuous 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 H U N T I N G 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 concealment 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 F E E D I N G 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
P L A T E 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)
P L A T E 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
P L A T E 71. Frigate Petrel Pe/agodroma marina, West Australia {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)
P L A T E JZ. Great Frigntebird Fregata minor (left) and Red-billed Tropicbird PhaetJjon aetberais, Galapagos, April (photos: Eric HosJk/ng); below, Shag Phalacrocorax 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 underwater feeders respectively, contrast with that of the tropicbird, a plunge-diver
P L A T E 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
P L A T E 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
P L A T E 75. G r o u p 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)
P L A T E 76. Above, Brown Booby Sitla lencugas/er, Galapagos, April (pbo/o: Eric Hosking);
left, Black-browed Albatross Diomedea mtlanopbris, 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, A D = 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
Species (plumage type in brackets)
Audubon's Shearwater Puffinus Iberminieri (2)
Madeiran Petrel Oceanodroma castro (1)
Red-billed Tropicbird Phaethon aethereus (3)
Yellow-billed Tropicbird Phaethon Upturns (3)
Brown Booby Sula leucogaster (1)
Masked Booby Sula dactylatra (3)
Red-footed Booby Sula sula (1, 3)
Ascension Frigatebird Fregata aquila (1)
Sooty Tern Sterna fuscata (3)
Brown Noddy Anous stolidus (1)
Black Noddy Anous tenuirostris (1)
Fairy Tern Gygis alba (})
Feeding while
flying
Feeding with
cessation of flight
?PS ?PD SD SF
A D CD P
?AP ?AD
H
PAP PAD
H
AP
(CD)
(H)
AP
AP AD
AD
AD
AD
AD
CD
H
CD
CD (P) H S
CD (P) H S
PD
PD
PD PSD (SF)
PD
PD
PS
PS
PS
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'): aerialpursuit, 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, surfacediving and surface-feeding. In aerial-pursuit the bird flies down its
prey (or victim); this method of feeding is characteristic particularly
(SF)
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 (aerialdipping). 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 characteristic 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 surfacediving 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 protection 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 characteristically 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 conditions 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'). Examination 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)