oology
The study of birds' eggs.
uncountable
Yet it is not difficult to see that Oölogy promises to become an important auxiliary both in aiding to determine natural divisions, and to enable us to decide in regard to varieties the specific identity of which is in doubt. Let us take an instance. [...] Without looking at all to the external structure of these birds, Oölogy would seem to indicate that the Cat-Bird, though closely allied to the true Mocking-Bird, is at least one step, and that the Brown Thrush is even farther, removed.
The hobby or practice of collecting birds' eggs, especially those of wild birds.
uncountable
Besides his life as a student, Han [Henri Jacob Victor Sody] continued to be interested in ornithology, particularly oology. Apparently, he was a diligent collector of birds' eggs, both in the vicinity of Wageningen, and on excursion for this purpose to other places (Breda, Texel, etc.), and compiled a catalogue of eggs collected by him in The Netherlands.
Where it comes from
From oo- (“relating to eggs or ova”, prefix) + -logy (“branch of learning or study of a subject”, suffix). The word is cognate with French oologie, Late Latin ōologia.
Adapted from the Wiktionary entry oology (CC BY-SA 4.0).