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Local and Global Climate Effects on Storm Petrel Demography
to be under any threat and is thus categorized as Least Concern
[23], but if we focus only on the Mediterranean subspecies, it may
be endangered mainly due to habitat degradation and introduc-
tion of predators, such as rats and cats, requiring conservation
awareness [24]. Breeding distribution and abundance is probably
under estimated, due to the highly pelagic attitude and nocturnal
behaviour [25]. The known Mediterranean population is estimat-
ed at between 11 000–16 000 breeding pairs, with three main
population cores identified in Malta, Sicily and the Balearic
Islands [26]. The current trend of the Mediterranean storm petrel
populations is stable [26], although there is a lack of basin wide
studies. In the few sites where mid-long term monitoring has taken
place (Marettimo, Filfla and Benidorm), it has been observed that
the colonies remain stable.
Dietary needs have evolved according to Mediterranean basin
food-web and are mainly supplied by small pelagic fish [27].
Considering fluctuations of small pelagic fish whose biomass
estimates resulted to be negatively correlated with the mean sea
surface temperature [28,29,30,31] we predict that climatic
variation both in sea surface temperature and in environmental
temperature and rainfall may affect storm petrel survival at
different stages of their life cycle. Our hypothesis is that conditions covariate) to test climate effects on the recruitment probability and
experienced at fledging can influence first year survival and
breeders may be affected by environmental conditions during the
pre-breeding and breeding period with effects on age of
recruitment and/or adult survival. We used temperature and
rainfall in the pre-breeding and breeding period and sea surface
temperature during winter in testing whether and in what measure
birds are affected directly, chlorophyll concentration was used as
indirect covariate.
We investigated the effects of climatic conditions on the
demography of this taxon by analysing the life histories of almost
5000 individuals monitored since 1991 in the Egadi Archipelago,
Italy. We then inferred that local conditions from the different
areas during the winter can affect vital rates and analysed
correlations between trends in different Mediterranean upwelling
zones and storm petrel demography in order to predict the
wintering areas.
Materials and Methods
Study area and species
The storm petrel (Hydrobates pelagicus) is the smallest procellarii-
form present in the Mediterranean Sea, where the subspecies
melitensis lives. This species lays one egg in May-June and chicks
fledge in late August-September. Mark-recapture data were
obtained from Marettimo Is. (37u589N, 12u039E, Figure S1), Italy,
between 1991 and 2012. The main colony, placed in a large cave
accessible only from the sea, is the only accessible colony consisting
of approximately 2500 pairs [25]. Recruitment in Marettimo is
observed earlier than in other colonies [32,33]. Each year, the
colony was visited at least once between June and August, and
adults and chicks were captured at nest and banded with metallic
inox rings for a total of 4926 marked individuals. Sexing of the
birds began in 2007 on chicks and adults following the methods
already described in Albores et al. 2010 [34]. Adults caught at the
colony are all breeders, as pre-breeders and skippers are not
present.
Climatic covariates
We used two types of covariates: time-dependent individual
covariates and environmental covariates (Table 1). Individually
based covariates follow individuals life history with no relation
with other individuals while values of environmental covariates are
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shared by all individuals with no reference to their life history. In
the case of time-dependent individual covariates, we had one value
per individual, referring to the environmental conditions experi-
enced at birth and the subsequent covariate values will charac-
terize individual life history: the sequence of covariate values for
each individual starts in the year of birth, hence birds born in
different years have different sequences for the same covariate
(e.g.: WNAO for the first 5 years of life of a bird born in 1995 will
be: 1,363; 20,620; 20,067; 20,227; 0,643; while for a bird born
in 1998 WNAO values will be: 20,227; 0,643; 1,303; 0,040;
0,237). We assumed these covariates to characterize individuals’
life history (as for example in the case of individuals’ weight),
although they are not dependent on an individual’s life history. In
the case of environmental covariates, we had a single value per
year that was shared by all individuals. Integrative indexes used as
individual and external covariates were obtained performing
Principal Components Analysis, PCA, see below for details on
variables used [35].
We defined four time-dependent individual covariates (WSR,
western Sicily rainfall, WNAO, winter NAO, PB, pre-breeding
period integrative covariate, and B, breeding period integrative
survival of chicks and adults.
N WNAO: we selected the December through February winter
North Atlantic Oscillation [36] due to its broad geographic
coverage and used the average value of the selected period.
N WSR: to define the integrative covariate a PCA was performed
on rainfall data from the four meteorological stations closest to
the island of Marettimo: Erice, Trapani, Marsala, and Mazara
del Vallo (Figure S1) [37,38]. The analysis considered the total
rainfall recorded in 12 months relative to the life cycle year
(May to April, considering the year starting with egg laying).
N PB and B: to incorporate the uncertainty with which climatic
variables affect the pre-breeding (March-April) and breeding
(May-August) periods, we included temperature, rainfall, and
NAO anomalies in the PCAs to calculate the integrative
covariates as Pre-Breeding, PB, and Breeding, B, indexes.
To test winter period climate effects on adult survival and
identify potential wintering sites for the species, we considered two
external covariates defined as environmental covariates: SST, sea
surface temperature, and CHL, chlorophyll concentration. We
considered time series from 2001 to 2010 of SST and CHL as
proxies of environmental conditions [39] from five areas with
upwelling conditions in different regions of the Mediterranean Sea
(Alboran Sea, Balearic Sea, Sicilian Channel, Adriatic Sea and
Aegean Sea) [40]. To test the effect of winter conditions, we
analysed data from December to February, performing 2 PCAs for
each sea area (1 PCA for sea surface temperature, SST, and 1
PCA for chlorophyll concentration, CHL, each one based on 5
points). We then used the PC factors as external covariates in the
model for a total of 11 covariates tested.
Survival and recruitment estimate from encounter history
We applied a multistate mark-recapture analysis to estimate
chick and adult survival, to assess recruitment and to evaluate the
potential effects of time and sex on these variables [41,42].
Recruitment probability is given by the probability of transition
from the non-breeder to the breeder state and can vary according
to age and to time. We constrained variation with age by
considering that recruitment probability stabilizes to a constant
value from a given age onward. We defined first year survival after
fledging as chicks survival and adults survival that of adults of
2 April 2014 | Volume 9 | Issue 4 | e94526