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Body size variation in the European Storm Petrel 79
usually two birds are involved in this display this trait (SSD = -0.3) what might have been
(Cramp 1998), probably the female also takes part attributed to different vocalization of the two sub-
in this performance. Thus, similar body size in species (Bretagnolle 1992) and different breeding
both sexes should have a selective advantage for site with different acoustic (caves mainly in H. p.
performing display behaviour. Wing and tail melitensis). Further studies on this subject are
length are directly related to flight performance, needed as head-bill length was measured only in
which might be relevant for foraging optimization AEG, POR and FAR.
by differential niche-utilisation. There are no data
of sex differences in the European Storm Petrel In contrast to the present study, rump band
foraging. However, in the Wilson’s Storm Petrel length was the feature differing between the sexes
Oceanites oceanicus, also exhibiting female-biased in AEG (Table 4). Albores-Barajas et al. (2010) sug-
SSD in the wing and tail length, there was gener- gest that a white rump band, highly contrasted
ally no significant difference in the strategies used against the black of the rest of the feathers, could
by males and females (Gladbach et al. 2009). Thus, indicate the sex of an individual under low light
it seems that the fecundity hypothesis may conditions. Considering that European Storm Pet-
explain better female-biased SSD in the studied rels nest frequently in caves in the Mediterranean
species. Selection may favour for the increased region (offering worse light conditions compared
female size considering the extremely small size of to nesting in open talus more common in the
the European Storm Petrel (mean 23–28 g; this Atlantic region), it is possible that this SSD is char-
study) and production of a large egg constituting acteristic only for the Mediterranean subspecies.
25–30% of the adult body mass (Warham 1990,
Croxall 1995). Discriminant function analysis has proven use-
ful for identifying dimorphism in a range of
SSD in European Storm Petrels captured in the Procellariiformes (e.g. Johnstone & Niven 1989,
Faeroes was not consistent in all measurements Lorentsen & Røv 1994, Weidinger & Van Franeker
because they exhibited male-biased SSD in the 1998, Genovart et al. 2003, Bourgeois et al. 2007,
head-bill length. Bigger heads of males may be Einoder et al. 2008). The correctness of so far pro-
attributed to investments in courtship displays posed functions for sexing the European Storm
and territory defense and acquisition (Warham Petrel was 63–85% (Table 4). However, many of
1990, Genevois & Bretagnolle 1995). A bigger head them were based on the small sample size. In par-
may be promoted by sexual selection if that is ticular, the work of James (1983) with the highest
associated with a vocal advertisement of a given correctness (up to 85%) was based only on 46 indi-
sex. Study on the Herring Gull Larus argentatus viduals. Moreover, sexing was not based on
identified a positive relationship between the molecular sexing but on cloacal inspection, which
head-bill length and the length of the trachea is not fully reliable (e.g., Boersma & Davies 1987).
(vocal tract filter) in males (Hardouin et al. 2014). Some functions were constructed based on meas-
Even though the mentioned study failed to iden- urements that were probably correlated (wing
tify corresponding sex- and body-size related vari- and tail length which were significantly correlated
ation in the acoustic components of the gull alarm in the present study; Pearson correlation coeffi-
calls, the authors suggested that such relation- cient: r154 = 0.30, p = 0.0001) which is a violation
ships are more likely to be expressed in sexual of discriminate function analysis assumption. The
calls. It has been hypothesized that male-biased function proposed in our study classified correct-
SSD in Snow Petrels could increase the range of ly 75% of individuals. That is quite a low level,
vocal frequencies and thus, the intra-specific despite the reasonable sample size used in the
repertoire (Croxall 1982). In the European Storm analyses (n = 156). Given this, and the concerns
Petrel, apart from the call shared by both sexes, with functions proposed by other authors and
males give a prolonged “purr” call associated with geographical variation in body size, it should be
sexual advertisement that is analogous to a territo- recommended to use alternative methods (e.g.,
rial song (James 1984). If this sex dimorphism in molecular tools) of sex discrimination in the
the head-bill length has indeed functional signifi- European Storm Petrel.
cance, one may expect male-biased SSD across the
whole range of the species (Fig. 6). Indeed, this ACKNOWLEDGEMENTS
has also been observed in Atlantic birds captured
in POR (SSD = 0.3). However, birds breeding in This study was supported by grant from
AEG exhibited rather slight female-biased SSD in University of Gdańsk (538-L120-B081-13). Birds
