whose production

As a monk called the lector had done in the monasteries of the Middle Ages, Daffy often read aloud to us as we ate our meals. We had recently been treated to Henry Savage Landor’s description, in Across Coveted Lands, of the Towers of Silence, in Persia, on top of which the Parsees placed corpses in a sitting position, with a stick under the chin to keep them upright. When the crows arrived to squabble over the body, it was considered a ticket to Heaven if the right eyeball was the first one consumed. The left was not quite so auspicious.
I could not help thinking of this now, and of the author’s account of the curious pigeon towers of Persia, each with a deep central pit for the collection of guano, whose production was the sole reason for keeping the birds.

What’s the meaning of ‘whose production’?