The role of ecological compensation areas and conspecific density for the persistence of the largest Swiss Corn Bunting population

Agri-environment schemes (AESs) are Europe’s main policy response to halt farmland biodiversity loss. However, studies investigating the long-term effects of AESs on the population dynamics of endangered avifauna remain scarce. Qualitative and quantitative data were collated to determine population trends during 1995-2020 of the largest Swiss Corn Bunting (Emberiza calandra) population. Bayesian multinomial models were utilised to assess the changes in habitat associations and dynamic occupancy models were used to infer how colonisation, extinction and equilibrium occupancy rates varied across habitat types and with conspecific density. The results documented an 81% population decline from 68 to 13 occupied territories and that their previous preference for arable land has changed to the ecological compensation area (ECA) flower strip. The best occupancy model supported a time-invariant, additive effect of habitat type and conspecific density on territory colonisation and extinction. Conspecific density was the strongest driver of occupancy, showing a strong positive effect on colonisation and a weaker negative effect on extinction. Overall, colonisation probability was relatively similar among habitat types, but highest in the ECA flower strips. At average conspecific density and equilibrium, flower strips had roughly 3.5 times the occupancy rate compared to arable land. The results suggest that perennial flower strips located in close vicinity to existing populations would increase the Corn Bunting population in the Grosses Moos, given the high probability of colonisation and persistence. This study highlights the value of long-term monitoring schemes to make informed farmland management decisions and to optimise species-specific conservation measures.

Keywords: Occupancy models, Bayesian multinomial models, Agri-environment scheme, ecological compensation area, intensified agriculture, population dynamics, Corn Bunting