In the autumn, as days shorten and forests display their colors, another seasonal change unfolds overhead: the great journey of millions of birds to their wintering grounds. For many species, September and October mark the post-nuptial migration, a voyage that sometimes takes them thousands of kilometers from their breeding sites.
Contrary to popular belief, it's not just the cold that drives birds to leave, but also the decrease in food resources. Insects become scarcer, some plants stop producing seeds or fruits, and northern European wetlands gradually begin to freeze. Each species adopts its own strategy, traveling a few hundred kilometers or crossing the Mediterranean and Sahara. Some travel by day, others use the night to advance in cooler, less turbulent air.
The reliefs of the Chartreuse mountains serve as excellent observation posts. The Col des Ayes, located between Saint-Pierre-de-Chartreuse and the Plateau-des-Petites-Roches, is a particularly interesting site, the only pass in Chartreuse known for its specific bird migration monitoring. Its configuration, at the mouth of a north-south valley, can channel part of the migratory flow, attracting mainly small passerines such as Chaffinches and Bramblings, as well as various tits (blue, coal). Large soaring birds, like certain raptors or storks, benefit more from the thermal updrafts of the Isère valley.
One should not expect immense flocks; migration can manifest as the rapid passage of a few birds, sometimes flying low over the pass. Contact calls then become almost as important as binoculars for ornithologists, allowing them to detect and identify migrants even before seeing them.
This year, on October 10 and 11, 2026, the ABC de Chartreuse is highlighting migration at the Col des Ayes. An observation point will be set up with ornithologists and the team from the Chartreuse Regional Natural Park. The program includes telescopes, discussions, and bird discovery, with free access to the stand. Accompanied ascents from the Col du Coq are also offered by reservation.
To track these movements across Europe, the EuroBirdPortal tool, presented on the Faune-France website, gathers observations from numerous European naturalist networks. Since September 2026, it allows visualization of the weekly distribution of nearly 600 bird species in Europe, based on over 750 million observations collected since 2010. The maps are updated daily, allowing near real-time tracking of bird movements. By selecting a species, such as the Chaffinch, one can observe its distribution changes week by week across Europe. Data is represented in 30x30 km grids.
This tool helps understand that the few birds spotted over the Col des Ayes are part of a continental phenomenon. It's important to note that a lack of data on the map doesn't necessarily mean an absence of birds, as observation effort varies by region. In October, look up: a flock of finches or a few tits might signal the start of a journey spanning hundreds, even thousands, of kilometers. To extend your observation from home, EuroBirdPortal is a valuable resource.
