Colour morph variation is widely observed in owls and several studies have described differences In life history traits among the morphs. Traditionally, Glaucidium owls are described as having two colour morphs, rufous and grey, which are supposedly independent of age or sex. During 2010–2012, nest boxes were set up in order to study the basic life history of Collared Pygmy Owls Glaucidium brodiei. We banded the population and followed some of the fledglings for nearly two years. Our results — in combination with data from museum specimens — clearly show that the colour morph In Collared Pygmy Owls is age-dependent, changing from the original fledgling plumage (2–3 months after hatching) to the rufous morph (4–7 months after hatching), and finally to the definitive grey plumage. Our results reject the long-standing polymorphism hypothesis for the pygmy owl. We conclude that the age of the birds should be considered before a colour polymorphism can be confirmed, especially in secretive nocturnal species. The case of within-species polymorphism might be overstated in Strigiformes.