After a couple of nights drawing blanks it takes more than faith to give up a warm comfortable evening to a session out in the cold, lasering your retina for the illusive hope of bird migration, rationed one shadow at a time, at intervals that span hours.
Before you tuck in under that quilt again, have a look at these numbers:
1. Consider an imaginary massive mist net, spanning 2365 km across Europe and assume an evenly distributed migration across this imaginary line.
2. It is estimated that there are about 2.1 billion songbirds and near-passerines migrating between Europe and Africa. I have not had the luck of counting them myself, but these guys and these guys did.
3. Assume that birds migrate between 100 and 200 meters off the ground. If I got my basic trigonometry right, at that distance, the diameter of a full moon in a telescope should cover between 8 to 16 meters of this imaginary mist net (note that probably 300-500m is a more suitable range, and somoene even quoted 1500m. One still needs to validate the actual visibility of small passerines at such distances. In any case, the higher up, the more we can catch within our blinkers)
4. If 2,100,000,000 birds cross our imaginary line of 2,365,000 meters then that gives you 887 birds per meter or 7000-14000 across the diameter of the moon.
5. Now consider a migration over a period of 80 days and the fact that you will miss 50% of the sightings because most of them will pass a different focus plane to where you are or they are too small to see (you will see some in focus, some out of focus and many you will miss).
Mother nature should yield an average of 40 – 80 bird sightings per night. Assuming a migration window of five hours that equates to 8 – 16 birds per hour. And if you live anywhere near the coast and see wildfowl then you can effectively double that number (these migrate during day as well).
Assumptions aside, in principle, that is not too bad. Particularly for those that have acquired the zen to endure the barren spells typical of sea watching.
Just try not to hold on to your breath for too long.