Ashinano, Hitoshi. 1995. Yokohama Kaidashi Kikou. Chapter 3. Vol. 1, Page 66, Panel 4.
'De-de-po-po' is the call of a kind of dove, kijibato (Oriental Turtle-dove, Streptopelia orientalis). It is a common bird in Japan. It calls repeatedly on mild calm days.
The kijibato may look similar to the rock pigeon which can be seen in cities all over the world, but the call is completely different.
Play the video and you can listen to the call of a kijibato.
[play video] Windows Media
[play video] QuickTime
Some people seem to like the call but I would say it's boring.
De-de-po-po would be Ashinano's own way to express a mild day (Ch. 3), or a boring day (Ch. 44).
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YKK - Behind the Panels > De-de-po-po
2005-04-21 KIMOTO Go