Well, it took about 10 times of listening to this to realize, "yea, they really are saying 'The ducks are birds'" ....
I still don't get the difference between "de" and "het"... is it like "der" and "die" in German, somehow?
Yes, it is related to the grammatical gender of the noun. For the most part, you just have to memorize which are het words and which are de words.
If you want to compare to German then "het" is equivalent to "das" and "de" is equivalent to all other forms.
I went with that because "The ducks are birds" doesn't really make sense as a sentence in English.
I guess we lose "The" from sentences when we're talking about an unspecific amount of animals. If I owned five ducks I could say "The ducks are eating their lunch" but if I was talking about ducks across the world I'd say "Ducks eat bread". However if my ducks ate something unusual I might say something like "These ducks eat bananas."
The only - ridiculous - situation where you might say "The ducks ARE birds" would be when the suggestion had been made that the ducks in question were plastic decoys. Even then it would be more natural to say, "No they are real, live birds"
Usually these discussions only talk about the English. I would like to know if in Dutch also is it more natural to say "Eenden zijn vogels." without the article? Italian would keep the article even for things (the things) in general but I suspect Dutch follows the English pattern.