Rieger, M. R. | Search bioRxiv

bioRxiv2024

Analysing bird population trends from monitoring data with highly structured sampling designs

Population trends derived from systematic monitoring programmes are essential to identify species of conservation concern and to evaluate conservation measures. However, monitoring data pose several challenges for statistical analysis, including spatial bias due to an unbalanced sampling of natural regions or habitats, variation in observer expertise, frequent observer changes, and overdispersion or zero-inflation in the raw data. An additional challenge arises from so-called rolling survey designs, where each site is only visited once within each multi-year rotation cycle. We developed a GAMM-based workflow that addresses these challenges and exemplify its application with the highly structured data from the Ecological Area Sampling (EAS) in the German federal state North Rhine-Westphalia (NRW). First, we derive a routine that allows informed decisions about the most appropriate combination of distribution family (Poisson or negative binomial), model covariates (e.g., habitat characteristics), and zero-inflation formulations to reflect species-specific data distributions. Second, we develop a correction factor that buffers population trend estimates for variation in observer expertise as reflected in variation in total bird abundance. Third, we integrate region-specific trends that adjust for between-year variation in the representation of habitat or natural regions within the yearly subset of sampled sites. In a consistency check, we found good match between our GAMM-based EAS trends and TRIM-based trends from the German Common Bird Monitoring scheme. The study provides a template script for R statistical software so the workflow can be adapted to other monitoring programmes with comparable survey designs and data structures.

ecology↗

bioRxiv2020

Criteria for effective fallow field eco schemes for farmland birds during non-breeding

O_LIFarmland eco schemes implemented under the current Common Agricultural Policy (CAP) of the European Union are often considered ineffective in halting farmland bird declines. Fallow fields, often seeded with dedicated seed mixtures, rate among the more beneficial eco scheme types. Yet, the CAP currently defines no minimum criteria for fallow fields to qualify as eco scheme, likely jeopardizing their potential biodiversity benefits. C_LIO_LIWe investigated the attractiveness of four fallow field types established under CAP eco schemes and dedicated bird conservation programs in Southern Germany. Our 2-year surveys on > 100 fields focused on the non-breeding season, where food limitation can become particularly problematic. We modelled bird incidences also in response to vegetation structure and adjacent landscape features to derive minimum criteria for effective fallow field eco schemes. C_LIO_LIFallow field types varied only mildly in overall species richness but showed striking differences in the attracted species. Finches in particular tended to preferentially visit 1-year fallow fields, while buntings tended towards 2-year and older field types. 1-year CAP fallows, however, are typically removed before mid-winter, and thus potentially act as a trap to farmland birds and other wildlife. C_LIO_LIThe investigated species consistently preferred larger fallow fields with a more differentiated vegetation structure. Placement close to woods and hedgerows positively affected birds inhabiting woodland ecotones, while classic farmland species showed higher incidences on fallow fields embedded in open landscapes. C_LIO_LI Policy implications Our findings call for the ongoing CAP revisions to specify minimum requirements that qualify fallow fields as eco schemes. These should include an at least biennial cycle, a diversification of seed mixtures, standards for fallow field size, and criteria for their placement in the landscape matrix. C_LI

ecology↗