[Spindleshanks]

Hi Martin.

This is seasonable and sure to appeal to the believers (count me in).

I see some logic problems in the octave's order of progress. As it stands, "this chain" follows the eagle's nest and would appear to include it among the human indiscretions. Can I suggest reversing the def. article and the pronoun "this" as so:

. . . . An eagle’s nest,
a mansion on a lofty bough, is blest
with two young growing chicks. In time the chain
of indiscretions by the human brain
will put half of this biosphere to rest.

I have trouble identifying the fledglings with humans. Where is the connection? What am I missing?
You have a tautology with "young chicks." "Growing" might pass as rhetorical, but I doubt you will get away with "young."
I wonder, too, about "mansion," which seems a tad overdone.

Best with this,

Peter