Introduction
Over 60% of an Afro-Palearctic migrant’s annual cycle occurs at
non-breeding grounds (McKinnon, Fraser, et al., 2013), where migrants
experience unstable and challenging environmental conditions. What
ensues during this period will have significant carryover effects on
many aspects of their survival and reproduction (Both et al., 2006;
Pulido, 2007) and on the overall population dynamics of a species.
Nevertheless, this period has been insufficiently studied (Marra et al.,
2015) and details regarding fine-scale spatio-temporal movements are
lacking. Understanding site persistence and the degree of between-years
site fidelity will contribute to a better understanding of migratory
connectivity and of how birds may respond to longer-term habitat and
climate changes that, in turn, can lead to appropriate conservation
efforts (Sanderson et al., 2006).
For many years, there was a largely evidence-free assumption that small
migrants tended to move across Africa, tracking changing seasonal
conditions in a generally itinerant way (Moreau, 1972). More recently,
there has been increasing evidence that this is strongly species- and
population-specific (Bulluck et al., 2019), with some species visiting
several sites, others spending longer periods at fewer sites,
establishing and defending territories, and, in some cases, showing both
strategies (Belda et al., 2007; Blackburn & Cresswell, 2016; Catry et
al., 2003; Thorup et al., 2019). To remain at a single site and maintain
a territory confers advantages regarding local knowledge such as
foraging locations, competitor densities, resource fluctuations, and
predators (Catry et al., 2004; Piper, 2011), and avoids high costs and
unpredictability associated with moving long distances, likely leading
to higher survival rates (Cresswell, 2014; Yoder et al., 2004). On the
other hand, itinerant individuals track ephemeral resources over a large
area and are likely to move as environmental conditions change with the
progression of the season, to optimise food availability (Ruiz-Gutierrez
et al., 2016).
Many Afro-Palearctic migrants not only remain for prolonged periods at
non-breeding sites but return to them year after year, especially
territorial individuals (Blackburn & Cresswell, 2016; Cuadrado, 1992).
Familiarity with these sites confers similar advantages as longer
residency. Furthermore, fidelity has also been detected at a temporal
scale, where individuals return to the same sites during similar times
of the year (Stanley et al., 2012; van Wijk et al., 2016).
According to the serial residency hypothesis (Cresswell, 2014), many
Afro-Palearctic migrants are likely to be faithful to any site(s) that
promotes their survival, thus we expect strong residency differences and
return rates amongst individuals of different ages. This hypothesis
predicts that first-years, which lack knowledge of small- and
medium-scale locations of where to arrive, will reach the non-breeding
grounds stochastically. Some will
find a site and remain at it until migration, while others will continue
their search elsewhere, many of them arriving at less suitable sites or
even discovering new unknown suitable habitats. Individuals will then
reuse those successful sites during subsequent years as adults.
Therefore, if an individual gets older, it becomes more site faithful
because of natural selection removing those that did not locate suitable
sites. In any population therefore, older birds will be more site
faithful.
Studying site persistence and between-years site fidelity, however, is
problematic. First, few species are likely to be so noticeable that they
will always be detected at a site when present, leading to false
negatives, particularly with low sampling effort. Second, determining
site persistence and return rates greatly depends on when individuals
are first marked and on their duration of stay, because passage birds
will have lower detection and overall capture probabilities than more
resident birds. Third, data is
highly dependent on the methods used (e .g . ringing
schemes, geolocators, resightings). Ringing studies, for example, are
usually undertaken at the beginning and end of the season at constant
sites. This increases a birds’ ‘net-shyness’ and reduces capture
probability and makes it difficult to detect short duration stays.
Results from tracking studies, on the other hand, are potentially the
solution, except that small passerines can only be tracked with archival
tags, where data is only recovered if an individual has some degree of
site fidelity and at a very low spatial resolution.
The Common Whitethroat Curruca communis (henceforth
‘Whitethroat’) is a widely distributed small Afro-Palearctic migrant.
Studies based on ringing recoveries and sporadic encounters have
speculated that Whitethroats are faithful to their non-breeding grounds
and remain there for a considerable period. Whitethroats inhabit dense
thickets, show inconspicuous behaviour, and are relatively quiet during
the non-breeding period (Zwarts & Bijlsma, 2015), all of which make
them hard to detect. In this study, we use intensive resighting efforts
throughout three non-breeding periods at a single site in Nigeria to
understand, at an individual level, how Whitethroats use the
non-breeding grounds at a fine spatial and temporal scale. We first
calculate the probability of detecting an individual during a resighting
visit and investigate site persistence of individuals of different age
and sex groups. We then describe return rates and the degree of
between-years site fidelity and determine whether individuals depart the
area at similar times every year. To our knowledge, this is the first
study to research winter residency and site fidelity of Whitethroats at
a very fine spatial scale during the entirety of multiple non-breeding
seasons.