The Dutch word 'duif' in English is both 'dove' and 'pigeon'. Still it is hard for me to distinguish the difference between these two English translations, as in Dutch we name them both simply 'duif'.
Of course mail lovers will love the Postduif (Mail Pigeon).
Dutch PostNL happened to have issued two shaped stamps in honour of the 'youth philately's day' in 2014. One of these shows the Postduif:
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The fact that postduiven ('duiven' is plural of 'duif') have to do with mail, is also shown on the following Europe stamp from Kazachstan. In 2008 the Europe stamps' theme was 'letter writing', and here he/she is: the postduif!
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Many more duiven have been issued on stamps.
This (post)duif is from Indonesia:
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And an other kind of duif from Malaysia:
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Duiven also appear on cancellation stamps. Here is one on an official postmark from Taiwan:
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And an unofficial postmark, but real rubberstamp print, from the USA, showing this most well-known bird to deliver mail:
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See more stamps on the letter 'D' at and via the 27 August Sunday Stamps.