Aberrant Balearic Shearwater inthe Strait of Gibraltar in July 2009

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200
[Dutch Birding 33: 200, 2011]
Aberrant Balearic Shear water in
Strait of Gibraltar in July 2009
On 1 July 2009, an unusually pale Balearic Shear-
water Pufnus mauretanicus was observed in the
Strait of Gibraltar from Isla de Las Palomas, Tarifa,
Cádiz, Spain, during the species’ postnuptial mi-
gration monitoring programme run by the Migres
Foundation in Tarifa (Programa Migres 2009). It
was ying west in a ock of seven and seen during
c 80 sec at a minimum distance of 150 m with op-
timal light conditions and calm sea, using 60x eld
scopes. The bird did not differ in size, shape or
ight action from the other birds in the group and,
therefore, we consider it safe to assume that the
aberrant bird was a Balearic Shearwater as well,
and not some other Pufnus species.
At a distance and in direct sunlight it looked pure
white, standing out against the other birds. Detailed
observation showed that the plumage had a homo-
geneous pale sand colour without apparent con-
trast between upper- and underparts. No colour
abnormalities were detected in the bare parts. The
plumage was fresh without evidence of moult, in-
dicating it was juvenile.
Aberrant coloration (eg, melanism, leucism, al-
binism) has a low frequency of occurrence in
Procellariiformes (see Bried et al 2005 for a review).
Five previous cases are known for Balearic
Shearwater:
1 pure white bird with brown bill and eyes; off Vendée,
France, 28 August 1984 (Mackrill & Yésou 1998, Yésou
& Bentz 2007)
2 ‘leucistic’ bird with pale grey and pure white parts; off
Vendée, France, 7 September 1984, considered same
as previous (Mackrill & Yésou 1988, Yésou & Bentz
2007)
3 ‘partial albinistic’ (sic) bird showing white patches in
upperwings; Balearic Islands, Spain, May 1985
(Mackrill & Yésou 1988)
4 ‘leucistic’ bird; off Vendée, France, 27 August 2005
(Yésou & Bentz 2007)
5 ‘leucistic’ bird, mainly white, with pale grey-brownish
parts; Baie de Lannion, Trédrez-Locquémeau, Bretagne,
France, 29 August 2006 (Yésou & Bentz 2007)
6 ‘leucistic’ bird showing white patches in upperparts,
Strait of Gibraltar, 17 July 2007 (Juan Ramírez in litt).
Pale (close to white) colour patterns in birds are
usually regarded as ‘leucism’ or ‘partial albinism’,
the latter being an erroneous term (cf van Grouw
2006). The bird described here better ts a case of
the so called ‘brown’ colour aberration (Hein van
Grouw pers comm), which is often mislabelled as
leucism; this aberration has not been claimed for
Balearic Shearwater before. Both are widespread
genetic mutations. Leucism involves a variable
amount of pure white feathers whereas ‘brown’
involves a qualitative reduction of eumelanin (ie,
incomplete oxidation of eumelanin) making dark
feathers turn brown and giving the bird a dis-
coloured appearance. ‘Brown’ is the most common
colour-related mutation in birds. It is recessive and
sex-linked in all species, so ‘brown’ birds are al-
ways females (van Grouw 2006).
Pale and unpigmented plumage parts are espe-
cially prone to wear, rapidly bleaching to white by
weather and sunlight (van Grouw 2006). Therefore,
‘brown’ Balearic Shearwater like the one we de-
scribe here will progressively acquire whitish plum-
age when it moves away from the Balearic natal
areas to the Atlantic wintering quarters, where such
a bird could easily be misidentied as leucistic. We
propose that this could be the case in the examples
1-2 and 4-6 mentioned above. However, given the
large numbers of Balearic Shearwaters observed at
close range both in postnuptial and prenuptial mi-
gration through the Strait of Gibraltar (Programa
Migres 2009), and the low rate of aberrant individ-
uals, it appears that this is a rather infrequent phe-
nomenon. Although our report is anecdotal, it may
be useful for further reviews exploring the preva-
lence of colour aberrations in wild birds.
We thank Hein van Grouw (Natural History
Museum, Tring, England) and Andrew Paterson for
their valuable comments.
References
Bried, J, Fraga, H, Calabuig-Miranda, P & Neves, V C
2005. First two cases of melanism in Cory’s Shearwater
Calonectris diomedea. Marine Ornithol 33: 19-22.
van Grouw, H 2006. Not every white bird is an albino:
sense and nonsense about colour aberrations in birds.
Dutch Birding 28: 79-89.
Mackrill, E J & Yésou, P 1988. Leucism and partial albi-
nism in Balearic race of Manx Shearwater. Br Birds 81:
235-236.
Programa Migres 2009. Seguimiento de la migración de
las aves en el Estrecho de Gibraltar: resultados del
Programa Mires 2008. Migres J Ecol 1: 83-101.
Yésou, P & Bentz, G 2007. Anomalies de coloration chez
le Pufn des Baléares Pufnus mauretanicus. Ornithos
14: 63-64.
Javier Elorriaga, Fundación Migres, Ctra N-340, Km 96.7, Huerta Grande, Pelayo, Algeciras E-11390
Cádiz, Spain (javielor@hotmail.com)
Blanca Pérez, Fundación Migres, Ctra N-340, Km 96.7, Huerta Grande, Pelayo,
Algeciras E-11390 Cádiz, Spain