Having been a bit of a birding anorak for the last 30 years, I have to confess to having a mental list of 'Birds seen from the Cockpit'
Most birds fly below about 300' so the chances of a birdstrike are highest on landing or takeoff. Birds travelling between feeding and roosting areas, eg. Gulls moving from reservoir to coast tend to fly higher, up around 500-800' and are often encountered i nsmall groups.
The highest I've ever encountered birds is a flock of Swifts at 1200', but they are the true masters of the airand spent all day on the wing, so thats only to be expected.
Migrating birds travel high and at night, normally during April-May and August-September.