1. INTRODUCTION
Several farmland bird species in Western Europe are declining rapidly primarily due to the intensification of agricultural practices (Burns et al., 2016; Jerrentrup et al., 2017; Tryjanowski et al., 2011). These practices include for example building sub-surface soil drainage systems, which cause habitat loss and change, increased fertilizer use leading to denser vegetation, fast crop growth and short turn-over rates, increased pesticide and herbicide usage resulting in lower abundance and diversity of food/insects/arthropods, and the specialization and standardization of agriculture, which tends to create homogenous landscapes with large fields of low crop diversity (Schifferli, 2001). The European Union has initiated several schemes with the aim to prevent the loss of biodiversity in agricultural habitats, but these schemes seem to have had limited success, preserving biodiversity only at the local scale and to moderate extent (Gamero et al., 2017; Kleijn et al., 2011).
The Ortolan Bunting, a small Palearctic migratory songbird, is an example of a species experiencing an extreme population decline. It breeds from Spain to Mongolia and from Iran to northern Finland and migrates to spend the winter in sub-Saharan Africa (Jiguet et al., 2019). Across Europe, the species has faced continuous population declines, with a general drop of 88% since 1980 (Jiguet et al., 2016). The status of the species is particularly alarming in Northern Europe, where it is critically endangered (Hyvärinen et al., 2019). In Finland, the decline has been approximately 99% within the past 30 years (Väisänen and Lehikoinen, 2013), with only 2,600 pairs remaining in 2020 (Piha and Seimola, 2021). In Fennoscandia, Ortolan Buntings occur at their northernmost range limit and are genetically and demographically isolated from the rest of the European population, making them particularly sensitive to potential threats with no immigration to counteract the declines (Moussy et al., 2018).
Understanding the reasons behind such dramatic declines as the one of the Ortolan Bunting can provide highly valuable information on how to manage farmlands in a more biodiversity-friendly way. Numerous threats such as habitat loss and degradation, climate change, and illegal hunting, are believed to explain the declines of Ortolan Buntings (Menz and Arlettaz, 2012). Especially the threats along the flyways have been studied intensively (Jiguet et al., 2016), and recently, the wintering habitats have been under investigation (Gremion et al., 2022).
Survival studies done in Norway and Finland show that Ortolan Buntings may not be suffering exceptionally great losses during their migration or wintering in Africa. In a study done by Nousiainen (2020) during 2013–2019, Finnish male Ortolan Buntings had an apparent annual survival rate of 43.3 % (41.9–58.0 %; in 15 other similarly sized passerine species). Similar, or somewhat higher, survival rates were found in the Norwegian Ortolan Bunting populations (Dale, 2016). Additionally, Nousiainen (2020) found that the survival was equally good throughout all the three main regions where the species occurs in Finland. Based on this result, the survival of the male Ortolan Buntings is likely not particularly low, and thus does not appear to be the most important factor explaining the declines. In contrast, there were regional differences in population declines, with the steepest declines in southwestern parts of the population, where more than fourth of the population was lost annually (Nousiainen 2020, Piha & Seimola 2021). In summary, although the survival of females and juveniles were not examined in this study, this result already in itself hints that there might be some regionally varying factors regulating breeding success that might, at least partly, explain the dramatic declines in Ortolan Bunting populations. Hence, it seems that important reasons for the declines are to be found within the breeding areas.
The breeding habitat requirements of the Ortolan Bunting have been intensively studied, mainly in the agricultural habitats where the decline is steepest (Menz and Arlettaz, 2012). In general, the Ortolan Bunting has largely varying habitat requirements. In the most southern parts of its current distribution, in the Mediterranean and sub-Mediterranean Europe, Ortolan Bunting breeds in open shrubland and steppe-like habitats, and montane zones up to 2,500 m above sea level. In temperate Europe, it is primarily associated with cultivated land and shrublands in historically burnt habitats (see references in Menz and Arlettaz, 2012). In Nordic countries, the species is present in a range of different breeding environments; not only in farmland habitats, but also in clear cuts and peat production bogs (Dale and Christiansen, 2010; Dale and Olsen, 2002; Vepsäläinen et al., 2007). In Finland, the species is rarely observed in small, fragmented fields within forest dominated landscapes (Piha and Seimola, 2021). Within both natural and agricultural landscapes, the Ortolan Bunting breeds primarily in relatively warm, dry areas, with well-drained soils and an annual rainfall below 600–700 mm (Cramp and Perrins, 1994). The fact that the species becomes often locally common after disturbance, for example after a fire or a clear cut, indicates that the species may behave like a pioneer species, typically colonizing the early stages of vegetation succession (Pons and Clavero, 2010).
At the territory level in agricultural habitats, the Ortolan Bunting is associated with field margins featuring structural elements such as isolated trees and hedges which are used for perching and singing (Grützmann et al., 2002). High crop type diversity seems to be important for the species since the species primarily nests and forages on the ground (Berg, 2008), and a single crop type rarely provides a suitable vegetation structure throughout the breeding season. Low vegetation structure with patches of bare ground is more easily accessible to birds foraging and moving on the ground than uniformly dense growing crops (Schaub et al., 2010; Schifferli, 2001), while higher and denser vegetation is needed for hiding the nest.
Despite extensive studies on Ortolan Bunting breeding habitat requirements, few have directly linked habitat factors to the population growth rate of the species. Furthermore, many of the previous studies have been limited either to small geographical areas, conducted solely at the territory scale or focused on the occurrence probability (or density of territories) over a single year or a few years.
We investigated the relationship between changes in the Ortolan Bunting’s breeding habitat characteristics and the rate of decline of the species in a multiscale context where both territorial and landscape levels are considered simultaneously. We used an exceptionally large data set which covered approximately the whole Finnish breeding range area and nearly two decades of observations, collected with a comprehensive territorial mapping approach.
Specifically, we studied whether the rates of decline in Ortolan Bunting populations were associated with i) proportion of agricultural land cover within the landscape surrounding the subpopulation (landscape variable), ii) crop plant type diversity and the proportion of bare ground (crop variables), iii) density of roads, ditches, main drains, riverbanks and buildings (small-scale structure variables), or iv) temperature and rainfall from the previous year (weather variables), while also assessing v) potential regional variation in the declines.
We hypothesized that the reduction of small-scale structures, loss of diversity in crop types and reduction in the amount of bare ground have had negative effects on Ortolan Bunting population growth rate. We also expected that population growth rates are more negative in areas with smaller proportion of farmland in the landscape. In addition, we expected that cold and rainy breeding seasons reduced population growth to the next year.