Discussion
Detection probabilities
The probability of detecting a colour-ringed Whitethroat at our study
site, when it was present, was 33%. Our results are consistent with theSylviidae family having relatively lower detection rates than
other passerine birds (Johnston et al., 2014; Zwarts & Bijlsma, 2015)
and are similar to detection probabilities at their breeding sites in
the UK (30%; Johnston et al., 2014). This rate is relatively low when
compared to detectability at the non-breeding grounds of other
Afro-Palearctic migrants such as Whinchats Saxicola rubetra (63%
detection probability; Blackburn & Cresswell, 2016b) and ChiffchaffsPhylloscopus collybita (recapture probability 66%; Catry et al.,
2003), though there are few studies that have addressed and calculated
detection probabilities during this period (e .g . Zwarts &
Bijlsma, 2015). Nevertheless, despite Whitethroats having relatively low
detection probabilities, we consider that our high sampling effort
(sites were visited at least once a week, for over 20 weeks each year)
was sufficient to compensate for this.
Site persistence
Site persistence varied significantly amongst individuals, ranging
greatly between one and 165 days.
Because of the high sampling
effort as well as similar return rates between individuals of different
residency categories, our evidence is fairly compelling that shorter
stays truly reflect shorter residencies and not detectability issues or
mortality. Overall, the mean persistence duration was similar throughout
years, so residency dynamics at a species level may not be changing
strongly with time. In the Gambia, Whitethroats were also observed to
have different degrees of site persistence with 45% of captured
individuals remaining in the area between two and 84 days, though most
individuals were caught less than a month after ringing (King &
Hutchinson, 2001). In Senegal, however, most individuals were on passage
(King & Hutchinson, 2001).
Different wintering strategies of individuals at the same site have also
been recorded for other long-distance migrants: 27% of BlackcapsSylvia atricapilla in Spain (Belda et al., 2007) and 8% of
Chiffchaffs in Portugal were winter residents (Catry et al., 2003),
whilst the rest were categorised as transients. This could have several
explanations. First, individuals could have genetic differences due to
parallel evolution of morphological and behavioural adaptations, making
some individuals more inclined to lead either a nomadic or a resident
lifestyle (Senar & Borras, 2004). However, we cannot be certain whether
individuals that were categorised as “passage” or “unknown” at our
study site remained itinerant throughout the season, if they were in
fact en route to a stationary non-breeding site elsewhere, or if
they died during the period. These genetic differences could also
reflect individuals from different breeding populations although this
seems highly unlikely because individuals switched strategies across
years, and Whitethroats seem to have a somewhat low migratory
connectivity (Tapia-Harris et al., n.d.). A second explanation could be
due to habitat quality changes throughout the season. The broad
residency spectrum, from continual and variable movement to winter
residency, appears to reflect a gradient in predictability in food
supplies (Newton, 2008). As time passes, habitats dry and resources
change (Moreau, 1972), so competition could increase. Some individuals
may decide to leave the area to find other more suitable habitats
elsewhere, while others may risk staying (for example those that are
better competitors or are already in the best territories).
Short-term residencies, during both autumn and spring, may indicate the
use of multiple important non-breeding sites. Nigerian Whitethroats
deployed with geolocators remained at a first stationary non-breeding
site in the Sahel before arriving at our study site in November
(Tapia-Harris et al., n.d.). If some of these birds were then to be
short-term residents, individuals could have more than two important
stationary non-breeding sites.
Multiple site use by individuals
has been well described for several migrants in the Palearctic systems
(McKinnon, Stanley, et al., 2013).
Adult birds’ site persistence was longer than first-year birds likely
due to their previous experience and potential
dominance. Many first-year birds
probably arrive stochastically at non-breeding sites looking for
suitable habitats. Many will need to explore the terrain and scout for
resources, and whilst some will remain at it until migration to a second
site later in the non-breeding season, or back to Europe in the spring,
others may continue their search elsewhere, making a relatively
immediate migratory scale movement.
Between-years site
fidelity
Many long-distance migrants return to the same non-breeding sites year
after year both in the Nearctic-Neotropical and Afro-Palearctic systems
(Blackburn & Cresswell, 2016; Moreau, 1969; Salewski et al., 2000).
Here we found that a minimum of 19% of individuals returned from one
year to the next, an intermediate return rate in comparison to other
Palearctic migrants in Africa (Kelsey, 1989; Salewski et al., 2000;
Thorup et al., 2019) and Whitethroats at their breeding grounds
(0–64%; da Prato & da Prato, 1983, 14.5%; Boddy, 1992). Not only did
a significant proportion of individuals return the following year, but
individuals moved, on average, only 300 m. These results suggest that
many individuals have a high degree of between-years site fidelity at a
very small spatial scale, though less than Whinchats at the same study
site, which moved < 30 m (Blackburn & Cresswell, 2016).
Fidelity across years confers the same advantages as longer residency
patterns, especially regarding knowledge of local and fluctuating food
sources, competitor densities, and location of refuges, and this seems
to secure and increase an individual’s survival.
Return rates were different amongst individuals from different age
categories: first-years had lower return rates than adults, at least
from year2 to year3. First-year birds lack knowledge of small and medium
scale locations of where to arrive, thus their first non-breeding period
is full of uncertainty and stochasticity (Cresswell, 2014). Some
individuals will find a suitable site immediately and remain at it until
spring migration, while others will continue their search elsewhere,
many of them arriving at less suitable sites or even discovering new
unknown suitable habitats. Older birds will tend to reuse non-breeding
sites, so becoming more site faithful over time. Some studies have
argued that higher return rates in adults could be due to greater
survival from the previous year. Though this may be true for a few
individuals, we think that because individuals had already undertaken a
first migration, when high mortality rates occur, then overall,
age-related survival differences are unlikely at this stage.
Even though most of the long-term winter residents remained as such the
following year, some remained at the site for less time the following
year. We expected individuals with a successful previous experience to
repeat the same behaviour in the following years to guarantee their
winter survival. When we further investigated these individuals, we
found that most of them were seen in spring of the following season.
Individuals may have remained longer at their previous wintering site
due to better yearly habitat conditions that year and delayed the return
to our site. If this were correct, we would expect that many individuals
do not return to our study site when conditions are better further north
and only return when conditions are suboptimal. On the other hand,
passage birds increased the duration of their stay the following year.
Given many of them were first-year birds, the lack of experience and
dominance would have become a disadvantage, arriving at non-breeding
grounds after adults, by which time fewer sites will have been
available. By the following year, returning earlier in the season and as
adults, they could occupy empty territories that allowed them to remain
for longer periods.
Departure dates
Individuals did not seem to change their departure timing from year1 to
year2, although further information over a span of many years is needed
to draw stronger conclusions. Timing of migration is of critical
importance in migratory species and is key for securing fitness (Drent
et al., 2003; Kokko, 1999). Departure from the non-breeding grounds has
been seen to correlate with arrival at breeding sites (Kristensen et
al., 2013; Ouwehand & Both, 2017), though later departing individuals
can migrate faster to compensate for lost time (Yohannes et al., 2009).
As our study site is located at the southern part of the distribution
and individuals have different breeding sites, the first individuals to
depart are not necessarily the first to arrive at their respective
breeding grounds (Tapia-Harris et al., n.d.).
Individuals at our study site showed lower intra-class correlation
departure coefficients (r = 0.15) than most other Afro-Palearctic
migrants (range 0.06 – 0.82, Both et al., 2016). Low repeatability fits
well with the idea that individuals do not always remain for similar
periods across years, and therefore it is perhaps not surprising that
some individuals left the area on different dates, but there are a few
things to consider with these results. Firstly, repeatability indicates
how consistently individuals differ from each other and is not
necessarily a measure of individual repeatability across years (Conklin
et al., 2013). Secondly, departure from our study site does not
necessarily imply that individuals commenced spring departure, they
could have moved to another non-breeding site.
Conclusion
Our results are consistent with the serial residency hypothesis
(Cresswell, 2014) but also suggest small modifications. Instead of
individuals repeatedly settling at the same wintering sites during the
same periods year after year, we hypothesise that individuals may vary
their timing at the sites depending on yearly conditions. We predict
that spatial site fidelity and repeatability are high with little
variation across individuals, but temporal fidelity varies individually
and according to yearly conditions.
Individuals stay stationary at a
suitable site and remain there for as long as possible. Some proportion
of the population will stay there all the northern winter, most likely
those that are better competitors and are at overall higher quality
sites and better environmental conditions, whilst others will move to a
second site. This might never happen to an individual, and for some
individuals this might only happen in their second or third autumn
non-breeding season, depending on the specific year’s conditions. The
following year the same first site will be revisited; if conditions are
stable and the site adequate to secure its survival throughout the
period then there would be no need to migrate elsewhere. If not, then it
would migrate to the same second site, and so on. When individuals reach
the limits of their potential distribution, such as our study site, then
they will return only in occasional years of widespread food shortage or
suboptimal conditions in the previous sites (Newton, 2008). We would
therefore predict that apparent survival at a sub-Saharan non-breeding
site will correlate positively with latitude. Some individuals may get
unlucky and will need to change sites several times, which makes them
appear itinerant, whilst others will remain as long-term winter
residents at few sites. Regardless of any particular year’s conditions,
the number of migration steps and distance of migration is minimised and
matched to environmental conditions.
In summary, results suggest that
spatial fidelity is high and constant through years, but temporal use or
temporal fidelity and site persistence may vary and a possible
explanation for this might be variation in yearly and seasonal
conditions. In other words, timing is important: individuals revisit
locations at very precise scales but do not necessarily repeat them at
the same time. There is temporal flexibility but not spatial
flexibility, except in the sense there is always an option to make a
potentially dangerous further migration to an unknown area if conditions
became untenable. But these hypotheses can only be tested fully when
small birds such as Whitethroats can be tracked with non-archival tags
so that wintering locations regardless of site fidelity and long-term
survival can be seen.