Methods
Field work
Selvagem Grande (Lat 30.140556, Long -015.868889) is an island nature
reserve that forms part of the Madeiran Archipelago, Portugal. Around 30
000 pairs of Cory’s Shearwaters breed on the island, arriving at their
nest burrows between February and March to undertake a long period of
nest defence and mating (the pre-laying period) before laying a single
egg in early June, which they incubate for around two months. Over 450
regularly occupied nest cavities within rubble walls are the subject of
long-term research, with annual monitoring of nest occupancy and
breeding output resulting in hundreds of ringed individuals
(Granadeiroet al. 2006). In 2016 and 2017 we deployed a large number of
geolocation loggers (GLS, Migrate Technology model C330; 122 in 2016,
104 in 2017) to track individuals’ migratory movements over winter,
revealing the migratory strategy of 110 males (98 overwinter movements
2016-2017 and 83 overwinter movements 2017-2018). GLS were leg mounted
and weighed less than 1% of the birds’ body weight, which, given the
attachment method, can be assumed to bear minimal or no effects on
survival and fitness
(Bodeyet al. 2017). The resighting rate of birds fitted with GLS is
consistent with the rate of return of breeding birds from one year to
the next, at ca. 85%
(Mouginet al. 2008).
Behavioural tests
We carried out behavioural tests over three breeding seasons between
2016 and 2018.
We assessed exploration in an emergence/novel environment test. Birds
were extracted from their nest, transported in a closed box to a small
room which served as the novel environment, and transferred into a box
with a removable flap door situated in a corner of the room. Over a 10
minute time bracket, birds could leave the open box into the empty room
while being filmed. The whole process lasted 20 minutes and had no
implication on the breeding success of the individuals tested. Thirty
two birds were tested during the incubation period of 2016, 69 during
the pre-laying period of 2017, and 43 during the incubation period of
2017. Twenty seven individuals were tested twice over the two years.
From this test we extracted two scores; whether or not the bird left the
box within the stipulated time (binary “emergence” score) and, if it
did, the latency (in seconds) for the bird to leave it.
In 2017 and 2018 we tested the reaction on extraction from the nest in
male Cory’s Shearwaters on first encounter, either when retrieving GLS
in the pre-laying period or as part of annual nest monitoring during
incubation. A binary “extraction score” was assigned depending on the
birds’ reaction to an approaching hand, reaction on being caught, and
resistance to being pulled out of the nest (Table 1), classifying birds
as either “reactive” (1) or “unreactive” (0). Behaviours that could
not be classified into either of the two classes according to this
classification were not considered. Around 60% of all assessments
resulted in a classification of either “reactive” or “unreactive”.
All assessments were performed by the same person to minimise
subjectivity, and were only carried out at nests where the target
individuals were the sole occupants of the nest at the time of testing,
were facing towards the nest entrance, and could be reached easily
without the use of a noose – this is the scenario for the vast majority
of nests in the study site. Extraction scores were obtained during
pre-laying (2017: 56 individuals; 2018: 147 individuals) and incubation
(2017: 168 individuals; 2018: 107 individuals). 17 individuals were
assessed twice during pre-laying in 2018 to determine short-term
repeatability. In total, 153 males were scored; 43 males twice, 17 three
times, and 11 more than three times.
Statistical analysis
Statistical analyses were performed on R Statistical Package, version
3.5.1
(R
Core Team 2018).
In order to identify whether age and annual cycle stage (pre-laying vs
incubation) could influence the results of the behavioural traits, we
ran generalised linear (mixed-effect) models (function glm, package
stats or function glmer, package lme4). Age was treated a factor of six
5-year age classes. The latency to leave the box was fitted with a
quasi-poisson error structure, while the emergence and extraction scores
were fitted with a binomial error structure. Individual ID was added as
a random intercept in models of emergence and extraction due to repeated
assessments on the same individuals. To control for the effect of
migratory strategy on extraction score (see Results), we fitted this
model only on migratory individuals. Previous breeding success does not
influence extraction score (see Appendix 1, Supporting Information).
We estimated individual repeatability in temperament over the entire
period of study to ensure that individuals were consistent in their
behaviour in the short term, and that therefore temperament could be
related to migratory strategy at least over this time period.
Repeatability was calculated as the intraclass correlation coefficient
with a binomial error distribution
(package
rptR, Nakagawa & Schielzeth 2010) using bootstrapping without
randomisation (1,000 iterations) to estimate confidence intervals. For
assessing exploration behaviour, this was calculated on the emergence
score using a binomial error distribution.
To investigate links between individual temperament and migratory
strategy (migratory/resident) we ran generalised linear mixed effect
models for the binary emergence and extraction scores as dependent
variables in separate models, with individual as a random intercept. Age
and annual cycle stage were added as fixed effects in the model
explaining emergence.
Linear regression estimates (“Est”) are presented with their standard
error.