Δε is used when the following word begins with a vowel or a plosive consonant (stop consonant). Δεν is used when the following word begins with a vowel. In English a similar thing happens with the words 'a' and 'an'.
Here is a link to information on plosive consonants: https://en.m.wikipedia.org/wiki/Stop_consonant
Could this also mean "pelicans do not speak"? I'm asking this because I was taught that in French when talking about an animal in general you use the singular. For example, 'elephants are herbivores' would be 'l'éléphant est un herbivore', instead of 'l'éléphantes sont des herbivores'.