See the link provided by Ripcurlgirl above. The article points out a major source of confusion over use of the term "penguin". It turns out that the "pingouins" of the Arctic region and the "manchots" or Antarctica are not related. They look somewhat similar at a glance. As you point out, the arctic birds may be commonly referred to auks and are capable of flight. The Antarctic penguins are flightless. I suspect that most English speakers, seeing the word "penguin" will tend to think of the Antarctic bird. It is also not at all helpful that the word for the Arctic bird looks like "penguin" so we are undoubtedly going to see a lot of confusion over this word. Duo accepts both "pingouins" and "manchots" here.
Le pingouin cherche un poisson → The penguin is looking for a fish.
Les pingouins cherchent des poissons → The penguins are looking for fish.
des is the French plural of the indefinite articles un / une.
In French, articles are mandatory → every noun requires an article, however this is not always so in English so there is no need to translate des as some. It is superfluous in the English sentence.
This doesn't mean that your translation is intrinsically wrong, just unnecessary.
If you feel it should be added to the list of accepted translations then report it however I feel you should get used to not translating des as some.
Bon courage !