Thanks John. Were you really a subscriber to the Spectator in 1909? An interesting letter, though, and I especially liked: In our pedestrian way, we can but stump along from one determinate note to another, while the birds in their song, as in their flight, soar freely in "a diviner air." The last phrase turns out to be Wordsworth - “More pellucid streams, An ampler ether, a diviner air, And fields invested with purpureal gleams.” - which doesn't seem to describe modern-day Lincolnshire.
But yes, I take the point that cuckoos can vary the song's pitch and interval, and have now found another discussion of this subject at
http://entartetemusik.blogspot.co.uk/20 ... uckoo.htmlHowever, I still haven't come across any mention of an interval as small as I heard. The bird involved may be at Alkborough now: we saw it flying west across the river at 8 p.m. yesterday.