Felix, taking inspiration from his father's failed 'war engine' seems to be describing a Mangonel.
This is almost a direct quote from John Keats' famous poem, 'La Belle Dame Sans Merci':
The sedge has withered from the lake,
And no birds sing.
The novel takes on a distinctly environmental tone in the following pages, as though Jefferies is describing a modern environmental disaster, such as those Rachel Carson warned of in her seminal 1962 eco-classic, Silent Spring.
On the preceding page Felix, as he sails along in his canoe, watches flocks of birds pass overhead, flying west away from his destination. It is the nesting season and clearly the action is unnatural.
It is one of several ominous signs and portents that he witnesses as he approaches the toxic wasteland of London. Nature is no longer following its ancient patterns. This could be seen as a prophecy of Climate Change, when awry seasons cause havoc with wildlife migration.
The imagery of this sequence is an echo of the Arthurian Wasteland, caused by a disruption to the natural order of things. TS Eliot consciously plays with this trope in The Wasteland:
A heap of broken images, where the sun beats,
And the dead tree gives no shelter, the cricket no relief,
And the drystone no sound of water.
Later on, when Felix reaches the ruined city of London, in a terrifying vision of hell on Earth, Jefferies certainly does what Eliot promises to do:
I will show you fear in a handful of dust.