I was first introduced to the idea of an owl "detective" through the ASRI. I used the WhatBird.com site to collect pictures and facts about the following owls:
I then printed them according to size: A "large" owl was printed using a full page, a medium half a page, and a small a quarter page to help children identify owls by size. With a large group, I hang owl pictures up on a wall or chalkboard. With a small group, I spread them on a table.
I also wrote out "clue" cards with the following facts:
1. Size of owl
2. Color(s) and marking(s)
3. Ear tufts (or lack of)
4. Beak color
5. Eye color
6. Other distinguishing qualities
Keep an answer key with owl's image until you memorize them. With younger children (I've done this lesson as young as kindergarten), I read the clues and through a process of elimination, we identified the owl. This visual process helps them analyze details to improve their science drawing and even their writing. Older students can be given the clue cards, may work with a partner, and must figure out which bird they're looking for through reading comprehension and visual analysis. For this, it is better to keep images spread on a table children can circle.
Another fun activity when you introduce owls is to identify them through sound.
A short video here.