Yes, Birda is designed to work offline so that you can continue to log your bird sightings no matter where in the world you are; you just need to make sure your location services (GPS) are switched on! However, a few aspects of the Birda app will not work while you are offline.
iOS
In the iOS app, an offline mode screen is displayed within the sections of the app that require internet connectivity to function. Logging of sightings and sessions are unaffected by this so you can continue recording your sightings and sessions without internet coverage.
Android
The Android app unfortunately cannot reliably determine if your phone is offline, so sections of the app that require the internet will show a 'Network request failed' error if you try to access them without internet. Logging of sightings and sessions are unaffected by this so you can continue recording your sightings and sessions without internet coverage.