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72 D. Jakubas et al.
& Székely 2006). Sexual selection is expected to long-lived Procellariiformes breeding colonially in
favour small body size in males exhibiting aerial natural crevices, burrows or in caves. Its breeding
displays (Andersson & Norberg 1981, Székely et area comprises the northeastern Atlantic Ocean
al. 2000, 2004, Figuerola 1999); in contrast, in males and the western Mediterranean Sea. Based on dif-
displaying or fighting on the ground, a large size ferences in biometry (Hemery & d’Elbee 1985,
is often advantageous. Fecundity selection may Lalanne et al. 2001), mitochondrial DNA (Cagnon
explain female-biased SSD if large females have et al. 2004) and vocalization (Bretagnolle 1992,
higher reproductive success due to their higher Bretagnolle & Zotier 1998), the European Storm
capacity for laying eggs or if males prefer large Petrel has been divided into two subspecies: H. p.
females to small ones (Andersson 1994, Sander- melitensis for the birds that breed around the
cock 1998, 2001). Different body size in both sexes Mediterranean Sea, and H. p. pelagicus for the
may evolve as a mechanism to avoid resource Atlantic populations. Females lay one large egg
competition or enhance feeding efficiency by dif- (25–30% of adult body mass) per year, which both
ferential niche-utilisation (Selander 1966, Thom et adults incubate. Both adults also feed the chick for
al. 2004). Since body size or morphology is often about two months, until shortly before the chick is
evolved to adapt to different niches (Sandercock ready to fledge (Davis 1957, Warham 1990). There
2001), sexually dimorphic pairs can exploit a wider is no sex difference in plumage, however, females
range of resources than monomorphic ones were found to be larger than males (Albores-
(Figuerola 1999). Barajas et al. 2010, Medeiros et al. 2012).
Interestingly, a tendency towards female-biased
Seabirds are excellent organisms to investigate SSD in Procellariiformes has been found only in
both geographical variation in body size and func- Hydrobatidae (Serrano-Meneses & Székely 2006).
tional hypotheses of SSD. They range over wide
areas and return to specific sites to reproduce. As The prime aim of our study was to examine
a result, the breeding populations of many geographical variation in the body size of the
pelagic species are isolated geographically (King European Storm Petrel across a large part of its
1974). This isolation may favour phenotypic range. To understand patterns driving geographi-
and/or genotypic variation as a result of fitting to cal differences in body size, we examined the rela-
local environmental conditions. It has been tionship between body size and environmental
reported that the clinal and continuous size varia- (sea surface temperature, air temperature and
tion in the Leach's Storm-Petrels Oceanodroma leu- wind speed) variables that describe the different
corhoa is related to ocean temperature in accor- breeding areas. Using the line of reasoning of the
dance with Bergmann’s rule (Ainley 1980). Clinal heat conservation hypothesis, we expected a lati-
increase in the body size has also been reported in tudinal increase in the size of European Storm
Atlantic alcids: a north-westward increase in the Petrels from south to north reflecting variability of
Atlantic Puffin Fratercula arctica (Moen 1991) and environmental conditions. Also, we aimed to (i)
from west to east or north-east in Brünnich’s identify the degree of sexual size dimorphism
Guillemot Uria lomvia (Vaurie 1965), Razorbill Alca (SSD) in the European Storm Petrel and to deter-
torda (Barrett et al. 1997) and Little Auk Alle alle mine in which traits it is expressed, (ii) compare
(Wojczulanis-Jakubas et al. 2011). Sea surface tem- SSD among different populations of the studied
perature has been evoked as a possible force driv- species, (iii) develop a discriminant function based
ing the size differentiation in both Atlantic Puffin on the traits that show SSD.
and Razorbill (Moen 1991, Barrett et al. 1997).
Seabirds exhibit both male-biased and female- MATERIALS AND METHODS
biased dimorphisms (Serrano-Meneses & Székely
2006). The degree of SSD may also vary within Field procedures
species range. Such an intraspecifc variability of We collected biometrical data in the large
SSD has been reported for some Procellariiformes, European Storm Petrel breeding colony on Nólsoy
especially in species divided into subspecies like Island (61°59’N, 06°38’W) in the Faeroes, where
Snow Petrel Pagodroma nivea (Barbraud & about 90% of the known breeding population is
Jouventin 1998), or Cape Petrel Daption capense concentrated (BirdLife International 2013). We
(Weidinger & van Franeker 1998). captured birds in the colony during the incuba-
tion period (6–21 August) in 2013 using a mist-net.
In this study, we investigated a pelagic seabird, All caught individuals had an apparent brood
the European Storm Petrel Hydrobates pelagicus. It
is a small (25 g; the smallest Atlantic seabird) and
