Page 2 - Jakubas_et_al._2014_Storm_Petrels

ACTA ORNITHOLOGICA
Vol. 49 (2014) No. 1

Body size variation of European Storm Petrels Hydrobates pelagicus in
relation to environmental variables

Dariusz JAKUBAS1, Katarzyna WOJCZULANIS-JAKUBAS1& Jens-Kjeld JENSEN2

1Department of Vertebrate Ecology and Zoology, University of Gdańsk, Wita Stwosza 59, 80–308 Gdańsk, POLAND,
e-mail: biodj@univ.gda.pl
2Í Geilini 37, FO–270, Nólsoy, FAROE ISLANDS

Jakubas D., Wojczulanis-Jakubas K., Jensen J-K. 2014. Body size variation of European Storm Petrels Hydrobates
pelagicus in relation to environmental variables. Acta Ornithol. 49: 71–82. DOI 10.3161/000164514X682904

Abstract. Body size differentiation may have developed in response to environmental gradients. A pattern of large indi-
viduals prevailing in colder areas is often observed and is explained by the heat conservation hypothesis (Bergmann’s
rule). To understand patterns driving body size variation in a pelagic seabird, the European Storm Petrel Hydrobates
pelagicus, we examined the relationship between wing length, body mass and environmental variables in breeding areas
(sea surface temperature, air temperature and wind speed). As this species has been divided into two subspecies:
Mediterranean H. p. melitensis and Atlantic H. p. pelagicus, we performed the analyses at different scales (species, Atlantic
subspecies and regional North Atlantic). At the species and subspecies scales, there was a longitudinal increase in wing
length from west to east. At the subspecies and regional scale, we found a latitudinal increase in this variable from south
to north. This result and the significant increase of wing length with decreasing sea surface and air temperatures are
concordant with Bergmann’s rule. In addition, body mass at the species and subspecies scales decreased with increas-
ing wind speed, what may have a functional implication (small body mass may increase manoeuvrability over waves
in conditions of stronger wind). Both genetic (two subspecies differing in body size) and environmental factors seem to
be important forces driving intercolony variation in body size. Our study on sexual size dimorphism (SSD) revealed that
in 156 molecularly sexed adults from the Faeroes, wing and tail length, and body mass exhibited female-biased SSD,
while head-bill length showed male-biased SSD. The best discriminant function for sexing based on body measure-
ments correctly classified 75% of individuals. Considering low correctness of proposed functions and geographical vari-
ation of body size, use of alternative methods (e.g. molecular tools) is recommended for sex discrimination in the
European Storm Petrel.

Key words: seabird, body size variation, molecular sexing, sexual size dimorphism, discriminant function analysis

INTRODUCTION increases via a higher surface area to volume ratio.
Body size variation may be also a consequence of
Some vertebrate species, especially those occur- ancestral colonization and subsequent diversifica-
ring over large geographical areas, often show tion (Jouventin & Viot 1985, Barbraud & Jouventin
variation in body size within their range. Body 1998).
size differentiation may have developed in
response to physical and/or biological clines in the Regardless of geographical variations, some
environment. A pattern of large individuals pre- species show sexual size dimorphism (SSD).
vailing in colder areas is often observed (e.g., This phenomenon is widespread and variable
James 1970, Ainley 1980, Monaghan et al. 1983, among animals (Andersson 1994, Fairbairn 1997,
Moen 1991, Barrett et al. 1997), and commonly Blanckenhorn 2000) and take a form of male-
explained by the heat conservation hypothesis, biased (males larger than females) or female-
known as Bergmann’s rule at the interspecific biased SSD (females larger than males). Three
level (Bergmann 1847) or James’s rule for within- major processes have been proposed that produce
species patterns (James 1970, Blackburn et al. 1999). and maintain SSD: sexual selection, fecundity se-
According this hypothesis, heat conservation lection and avoiding resource competition (Jehl &
Murray 1986, Andersson 1994, Serrano-Meneses