Shifts in migration phenology under climate change: temperature vs. abundance effects in birds

In migratory birds, increasing temperatures have been linked to earlier arrival to breeding sites, enabling an earlier start of breeding and leading to changes in abundance. Long-term population trends may thus reflect a species capacity to respond to climate change. However, when a species is more abundant, it is also more easily detectable by observers, leading to an earlier detection of its arrival. Therefore, investigations of the drivers of shifts in apparent arrival dates to breeding sites and population trends remain challenging. Here, we formulate predictions aiming to disentangle the different drivers, analysing spring arrival dates and population changes of 52 migratory birds in a Central European country, the Czech Republic, from 1994 to 2017. If shifts in arrival dates are driven by increasing spring temperatures, migrants should arrive earlier in a warmer year and their abundance should increase in the subsequent year due to earlier breeding. If earlier migrant arrival results from their increased detectability caused by higher abundance, then migrants should arrive earlier in the same years when their abundance is high. We found clear support for the former prediction, indicating that climate change drives the earlier arrival of migrants irrespective of changes in their detectability. Moreover, species advancing their arrival to a greater degree had more positive population trends, and responses to rising spring temperatures in the Czech Republic became weaker with increasing migration distance. Therefore, climate change drives population trends of migratory species according to their capacity to adjust their arrival date to variations in spring temperatures.