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.