Cory's Shearwater
Cory’s Shearwater
 |
Cory's Shearwater
- A. Raine |
Two sub-species are recognised Cory's Shearwater Calonectris diomedea borealis
of the North Atlantic and Mediterranean or Scopoli's Shearwater C. d. diomedea
of the Mediterranean.
General Description:
The head is mostly greyish-brown, some with an invariable amount of white feathers,
merging into white chin and throat without obvious demarcation. Upper-parts greyish-brown,
long upper-tail coverts occasionally tipped white, forming indistinct horseshoe
on upper tail. Under-parts white, except for first and occasionally second year
birds with heavy barring on flanks and under-tail coverts. The upper-wing is greyish
brown; primaries and secondaries darker. During moult, the missing coverts on the
primaries and secondaries give the impression of a white wing-bar, when the bird
is in flight. The brownish margins of the underwing end in dark wing tips. The tail
is blackish-grey. Various degrees of albinism, especially on the head and primaries
occur, with at least one bird recorded from Maltese waters being completely white.
Voice:
 |
Cory's Shearwater
- A. Raine |
Dimorphic, males having a higher pitched call than female’s, which is more guttural.
The call can be best described as the wail of a human baby or as that of a strangled
cat! (Awwa-awwa, Awwa-awwa, Awwa-awwa, Awwa-awwa-ah). The last syllable taken as
inhaled breath. (Click here.)
Morphometrics:
Sexual dimorphism in measurements noted for this species with males being larger
having considerably larger heads and bills.
|
Wing
|
Bill length |
Bill height |
Tarsus |
 |
352 - |
 |
341 |
|
|
|
52.5 - |
|
41.8 |
|
|
|
13.7 - |
|
11.9 |
|
|
|
54.5 - |
|
52.5 |
|
.
Distribution:
Calonectris diomedea borealis breeds on offshore islands in the Canaries, Berlengas
and Madeira (Desertas and Selvagen). North Atlantic birds may occasionally enter
the Mediterranean. The Mediterranean polytipic C. d. diomedea is sedentary, erratic
and partial migratory. It is found breeding on many offshore islands throughout
the whole of the Mediterranean, Tyhrenian and Adriatic seas and most of the Aegean
.
Status in the Mediterranean:
Several colonies are scattered throughout the whole Mediterranean basin. Largest
concentrations are found in the Siculo-Melitensis basin with over 10,000 breeding
pairs on Linosa, 30,000 b.p. on Zembra, and around 7,000 b.p. on the Maltese islands.
Smaller colonies are found in the western Mediterranean, Adriatic Sea and Aegean
Sea.
Status in the Maltese Islands:
The Cory's Shearwater is a breeding visitor from late February to late October.
An increase in numbers offshore has been noted to occur in mid-November, with a
continuous north-westerly movement, it is possible that these latter birds may be
juvenile birds departing from eastern Mediterranean populations and moving west
towards the Atlantic. Single birds are observed during the winter months of December
and January.
Cory’s Shearwaters breed on appropriate sea-cliffs on Malta, Gozo, and Comino and
on Fungus Rock and Filfla Islet. All past authors have listed it as a breeding bird,
but numbers varied from a few scattered pairs to several thousands. The main breeding
concentrations on Malta are situated along the south and southwestern coast. The
western tip of Marsaxlokk bay is the extreme limit of its breeding range, and several
sub-colonies are to be found, almost uninterruptedly, all the way up to Ghar Lapsi.
Breeding takes place also along the Dingli Cliffs, and a few pairs have also been
located along the northwestern cliffs where some 200 pairs are estimated to breed.
On the northeastern cliffs at Rdum tal-Madonna , single birds have been heard calling
for several years, but actual breeding was confirmed in 1993, when a pair successfully
raised a chick. At least three to five other pairs have been observed in the area.
In the spring of 1998 a small colony estimated at about 10-15 pairs was discovered
on the northeastern cliffs of Comino.
The island of Gozo holds the largest number of breeding birds, the highest concentrations
situated at Ta'Cenc Cliffs in the southwest. This area has been estimated to hold
about 1,000 pairs. The cliffs to the west of Ta’ Cenc have been estimated to hold
a further 100-300 pairs. A small colony at Xlendi Bay was deserted when electricity
was introduced in the 1970s . Breeding also takes place between Xlendi and Wardija
Point where numbers have been estimated to be in excess of 1,000 pairs. Breeding
was also confirmed on Hagret il-General (Fungus Rock) in Dwejra Bay in 1987 and
a visit to Fungus Rock in May 2000 resulted in an estimated colony of 25-30 pairs.
Along the northwestern coast between Dwejra and St. Dimitri the number of breeding
pairs is estimated at 300-350 pairs, possibly more.
The colony on Filfla has been reported as having less than 30 birds (1954) to about
200 pairs (1975) Throughout the years it has been observed that the shearwater population
on Filfla fluctuates according to the availability of nesting sites. This availability
is mainly due to a friable and fragile boulder and screes habitat of the island,
thus being highly vulnerable to climatic agents. The main cause for these fluctuations
in the breeding population is the loss of established nests and the establishment
of new ones that may take up to three years to be occupied.
Breeding:
Cory’s Shearwaters breed on offshore islands and inaccessible seacliffs laying a
single egg inside crevices, burrows or amongst boulders. The Cory’s Shearwater is
synchronous in its breeding with all the birds in a colony laying within a period
of 12 days. In the Maltese Islands the peak egg laying date is the 28 of May (earliest
19th May and 01st June being the latest). The male takes over the first incubation
spell lasting on average six days. The incubation period lasts 52 days equally divided
by both partners. The female bird is present when the new hatchling emerges from
the egg. During the daylight one of the parent birds stays with the chick for an
average of five days (1 to 7 days) after which the chick is only visited at night.
Throughout the months of August and September the young birds weigh almost twice
as much as an adult bird. It is covered in a thick grey down and by September the
first true feathers start to emerge. All the colonies are deserted by the third
week of October.
Movements:
Most of the Mediterranean birds migrate into the Atlantic through the Straits of
Gibraltar and while young birds continue southwards towards the South African coast,
adult birds remain close to the north-western shores of Africa. An unknown number
of birds have been reported as wintering in the Mediterranean, mainly off the Tunisian
coast.
Threats and Conservation:
The persecution on Cory’s Shearwater by Maltese fishermen and shooters has been
first documented by Antonio Schembri in 1843 where he described how fishermen used
to cover the entrances of nesting holes with nets, catching the birds while entering
or leaving the nest. The bird’s flesh was roasted, mashed and used as fishing bait.
Charles A. Wright in his notebook noted that both Cory’s and Yelkouan shearwaters
where frequently on sale at the Valletta Market. Giuseppe Despott in 1913 describes
how fishermen used to catch the birds by means of nets to pluck the underwing feathers
to be used as lures for trolling. The birds were then set free. This practice although
on a much smaller scale still persists today. Despott in 1913 also recounts on the
increasing number of shooters going out to sea in boats to shoot the shearwaters
for pleasure and remarks that the massacre going on was so great that he feared
for the extinction of this bird from Malta.
Shooting at sea increased in the 1980's and early 1990's when shearwaters, mainly
Cory’s Shearwater were shot in large numbers. Stuffed bird collections and target
practice are the main reasons for this slaughter, while a number of breeding birds
is still taken for their axillaries to be used as fishing lures. Quarrying, urban
development, fishing by-catch and direct persecution are some of the threats that
these birds have to face on a daily basis.