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.