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Reprinted
from British Antarctic Survey Bulletin, No. 2, December 1963, p. 53-71
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND,
SOUTH ORKNEY ISLANDS
By N. V. Jones
ABSTRACT. A study of the sheathbill, Chionis alba (Gmelin), was
carried out at Signy Island, South Orkney Islands, between April 1961 and
April 1962. The general characteristics of the species and the annual population
cycle have been investigated. Breeding birds return to the island early in
October, form pairs early in November, and establish territories later in
November. There is a high level of fidelity to a previous year's mate and
considerable nest-site tenacity. Nests are concentrated around penguin (Pygoscelis
adetiae and P. antarctica) colonies, the closer association
being with the latter species. Both sexes participate in nest construction,
incubation and the care of the chick. Egg-laying begins in early December,
but full incubation starts only when clutches, which number from one to four,
are complete. The normal incubation period is 30 days. Chicks are fed largely
on "krill", obtained by disturbing penguins which are feeding their
own young, and the sheathbill's life cycle is suitably coincident with that
of the penguins. Chicks fledge when 50 to 60 days old and at this stage they
become shore feeders. Chicks and adults disperse from the island in about
mid-May, and there is evidence of a rather loose northward migration in winter.
No definite predators are known. A number of parasites have been recorded.
THIS paper describes work done on the sheathbill at Signy Island (lat. 60°43'S.,
long. 45°36'W.) in the South Orkney Islands, while the author served
as meteorologist with the British Antarctic Survey. Initial observations
were made around the Survey station during the 1961 winter. In the first
weeks after the sheathbills returned in the spring of 1961, their breeding
grounds on Gourlay Peninsula were visited on all possible days, and between
November 1961 and April 1962 long periods were spent in residence at a field
hut erected in this area. Some nests had already been marked and breeding
birds colour-banded, chiefly by P. R. Richards in 1958-59 and the late R.
Filer in 1960-61, and this preliminary ground work proved invaluable. Between
late November 1961 and late March 1962 many nests were visited daily. Individual
recognition of birds was made possible by banding with 10 mm. monel metal
rings, adults on the left and nestlings on the right tarsus, and by banding
breeding adults with coloured plastic spirals which were conspicuous at a
distance. Aircraft cellulose paint was used for temporary marking. Nest sites
were marked with paint and by stakes or cairns and eggs were paint-marked
within 24 hr. of laying. Eggs were weighed and measured, and certain broods
of chicks were weighed and measured regularly throughout their pre-fledging
period. Many adults were also weighed and measured, and a few were dissected.
Counts were made of the numbers of birds seen around the station during winter
and spring, and many photographs (still and cine) were taken at all seasons.
GENERAL BIOLOGY
Classification and distribution. The sheathbill is a prominent member
of the avifauna in the Scotia Arc-Graham Land region. The first specimens
were collected at Isla Ano Nuevo (lat. 54°40'S., long. 64°16'W.)
in 1775 during Cook's second voyage (Forster, 1777), and the species was
described as Vagina/is alba Gmelin in 1778 and re-classified in
the genus Chionis Forster in the same year. Today the Chionididae
(Charadriiformes) are considered a monogeneric family, Chionis (Peters, 1934)
including two species, alba and minor. Chionis minor Hartlaub
is represented by different subspecies at Prince Edward Islands, lies Crozet,
lies de
Kerguelen and Heard Island. The last of these islands is the subject of current
research by M. C. Downes and others (1959).
All the known breeding grounds of Chionis alba lie south of the
Antarctic Convergence; at South Georgia, probably the South Sandwich Islands,
the South Orkney Islands, the South Shetland Islands and the northern part
of the west coast of Graham Land (Fig. 1). Nonbreeding birds occur regularly
north of the Convergence at the Falkland Islands, and on the east coast of
South America as far north as the Rio de la Plata. The numbers in these regions
are reported to increase in winter (Murphv, 1936).
54 BRITISH ANTARCTIC SURVEY BULLETIN
Appearance of the adult. The plumage is pure white (Fig. 2)
over a dark grey layer of down. A pink bald patch lies below the eye,
and in adults this is quite convex and bears superficial markings. The
eyes are also surrounded by a bald rim. Adult birds also have a small
swelling in the feathered region just above and in front of the eye.
Dorsally and laterally around the base of the bill there is a cluster
of pink caruncles which vary in size and extent from bird to bird. These
show no evident sexual difference but they are larger in older individuals
(Fig. 3).
The beak is strong and stout with a deep broad base. The characteristic horny
sheath, usually greenish with local yellow and blue shadings, completely
covers the base of the upper mandible and extends forwards to just in front
of the nostril. Anterior to the sheath, both mandibles bear an orange patch.
The dorsal and ventral surfaces and the tips of the beak are black.
The legs are dark grey and bear unwebbed toes with strong black claws. The
wings have a short black spur.
Gait. Sheathbills run quickly for short distances,
with rapid strides and little movement of the body, presenting
a strongly galliform appearance. The birds often appear reluctant
to take to the wing even when chased. Breeding birds, which
spend most of their time in the nesting area, generally forage
and carry food to the young on foot.
Flight. Sheathbills fly with a flapping action reminiscent
of a pigeon. Over the breeding
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 55
grounds they generally fly below about 20 m. and they also fly very low when
crossing short stretches of water, but farther out at sea they seem to fly
at, or above, mast height.
Feeding. The species is omnivorous and exploits all available
food sources. Generally at Signy Island the birds are shore feeders,
taking algae and limpets from the intertidal and splash zones. Limpets
are common in the lower littoral zone and form an important element in
the diet during spring and autumn. The birds prise the molluscs from
the rocks with the sides of the beak and remove the flesh from the shell
by vigorous shaking or by holding the shell with the feet and pecking
out the contents. The alga most commonly taken is a green filamentous
species which grows abundantly in the upper littoral and splash zones
in summer
Fig. 2. A pair of sheathbills above their nest. and autumn.
This food is not available in spring because winter snow cover and frost,
and the lack of spray from the frozen sea, causes die-back and renders the
coastal rocks barren at the beginning of the open season. But during late
summer and autumn, especially when breeding is finished, this coastal algal
felt is an important food. The birds scrape it from the rock with a scissoring
action of the beak, the head being held on one side. The very long (20-25
cm.) rectal caecae of the bird may be a specialization for the digestion
of such food. During the penguin breeding season the sheathbills generally
forsake the seashore and feed among the penguin colonies. They are adept
at stealing poorly guarded penguin eggs and are suspected of killing very
young chicks. Penguin excrement—a food of doubtful nutritional value—is
eaten in quite large quantities. Any carrion, such as the remains of skua
(Catharacta skua) meals, is also taken. The most important food
source in penguin colonies is, however.
56 BRITISH ANTARCTIC SURVEY BULLETIN
regurgitated "krill" (mainly euphausids) which the penguins feed
to their young. This is the primary diet of sheathbill chicks and is discussed
in detail in a later section. During the breeding season of the Weddell seal
(Leptonychotes weddelli) in August- September, the sheathbills attend
pupping animals and feed on afterbirths and blood (see also Valette, 1906).
In winter the Survey station also provides a food source, the birds foraging
for kitchen refuse and among the dog spans. Finally, dissected birds invariably
have gravel in the gizzard and in many cases they also contain cephalopod
beaks. These are presumably picked up on the shore, either washed up or possibly
from seal faeces.
Fig.
3. Heads of sheathbills showing the facial characteristics.
A.
Adult bird. B. Juvenile bird (first winter).
Excretion.
The faecal material is generally rather wet. When excited, the birds evacuate
the rectal caecae which contain a mass of fluid, the nature of which depends
on the diet. This reaction commonly occurs when the birds are handled and
may be repeated several times within a short period.
Roosting. Outside the breeding season the birds tend to congregate
in small groups and roost on sheltered rocky ledges overlooking the
sea and usually only a few metres above it. During the breeding season
non-breeders continue to use communal roosting places, while breeders
roost near their nests. The birds usually roost on one leg, with their
heads tucked into the axilla. The habit of standing on one leg is common,
especially in winter when the birds even forage for food with one leg
tucked up into their ruffled feathers.
Bathing and preening. Sheathbills keep
their plumage in remarkably clean condition,
even when foraging in muddy penguin colonies
in summer. They bathe frequently in pools by
the and preen on the spot, or near to the nest
if they are breeding birds.
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 57
Voice. The voice is harsh and throaty
and, although there are several different calls,
the voice varies little.
ANNUAL POPULATION CYCLE AND MIGRATIONS
Although sheathbills are present at Signy Island throughout the year, the
population is by no means static. Fig. 4 shows the approximate maximum number
of birds around the area of the station between April and November (as a
block histogram) and the number of colour-ringed known breeders from Gourlay
Peninsula seen in the same area at the same time (dotted curve).
Very few birds occur near the station in the summer since most of them are
in the penguin colonies some distance away. During the second half of March
numbers increase and then remain high until May or June. This March-May peak
is partly due to an influx of breeding birds and the May-June drop is believed
to be correlated, at least in part, with their departure from the island.
The second increase, which occurs in June and includes no known breeding
birds from Signy Island, may be due to the arrival of birds from farther
south. The decline in July, which becomes more noticeable during and following
very cold spells, may similarly indicate a continued northward movement.
The very evident decline in numbers in late August has been reported before,
and is correlated with movement to the pupping areas of the Weddell seal.
The ensuing rise in numbers in October-November is certainly partly due to
the return of breeding birds, and therefore the population may be assumed
to be migrating southwards again at this season.
Finally, after mid-November the majority of the birds are concentrated in
the neighbourhood of penguin colonies. It has long been suggested that sheathbills
cross Drake Passage from the Antarctic to the Falkland Islands and South
America in the autumn, to augment the groups which spend the summers there
but are not known to breed. The general pattern of population increase and
decline at Signy Island, and the known disappearance and return of marked
breeders, supports such a concept of a rather loosely denned seasonal migration,
and there are some banding returns which provide more concrete examples.
One bird marked in the Falkland Islands
58 BRITISH ANTARCTIC SURVEY BULLETIN
in August 1961 was recovered at Signy Island in January 1962, and there has
been a sight record in the Falkland Islands of a bird believed to have been
ringed at Signy Island as a nestling.
These migrations are, however, not simply north-south movements as is shown
by the recovery near the South Shetland Islands of a bird banded as an immature
on Signy Island. This bird was banded in June 1961 and recovered on a Russian
ship in December 1962 in lat. 60°14'S., long. 62°07'W.
Some homing experiments have shown that sheathbills can return successfully
to their breeding grounds over large tracts of ocean, but that they do so
rather slowly. Two breeding birds from Signy Island were sent to South Georgia
in January 1961 by the late R. Filer, and returned to Signy Island in time
for the following breeding season. In another experiment before breeding
started in December 1961, two birds were released at sea to the south-west
of the Signy Island. One bird, liberated 72 miles (116 km.) away, returned
10 days later, but the second, released 180 miles (300 km.) off, has not
yet been recorded back. Finally, two unsuccessful breeders were transported
to the Falkland Islands in February 1962 and released in the outer harbour
while approaching Port Stanley. One of these birds was found dead on a nearby
beach soon afterwards, while the other returned to Signy Island in October
1962.
There is, therefore, good general evidence for a seasonal migration northwards
and southwards, but it is certainly not a universal or regular movement.
For example, while none of the 73 banded breeding birds at Signy Island was
seen there during the winter, a pair of breeders marked at the Argentine
Islands by the late C. M. Smith (1960) remained around that station during
the winter. At Signy Island many adult sheathbills of uncertain provenance
stay all winter around human habitation, and it is difficult to decide whether
they remain in the area only because the station is there, or whether the
station attracts birds that would under natural conditions be more widely
spread over the surrounding country. In winter these birds can probably find
natural food among the seals and penguins in the pack ice and where planktonic
Crustacea are washed up on to ice floes.
THE BREEDING CYCLE
Return and pre-nesting activities. In 1960-61 73 birds breeding
on Gourlay Peninsula were banded by the late R. Filer and R. Pinder. All
disappeared during the following winter and the first to be seen again arrived
at the British Antarctic Survey station on 3 October 1961, No colour-banded
birds had been observed during visits to the breeding area on the twc preceding
days.
Between 5 October and 5 November the number of marked birds on Gourlay Peninsula
increased rapidly (Fig. 5). At the beginning of this period the island was
still completely
Fig. 5. Build-up of the breeding population in the nesting area during the
spring of 1961.
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 59
surrounded by ice and, although leads and pools appeared during October,
the final break-up did not occur until early November. During this time,
the greater part of Gourlay Peninsula was snow-covered, and this was the
case even when the Adelie penguins (Pygoscelis adeliae) commenced
laying on about 24 October. The nest sites of most of the sheathbills were
not free of ice and snow until late November or early December. During the
pre-breeding period the sheathbills spent their time foraging among the penguins
and seals on the shore, or around the British Antarctic Survey station. They
also spent considerable periods in the vicinity of their previous season's
nests, but they did not establish territory and appeared to take little interest
in their past season's mates.
Pairing. In a few cases, pairs were
seen together in the early pre-nesting period,
but such associations were loose and temporary,
and probably occurred because of chance meetings
in the vicinity of the old nest. However, one
known pair was seen displaying to each other
at the station on 6 October, but this display
was at low intensity and the birds were later
seen performing the same ritual with several
other individuals. It was not until early November
that attachment to the nest area increased
significantly, and definite pairs formed and
began to defend a territory. In the majority
of cases the pair-bonds of the previous year
were reformed
and the same nest site was occupied.
Of the 73 colour-banded breeding birds of 1960-61, 66 returned, including
20 pairs and 5 individuals rendered "unattached" by the failure
of their mates to return. All the pairs that returned re-formed and re-occupied
their old territory. The "unattached" birds soon found new mates
and occupied the territory used by the banded bird in the previous season.
This suggests that the birds that fail to return are usually replaced by
unestablished breeders.
Some changes of nest site were recorded, mostly temporary ones necessitated
by the presence of ice in the original nest. The only other changes were
of sites within the same territory and these were suspected to be due, at
least in part, to the observer's interference at building time.
These observations indicate a considerable degree of faithfulness to the
previous year's partner, and a high degree of nest-site tenacity.
The most conspicuous display performed by sheathbills is the "bowing
ceremony" which is seen both during and after pair formation. The birds
stand facing each other about 1 ft. (0-3 m.) apart and bow to one another,
usually several times in quick succession, within a 5-sec. period. Bowing
is usually accompanied by a low "muttering" sound, and the birds
often peck at each other's bills between bows. The intensity of the ceremony
varies from an almost casual nod to a prolonged series of bowing periods.
The intense display is universal during the courtship period but it is also
very common between established pairs and seems to be important in maintaining
the pair-bond.
Territory. Establishment of territory
is a gradual process which follows pairing.
Although some pairs are formed fairly early
in November, the territory is often not really
established until two or three weeks later.
Its size is primarily governed by the situation
of the nest, although there is a good deal
of variation which depends on the birds themselves.
If the nest is sited inside a penguin colony,
the territory is often quite small, but otherwise
it may be rather extensive (of the order of
200-2,000 m.2). It was found difficult to map
territories accurately, except where the boundaries
lay close to nests, mainly because of the penguins
and the rough terrain. Most birds had a single
territory within which they both bred and fed,
but in a few instances separate breeding and
feeding territories were held and in one case
these were about 350 m. apart. This latter
situation arises in birds which feed in penguin
colonies offering no good nest sites. In such
cases one bird generally remains near the nest
while the other forages. The feeding territories
are poorly defended but the nesting ones are
easily looked after as they are quite small.
Judging from the existence of such cases, and
the presence of a number of non-breeders which
showed signs of maturity, the Gourlay Peninsula
colonies seem to be supporting about the maximum
number of breeding sheathbills they can accommodate.
Defence of territory. Boundary encounters
between the occupants of neighbouring territories
were frequently seen, especially where nests
lay close together. On these occasions the
birds stood facing each other in threatening
attitudes, each on its own side of the boundary,
and usually moved slowly along the boundary
in such postures before one turned and returned
to its nest. The threat posture of the sheathbill
is a forward-oblique pose which is usually
60 BRITISH ANTARCTIC SURVEY BULLETIN
accompanied by a "bill-wiping" action on the ground in front. The
posture is usually accompanied by a single "caw" and this call
is repeated during the bill movement.
Fights were seen occasionally, mostly during the courtship period, when territories
were still imperfectly established. While fighting, the birds faced each
other in extended upright postures, pecked at each others heads and often
beat at one another with their wings. When a fight was at its most heated
a bird sometimes rushed at its opponent, gripped any appendage with its beak
and hung on tightly. This resulted in vigorous fluttering and apparent submission
of the bird thus treated. Such fights occasionally caused minor injuries
around the head, but usually resulted only in muddied and bedraggled plumage.
The territory is generally watched from a prominent rock above the nest by
one or both birds. If an intruder appears, the occupant either runs or flies
from its vantage point towards it. In the latter case it alights close by,
finally attacking on foot. Threatening movements of this kind normally cause
the immediate retreat of an intruding sheathbill. Attacks are also made on
intruding brown skuas (Catharacta skua), penguins which approach
too near the sheathbill nest and humans; the author has many times been attacked
while handling chicks.
If a skua alights anywhere near a nest the occupants commence an excited
threat display with frequent "bill-wiping" and "cawing",
and in many cases one sheathbill will fly at the intruder. Such attacks are
generally made from behind and are only lunges followed by hasty retreat,
but they are usually sufficient to discourage the intruder.
Nesting localities. Sheathbills
were not seen in the interior of even so
small an island as Signy Island, and they
appear to travel and breed strictly within
the coastal zone (Fig. 6).
Fig. 6. Map of Signy Island showing the sheathbill breeding areas (excluding
Moe Island). These coincide with
penguin colonies except the two marked ®. Stippling represents permanent
ice.
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 61
All except two of the known nests were associated with Adelie (Pygoscelis
adeliae) or chinstrap (P. antarctica) penguin colonies. Both
penguin species breed on promontories which become snow-free in summer, the
Adelie penguins occupying flat ground while the chinstrap penguins concentrate
on rocky slopes overlooking the sea.
The majority of sheathbill nests are situated in crevices between, or in
holes below, rocks and some of them are very deeply placed. A few occur on
rather open ledges. The terrain prevents burrowing. The general requirements
seem to be:
| i. A sheltered position, preferably in a crevice, |
| ii. Proximity to a food supply, |
| iii. Suitable observation point above the nest, |
| iv. Proximity to the shore. |
Such
conditions are more commonly found on the rocky slopes occupied by chinstrap
penguins than among the Adelie penguin colonies, and it is therefore not
surprising that the majority of sheathbill nests occur close to, or among,
colonies of the former species (Fig. 7).
The nest and its construction. During
the last weeks of November 1961 there
were signs of scraping and rearrangement
of material in the nest holes. At this
time, however, many nests were still
ice-bound, and a number remained so until
mid-December. Under these conditions,
where the nest hole itself was accessible,
the birds often started a new nest on
the ice above or near the old one. The
subsequent thawing of this ice was often
followed by a shift of the nest to the
old position.
In one case, a pair of birds, whose original nest was still buried by ice
as late as 18 December, built and laid two eggs in a new nest 20 m. away
and on the other side of another territory. By 21 December, however, the
original nest had thawed out. Eggs were laid in it on 22 and 26 December,
and no further attention was paid to the new nest from which the eggs soon
disappeared.
Both birds participate in nest building, the male in general being the more
active in collecting material while the female does more of the arranging
within the nest hole. This division of labour is, however, a tendency rather
than a rigid or universal rule. The nest material is gathered within the
birds' own territory and foraging is a lengthy and apparently selective occupation.
Often many pieces of material are picked up and discarded before a bird finally
takes something back to the nest.
Nest construction involves simple actions. A bird sits in the middle and
picks up any material within reach of its beak, dropping it in front or over
its shoulder. This process is repeated on both sides of the body and while
facing in different directions, so that the nest acquires its shape and loose
structure. As might be expected, the nest is usually loose and untidy, and
made up of penguin feathers, egg membranes and shells (Fig. 8). The proportion
of seaweed and limpet shells used increases with proximity to the sea. Any
other available material, such as bones, moss, lichen, and in one case even
a handkerchief, may be incorporated.
Copulation. Only four acts of
copulation were observed, all from a
distance, and in all these cases the
birds were near their nests. The complete
behaviour sequence was never seen, and
the pre-copulatory display was witnessed
only once. On this occasion the male
strutted stiff-leggedly around the female,
which stood still with a slightly lowered
head and a raised tail. This pair were
disturbed before copulating, but Downes
(personal communication), who has described
similar behaviour more fully in Chionis
minor at Heard Island, states that
here, after a similar strutting behaviour
the male stands facing in the same direction
as the
female and after making quick and repeated clutching movements with his feet,
mounts. In both C. minor and C. alba the male does not
grip the head of the female with his beak. Downes also describes invitation
movements by a female C. minor, and one case of what might be auto-erotism,
but these phenomena were not observed in C. alba.
In the Signy Island sheathbills, one bird of a pair usually displayed tread
marks on its back prior to egg-laying, which helped in differentiating the
sexes. One or two birds showed marks which might indicate reversed positions
in copulation (or attempted copulation).
The egg. Sheathbill eggs resemble
those of a plover in colour and shape.
The ground colour is usually a dirty
white, but it may be fawn or even light
olive. The surface is liberally marked
Fig. 7. The Gourlay Peninsula penguin rookeries showing the distribution
of sheathbill nests during the 1961-62
season. CSurvev bv J. J. Cheal. 1951.1
THE SHEATHBILL, Chionis alba (Gmelm), AT SIGNY ISLAND
64 BRITISH ANTARCTIC SURVEY BULLETIN
with brown and grey blotches, which tend to be larger and darker at the broad
end. The dimensions and weights of 103 eggs are summarized in Table I.
TABLE I. MEASUREMENTS OF EGGS OF Chionis alba
Number
in
Sample |
Mean
|
Range
|
Standard
Deviation |
|
| Weight | 103
|
45-5
g.
|
52-40
g.
|
2-47
g.
|
| Length | 103
|
57-6
mm.
|
64-5-52-5
mm.
|
2-49
mm.
|
| Breadth | 103
|
38-5
mm.
|
40-5-36-0
mm.
|
1-10
mm.
|
The
first egg of the season was found on 7 December and thereafter the number
of nests containing eggs increased steadily. By 20 December most nests
contained eggs; the last recorded laying was on 14 January. Laying must
have commenced at about the same date in the Laurie Island birds studied
by Pirie (Wilton and others, 1908), since he found two clutches of three,
and two of two eggs on 11 December 1901. Roberts (1948) reported first
eggs as early as 29 November at Hope Bay in 1947. At Gourlay Peninsula
the minimum interval between the laying of two eggs in a clutch was one
day, and the maximum four days.
These data contrast somewhat with the statement of Bennett (1927) that a
week may elapse between the laying of two eggs. The interval between the
laying of eggs in a clutch was not constant. Clutch size varied between one
and four eggs (Table II), and this range of variation exceeds that reported
at South Georgia by Murphy (1936). The number of fledglings produced per
pair in the South Orkneys Islands also seems greater than at South Georgia.
TABLE II. CLUTCH SIZE OF Chionis alba AT SIGNY ISLAND
Number
of Eggs in Clutch
|
Number
of Nests
|
| 1 | 9
|
| 2 | 23
|
| 3 | 13
|
| 4 | 3
|
Incubation. It
has been stated (Bennett, 1927; Murphy, 1936) that incubation commences
as soon as the first egg has been laid, and this is clearly desirable
in a species nesting in such a cold climate. However, while the period
between the laying of the eggs in a clutch at Signy Island was often
several days, most chicks hatched within a couple of days of one another.
This near-synchronization of hatching occurs because, although the birds
do cover their eggs as soon as the first one has been laid, they do not
incubate them fully at this stage. During the first few days, the early
eggs of a clutch feel warm to touch, but when the clutch is complete
and incubation has commenced in earnest they feel very much warmer. Development
of the early eggs is thus held back until the clutch is complete. There
is no rigid rule about the commencement of incubation; however, in some
cases it starts before the last egg is laid and in a few others it does
not begin until some days after that event.
Table III summarizes the data collected for incubation periods of a series
of selected clutches of varying size. The "real incubation period" is
taken as the period from the laying of the last egg to its time of hatching,
and the average incubation period was calculated using these real incubation
times only. Incubation is shared by both sexes, although they do not usually
sit for equal periods. Generally, the male appears to take the longer spells
during the laying period and for a few
THE SHEATHBILL, Chionis alba (Gmelm), AT SIGNY ISLAND 65
TABLE III. INCUBATION PERIODS IN CLUTCHES OF VARIOUS SIZES
Nest |
Egg Colour |
Date Laid |
Date Hatched |
Apparent Incubation Period (days) |
Real Incubation Period (days) |
20 |
blue |
2 January |
2 February |
31 |
31 |
19 |
blue |
18 December |
18 January |
31 |
31 |
5 |
blue |
22 December |
21 January |
30 |
27 |
red |
25 December |
22 January |
28 |
28 |
|
18 |
blue |
22 December |
23 January |
32 |
29 |
red |
25 December |
24 January |
30 |
30 |
|
1 |
blue |
9 December |
11 January |
33 |
27 |
red |
12 December |
12 January |
31 |
28 |
|
yellow |
15 December |
lost |
— |
— |
|
6 |
blue |
11 December |
15 January |
35 |
30 |
red |
13 December |
15 January |
33 |
30 |
|
yellow |
16 December |
15 January |
30 |
30 |
|
17 |
blue |
11 December |
16 January |
36 |
30 |
red |
14 December |
16 January |
33 |
30 |
|
yellow |
17 December |
18 January |
32 |
32 |
|
7 |
blue |
12 December |
inviable |
_ |
_____ |
red |
14 December |
18 January |
35 |
29 |
|
yellow |
17 December |
18 January |
32 |
29 |
|
green |
20 December |
20 January |
31 |
31 |
The
real incubation periods for 29 last eggs of clutches ranged between 28
and 32 days, with an average of 29.9 days.
days afterwards, but during the remainder of the incubation period the female
spends the longest periods on the nest. Some males, however, persist in long
incubation spells. It proved difficult to measure the length of incubation
duties because the birds are easily disturbed, but the longest spell observed
reliably was 5 hr. 14 min., while short spells lasted in some cases only
1 or 2 min. The "off-duty" bird forages for food or nest material,
or stands guard on a nearby vantage point.
Nest relief is usually quick, and the relieving bird moves on to the nest
as soon as its partner comes off. Sometimes there is a "bowing ceremony" when
the birds meet.
Egg losses at Gourlay Peninsula were fairly high but their causes were obscure.
In a few cases eggs vanished without trace, and the only suspect agents were
the birds themselves or other sheathbills. Several eggs failed to hatch,
and these had fine cracks in the shells which were probably caused by freezing.
However, the importance of this factor under natural conditions is uncertain
since the eggs may have been chilled after the birds had been disturbed by
the observer. Some birds, especially early in the incubation period, tended
to stay off the nest for rather a long period after disturbance, and on a
cold windy day this may have caused egg mortality.
Hatching and brooding.
Most eggs hatch within three or
four days of showing the first" starring" of
the shell but in a few cases a
five-day interval was observed.
At hatching the chicks are capable
of movement within the nest and
they can open their eyes, but these
activities are only kept up for
short periods at a time.
The egg-shells are usually removed but they may be incorporated into the
nest structure. Both sexes share brooding duties, but to varying degrees,
as in incubation. The chicks are brooded quite closely for ten to eighteen
days, and less intensely thereafter. Although the chicks are fairly well
developed and capable of moving about soon after hatching, they usually
66 BRITISH ANTARCTIC SURVEY BULLETIN
remain in the nest for about a fortnight. Chicks hatched in the more open
nests (Fig. 9) leave them for better shelter long before the chicks emerge
from the more sheltered nests.
Nest hygiene is maintained by the chicks themselves, which generally move
to the rim of the nest and defaecate away from it, and in a few cases may
move right away from the nest to excrete. This habit was established in chicks
observed at four days old and it seems to begin even earlier as droppings
were already present near the nest at this time.
Feeding of chicks. The
omnivorous diet of adult sheathbills
is less suitable for the chicks,
and in fact these are fed largely
on a much richer food source, "krill",
which is available al this time.
This "krill" is obtained
from penguins, which bring it for
their own young, and this relationship
is probably the sheathbill's greatest
dependence on penguins.
There are several records of sheathbills feeding euphausiids to their chicks,
but there are few descriptions of how the material is procured. Hall (1900)
and Sladen (1958), however, describe a method which is commonly followed
at Gourlay Peninsula. While some "krill'' is collected off the ground,
after penguins have been feeding their young, more is obtained by direct
interference in the feeding process. This interference is most prevalent
when the penguin chicks are in creches and food chases (Sladen, 1958) are
common, since at this time a sheathbill can attend a parent penguin and chick
without interference from neighbouring penguins. The sheathbill waits until
the penguin chick is being fed and then either pecks at or flies on to its
back (Fig. 10). This disturbs the chick, which usually ducks away and breaks
off feeding. The parent penguin cannot stop regurgitating food immediately
and some usually falls to the ground where the sheathbill picks it up. This
source of food is of paramount importance to the Gourlay Peninsula sheathbills;
the few that did not rely on it for feeding their chicks were either late
breeders or unable to hold a territory covering a penguin colony.
The sheathbill's breeding cycle coincides with those of the penguins, in
that most chicks hatch about mid-January when the Adelie penguin chicks are
forming creches. The Adelie penguins disperse rapidly in late January or
early February, but by this time the chinstrap penguin creches are present,
and this penguin species disperses more slowly since the birds
Fig. 10. A sheathbill disturbing a feeding chinstrap penguin chick in order
to obtain food for its own young.
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 67
stay ashore to moult. Consequently, "krill" may be obtained from
mid-January onwards, almost into March.
Most sheathbill territories include colonies of both penguins, but some take
in Adelie penguin colonies only. In this case the food supply diminishes
abruptly in late January.
Fig. 11 shows the consequences for the two sheathbill chicks in nest 78.
These chicks were
increasing in weight normally until the sudden and complete departure of
the Adelie penguins.
The parents were then unable to obtain "krill", and the chicks
lost weight despite some feeding on algae. One chick vanished without trace;
it may have died of starvation or, in its weakened state, fallen an easy
prey to a skua. At this stage, however, the parents were able to establish
a feeding territory in the corner of a chinstrap penguin colony, and so obtain "krill" for
the remaining chick, which resumed normal development. The data in this figure
may be compared with those for the uninterrupted growth of two chicks in
a "normal" territory including colonies of both penguins (Fig.
12).
This example illustrates the close relationship between the life cycles of
sheathbills and penguins. Whether the sheathbill has adapted its breeding
cycle, or the relationship is coincidental, is a matter of conjecture. Its
occurrence is perhaps best regarded as a further example of the resourcefulness
of the sheathbill in exploiting the available food supplies. The dependence
is not absolute; one pair successfully reared chicks without any contact
with penguins and a number of late breeders had to rely on shore feeding
for a period before the chicks fledged. It is clear, however, that the food
made available by the penguin colonies permits the establishment of a far
greater sheathbill population than could otherwise occur.
Parent sheathbills carry food to their young in their beaks and, if necessary,
call them from the nest with a short "caw". The chicks take the
food directly from the beak but if any falls, they immediately peck it up,
even when they are only a few days old.
Appearance of chicks.
At hatching the chick weighs
about 30 g. and is clothed in
a dark brown down which bears
light-coloured, hairlike tips
and therefore presents a "smutty
brown" appearance. There
is a bunch of white, hairlike
feathers below the bill and a
similai smaller one beneath the
eyes. The down is in well-defined
tracts (Lowe, 1916), and the
bare patches at the wing bases
are very apparent in the young
bird. The eye is surrounded by
a bald white ring, and there
is also a bare patch of skin
below the eyes. The bill is strong,
dart brown, and with a pale tip
bearing a white egg-tooth. The
sheath is not clearly differentiated
BRITISH ANTARCTIC SURVEY BULLETIN
Fig. 12. Growth curves for the two chicks from nest 38. (Territory included
both species of penguin.)
A.
Weight.
|
C.
Tarsus.
|
B.
Culmen.
|
D.
Wing.
|
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 69
from the rhamphotheca except on the dorsal surface. The differentiation gradually
increases but the nostrils remain uncovered even after the young have fledged.
The legs are dark brown and the toes have strong dark claws. The wings bear
small black spurs.
Behaviour of chicks. The
chicks can leave the nest
when they are three or four
days old, and they often
do so to excrete. They soon
become familiar with the
nest precincts and when removed
a little distance from the
nest young chicks soon run
back if they are set down
facing the right way. Chicks
react readily to the parent's
alarm call, or to its sudden
departure from the nest,
usually by "freezing" in
the middle of the nest or
squeezing into its darkest
corners. When brooding has
ended the chicks may wander
from the nest, although they
avoid the open unless the
parents are nearby. Often,
especially if they have hatched
in an exposed
nest, the chicks use some other hole or shelter at this time.
If they are approached by other sheathbills, as may happen when the parents'
vigilance is relaxed, the chicks cower and give repeated high-pitched "cheeps".
This soon brings the parents, which chase away the intruders. The latter
are usually non-breeders which display interest in chicks and have several
times been observed to peck at them.
Fledging. Before
the white contour plumage
of the juvenile bird is acquired
a dark grey layer of down
develops which soon shows
through the initial brown
prepennae and becomes the
down layer of the juvenile
bird. The contour plumage
then develops from the same
follicles as the brown down,
so that the latter are pushed
out and ultimately fall off'
the white feathers. The last
areas to lose the remnants
of the brown down are the
head, neck and rump. The
juvenile plumage is acquired
in 7 to 8 weeks after hatching.
The main changes are summarized
in Table IV.
Most of the chicks fledge when they are between 50 and 60 days old. After
leaving the nest, they generally become shore feeders and their diet often
changes from "krill" to algae quite
TABLE IV. PLUMAGE DEVELOPMENT OF Chionis alba CHICKS
| Age (days) | P/umage | Other Features |
| 1-10 | Uniform smutty brown down. | Beak:
dark brown with pale tip bearing egg- tooth. Iris: dark brown shading to grey. Legs: dark brown/black becoming dark grey. |
| 10-20 | Dark grey
down appears about twelfth day and shows through the prepennae by the twentieth day. Contour feathers are developing and wing coverts and secondaries are now prominent. |
Iris:
appears grey with dark brown around the pupil. |
| 20-30 | Contour
feathers are now showing all over the body except the head, neck, breast and rump. |
Beak:
egg- tooth lost. Sheath differentiated on dorsal surface only. Sheath pale and culmen pinkish shading to pale green and then black with pale tip. |
| 30-40 | Brown prepennae
are only present on the head. |
|
| 40-55 | Full
juvenile plumage has been acquired, but a few prepennae are still present on the head. |
Beak:
sheath: greenish with bluish base and dark forward edge. culmen: creamy shading through green and dark to pale tip. Iris: grey with brown around the pupil. Legs: grey. |
70 BRITISH ANTARCTIC SURVEY BULLETIN
abruptly. Towards the end of March the young birds begin to wander away from
the Gourlay Peninsula area, and at this time some arrive in the neighbourhood
of the Survey station They have been seen around the island until mid or
late May, at which time they disappeared. Banding returns showed that they
did not return until they were at least 2 years old. Recently, however, Topliffe
(personal communication) reported the reappearance of six 1962 nestlings
in late July and early August of that year. No information is yet available
to suggest a reason for this and their subsequent movements are not known.
Post-breeding activity.
The adults leave the
nest area and territory
as soon as the young
are fledged. Although
they may return later
to the vicinity, they
then show no territorial
behaviour and usually
spend little time there,
since by this stage they
are largely shore-feeders.
They gradually become more scattered, appearing around the Survey station
in late March and disappearing from the island about mid-May.
Non-breeding population.
Non-breeding birds apparently
make up about one-quarter
of the summer sheathbill
population of Signy Island.
These are believed to
be immatures (but 2 years
old), or mature birds
prevented from breeding
by lack of success in
acquiring territory or
by late arrival (one
colour-banded bird did
not return until February).
These birds spend their
time foraging on the
shores or in unclaimed
or poorly guarded areas
in penguin colonies.
PREDATORS
No certain predation on adult sheathbills has ever been observed. The only
likely predator is the brown skua (Catharacta skua) and this probably
does not take healthy adult sheathbills. The behaviour of the sheathbill
chicks and adults when skuas approach the nests suggests that these birds
are regarded as a potential danger, and there are several records of skuas
diving at young sheathbills, although none of the chicks were certainly killed.
One chick studied at Gourlay Peninsula was apparently eaten by a skua but
the cause of death was not evident. Probably other sheathbills, especially
non-breeders which are believed to take eggs
from poorly guarded nests, are the principal active predators.
PARASITES
External parasites. Sheathbills at Signy Island are commonly infested
with lice and feather mites. Mallophaga are abundant on the head and neck,
and they have also been collected from the body. The species obtained (kindly
identified by Dr. T. Clay) are:
| Quadraceps antarcticus Timmermann, |
| Actornithophilus pauliani Seguy, |
| Saemundssonia sp. |
Analgesid
mites are mainly present in the rump feathers anterior to and over the
preen gland, and on the underside of the wings. The mites in the latter
position were more numerous in the summer.
Internal parasites.
The only internal parasites
collected were Acanthocephala,
found in the posterior
sections of the intestine.
However, a detailed
search was not made
and parasites such
as the trematodes reported
from the gall bladder
of Chionis minor by
Downes (personal communication)
may have been missed.
ACKNOWLEDGEMENTS
I gratefully acknowledge all who have helped to make this work possible.
They include Sir Vivian Fuchs, Professor J. E. Smith, Professor F. W. Rogers
Brambell, Dr. T. B. Reynoldson, Dr. R. J. Adie and Miss E. Todd, who have
helped during the writing up of the work, and Dr. T. Clay, who kindly identified
the Mallophaga. A special debt is owed to Dr. M. W. Holdgate for criticizing
and assisting with the manuscript. In the field, assistance was given by
numerous British Antarctic Survey personnel, and particularly by my long-suffering
companions on Signy Island; D. Clarke, R. Pinder, R. D. Thompson, B. P. Westlake,
and F. W. Topliffe, who is still collecting data on Signy Island, deserve
my special thanks. M. C. Downes of the Australian National Antarctic Research
Expeditions has been extremely generous
THE SHEATHBILL, Chionis alba (Gmelin), AT SIGNY ISLAND 71
with information on Chionis minor and in allowing me to use the
bibliography he has compiled for the genus.
I wish to pay a special tribute of thanks and admiration to the late R. Filer,
whose name this work should bear. It was the dedication that cost him his
life during his sheathbill observations that stimulated this work.
MS. received 29
March 1963
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