oology

thumb|alt=Old illustration, showing 23 birds' eggs of different size and colouring|Mid-19th century illustration showing the eggs of a number of bird species

Oology (; also oölogy) is a branch of ornithology studying bird eggs, nests and breeding behaviour. The word is derived from the Greek oion, meaning egg. Oology can also refer to the hobby of collecting wild birds' eggs, sometimes called egg collecting, birdnesting or egging, which is now illegal in many jurisdictions.