The relationship between Nigel and Gabi can be compared to that of Donkey and Dragon in the Shrek movies. They have many 'dronkey' children even though donkeys and dragons are very different! This will not happen to Nigel and Gabi though, as the Shrek movies are fantasy ones set in a imaginary place while the Rio series are adventure movies set in a real place. 

I've included a picture of Dragon and Donkey, sorry that I couldn't find one with their 'dronkeys' in it too! I think that a 'frogatoo' would be even weirder than a 'dronkey'!