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).