[eng] An emerging consequence of global climate change is a shift of seasonal biological events. In
animals, climatic divers have always been found to guide population trajectories and individual
life-history tactics, but it is not clear whether individuals will be able to adapt to the fast paste at
which temperature and rainfall are changing. The analysis of long-term data sets reveals
different degrees of plasticity in the phenological responses of individuals, within and between
species, but the pattern and the payoff of this plasticity are not always known. Understanding
individual responses is essential to reveal the mechanisms and implications of climate-related
changes on population dynamics and individual life-history tactics. We used longitudinal data
collected over eighteen breeding seasons (2001-2018) on the laying date of Cory’s shearwaters
Calonectris diomedea, at Pantaleu islet at Dragonera Natural Park, Mallorca (Balearic
Archipelago, Spain). The Cory’s shearwater is a trans-equatorial migratory seabird that spends
the winter (October-March) into the Atlantic Ocean before returning into the Mediterranean
Sea to breed (April-September). Individuals breed in colonies and egg laying occurs more
synchronous than in other seabirds, however, the laying date at population as well as individual
level change every year, possibly as a response to winter ocean state (e.g. through carrying over
effects) and/or conditions at the breeding grounds. We first considered the winter North
Atlantic Oscillation index (wNAO) as a proxy of environmental variability and assessed whether
its variation correlated with the variation in the average laying date at population level.
Subsequently we used longitudinal data to investigate whether individuals were consistent in
their laying date relative to the population average, i.e. repeatability of laying date. Finally,
using mother-daughter data we assessed whether the laying date has a heritable component.
We found that reproductive onset at population level correlated positively with the climatic
index. This was fully explained by individual plasticity because, despite a change in the average
laying date, individuals were consistent in their trait expression. Indeed, the repeatable of this
trait was high (R = 0.517), with some birds breeding consistently earlier than others.
Interestingly, trait repeatability was higher for those females born at the colony (R = 0.653)
compared with all females. The heritability of laying date assessed using 29 half parent- half
offspring pairs was high, but the value suffered of a large uncertainty due to a relatively small
sample size (h
2 = 1.14, R2 = 0.12, p=0.07). We thus used another approach and sorted females
into two groups, namely ‘early’ and ‘late’ breeders, according to whether laying date occurred
before or after the median of the population respectively. We compared the proportion of each
combination through a 2x2 contingency table. Results showed a statistically significant
difference (X2
1=7.79, p<0.01) confirming a high heritability of laying date. Results from general
linear mixed models indicated that early breeding birds had 27% more chance to breed
successfully than late breeders (breeding success: 0.83 and 0.60, for the earliest and the latest
laying date, respectively; z=-2.68, p<0.01). In conclusion, our work showed that changes in the
laying date correlate to environmental conditions and that despite a high plasticity, individual
repeatability is high as it is the heritability of this trait. Despite this, the advantage of an early
breeding seems small suggesting that inter-individual variability is probably the result of
differences in phenotypic quality intrinsic to the population.