When to migrate south? When there is a good tail wind.

A common noctule, Nyctalus noctula. Photo my own.

This week's paper addresses an age old question, how do animals decide when to migrate? The paper is entitled "Determinants of spring migration departure decision in a bat" lead-authored by Dina Dechmann at the Max Planck Institute. Pregnant female noctules in the spring migrate hundreds of kilometers Northeast from hibernation caves to insect-rich feeding grounds (this might be from Switzerland to Sweden or Bulgaria to Russia). But how do they decide when to leave? 

Over three years Dechmann and her colleagues fitted 29 female noctules with radiotransmitters and then used a small Cessna plane to search for the signals of bats that were migrating. They also obtained detailed weather information for the sites where the bats were tagged. In songbirds, birds tend to decide to migrate once they have put on enough fat. That was not the case for noctule bats, their decision to migrate was not affected by their body condition. Migration decisions in noctules were best explained by wind direction, wind speed, and air pressure. Bats were most likely to migrate on nights with faster tailwinds in the migration direction, and with higher air pressure. The tailwinds make a lot of intuitive sense, and generally high air pressure is associated with more stable weather conditions which might also be to a migrating bat's benefit. Songbirds generally migrate much further than bats, so the lack of putting on fat determining migration is likely due to the shorter migration distance.