While in the US, I made good use of Bill Evan's Old Bird reference guide to study and learn the nocturnal flight calls of migratory passerines. It has been very fun to review the evening recordings captured by the mic I have hooked up to my roof, uploading my results to eBird.
However, I will be moving to Europe later this year. I am keen to get started learning European flight calls. Are there any good reference materials, like Old Bird, that can get me started? Something besides just scanning xeno-canto.