Climate change, phenology and species detectability in a monitoring scheme

The knowledge of the state of biodiversity on the globe is based on a large number of monitoring schemes. Quite often the results of these schemes are sensitive to the timing of monitoring due to the phenology of species, which in turn may affect the detectability of species during censuses. As global warming has been shown to cause changes in phenology, there is an increasing risk that species detectability will be altered if the timing of monitoring is not adapted to this change. I tested how sensitive species detectability is to the timing of censuses and whether there are potential climate-driven temporal changes in the detectability of 73 Finnish land bird species monitored using single-visit line-transects in 1987-2010. This was done by investigating seasonal and annual patterns in the proportion of birds in the main belt and those detected by displaying activity. Over 20 of the study species showed significant changes in detectability within the census season. However, only three species showed a significant trend in annual detectability. According to multi-species analyses there was a slight but significant increasing trend in the proportion of displaying birds and a slight decreasing trend in the proportion of birds in the main belt. However, the observed species-specific annual changes in displaying activity or in proportion of birds in main belt were not associated with the observed population trends of species during the same period. Nevertheless, the findings highlight a strong potential risk that species detectability can change if climate change escalates in the future. I recommend researchers to investigate how sensitive their monitoring systems are for phenological changes and prepare tools for taking potential changes in detectability into account.