4.2 Importance of interconnected agricultural landscape
Our findings revealed a weak positive connection between the proportion
of farmland surrounding the singing groups and the population growth
rate. Indeed, in Finland, we see that Ortolan Buntings are still
surviving in singing groups located in landscapes that are characterized
by interconnected agricultural areas.
We propose two explanations for this result. Firstly, large and
interconnected farmland areas tend to be more intensively managed than
smaller, more scattered farmlands which usually include less intensively
managed or even abandoned fields. In Germany, in the Wendland region,
the Ortolan Bunting population was growing at the turn of the
21st century (approx. 1995–2007). This increase was
connected with a major change in land use: a notable proportion of
grassland was converted to arable land (Deutsch, 2007). Abandoned arable
land will eventually have higher and denser vegetation cover, and
possibly more predators, which will be unfavourable for Ortolan
Buntings. In Italy, the dense afforestation and grassing after the
abandonment of pastoral and agricultural activities are hypothesized to
be the causes of Ortolan Bunting extinctions in the hills of Brescia
(Leo et al., 2023). Also, in Sweden and England farmland birds are
declining possibly due to both agricultural intensification and
abandonment (Wretenberg et al., 2006). Also at its wintering grounds,
the Ortolan Bunting seems to prefer extensively managed, structured and
semi-open agricultural landscapes (Gremion et al., 2022). It seems
evident, that an intermediate level of management benefits the Ortolan
Buntings by providing a suitable set of habitat characteristics.
Secondly, Ortolan Buntings are social birds that form singing groups of
several males (Cramp and Perrins, 1994). Therefore the species shows an
aggregated distribution pattern, one which is only partly determined by
habitat structure and quality (Vepsäläinen et al., 2005). In large,
interconnected farmland areas there is more space and suitable habitat
for several singing males to establish territories. These males then, in
turn, can attract more singing males, and eventually also females to the
same area. A smaller, isolated farmland patch of otherwise ideal habitat
might not be as attractive to the species, because it has a lower
potential for formation of a new singing group. The disappearance of
group structure in declining populations might hamper breeding even
further. In Norway, the small Ortolan Bunting population has a strongly
male-biased adult sex ratio due to female-biased natal dispersal away
from the small and isolated distribution area. This skew logically
affects the pairing success of males. In the Norwegian population, about
half of all males are unpaired (Dale, 2011). In Finland, less than 70%
of Ortolan males were paired in an inventory study (Piha and Seimola,
2021). If females disperse away from good but fragmented habitats,
without finding better breeding sites or companions elsewhere, small
local population might be heading to extinction slowly and irrespective
of whatever conservation measures are made locally. The only measure
that can help in this situation would be the rapid habitat restoration
in the matrix between existing habitat fragments (Dale, 2001). Another,
in this case potentially disadvantageous, aspect of sociability, is the
strong site fidelity of adult Ortolan Bunting males. Adult males may
maintain singing groups despite the habitat transforming to suboptimal
for breeding. Site fidelity might hence further mask the results related
to habitat characteristics.
In summary, at the landscape level, large and interconnected farmland
areas represent open and sufficiently managed areas, possibly attracting
conspecifics, and might therefore function as an ecological filter which
can influence the preference for certain habitat characteristics.
Otherwise good breeding sites might be neglected if they do not occur
within interconnected farmland landscapes.
4.3 Positive growth rates only in North Ostrobothnia
Among all the regions, only North Ostrobothnia showed positive
population growth rates, while South Ostrobothnia, Tavastia, South-West
Finland, North Carelia, Uusimaa and Central Finland showed negative
population growth rates.
Several factors might contribute to the observed variance in growth
rates between regions. Firstly, North Ostrobothnia boasts a substantial
amount of newly established agricultural fields, mainly converted from
peatlands. These areas have not yet been under heavy agricultural
management practices, such as pesticides or turning of the soil. They
often feature exposed mineral soil and natural vegetation with extensive
growth of bushes (Salix sp.) and young trees (Betula sp.,Sorbus sp., etc.), which offer abundant food resources. Food
seems to be plenty during the feeding period along the stretches of land
between the peat extraction fields where willows and birches are growing
(field observations).
Secondly, the specialization of farms has been intensive in Finland
during the last decades, namely eastern and northern parts of the
country are mainly specialized in cattle farming and southern parts in
grains and vegetables (Hiironen and Ettanen, 2013). For example, in
eastern Finland large farmland areas almost exclusively grow fodder
grass hence leaving negligible amount of bare ground for the birds
during spring.
Thirdly, concerning species distribution, it is important to consider
that the presence of species in a particular location may not solely be
due to the suitability of the habitat but also influenced by the fact
that species had a means of getting there (spatial aspects of population
dynamics, metapopulation dynamics) (Storch et al., 2003). The fact that
the Finnish Ortolan Bunting population has declined less in the north
and the resulting shift towards north could be a mere result of the
deterioration of the southern habitats and the fact that the birds have
no other direction to go to. However, field observations do not support
this wholeheartedly. In Finland, there are two easily distinguishable
singing type populations (dialects) in Finland, the northern and the
southern, which are geographically isolated, and seem to be disconnected
from each other without any significant immigration between populations
(Piha & Seimola, 2021). As birds generally learn their songs from one
another, the persistence of these song types may indicate limited
dispersal (via recruitment), which is likely in the context of a
declining population.
Research on the breeding- and population biology of Ortolan Buntings in
different regions of Finland would elucidate whether the breeding of the
species is failing and the causes for it, and which regions currently
act as sources or sinks. Although, this research might already come too
late as the Allee effect often occurring in extremely small populations,
might impair deductions from such investigations. The fast decline of
Ortolan Buntings has continued after the study period, in all regions of
Finland (Piha and Seimola, 2021).