A method to estimate phenological variation using data from large-scale abundance monitoring programmes

Capsule Large-scale abundance monitoring programmes can be used to estimate annual phenological shifts. Aims Phenology refers to the timing of any annually repeated biological event. The method developed here aims at measuring phenological variation in an indirect way by modelling seasonal abundance variations. Thus, it provides the opportunity to use a large number of datasets which have rarely been used in phenological studies. Phenological variations computed using this standardized method are comparable between species. Methods The data used for the development of this method originates from the French Breeding Bird Survey, a large-scale abundance monitoring programme launched in 2001. For each species, the phenological shift between two seasonal abundance trends is computed using maximum likelihood. Results Phenological shifts relative to the year 2005 (reference year) were estimated for 46 species over a 5-year period (2001-6). The standard deviations of the shifts do not differ significantly between species with different migratory status. Moreover, at the species level, the computed phenological shifts relate to the shifts of the mean date weighted by abundance. However, mean date, cannot be used in studies incorporating species with different migratory status (e.g. trans-Saharan migrant, sedentary) because of ambiguous changes for the some biological shift in timing. Conclusions The method described here is of particular value in determining how the phenology of common bird species changes in relation to climate. It offers the opportunity to increase the spatial scale of phenological studies and to include multi-species analyses. This method could be applied to any abundance or constant effort site programme to study the timing of any biological process for which a seasonal distribution is available.