Translation bug: "Ducks are birds"
My answer: "Patos son pajaros," whil it should have started with "Los patos.." Is this a bug or is there a specific reason that requires "Ducks" to be translated as "Los patos"?
Spanish, like French, has what you might call a 'generic' use of the article. When you're talking about, say, ducks, as a rule, you use the plural article, 'los'.
Spanish is my mother tongue and is not only grammatically incorrect to omit the corresponding article when writing a word but it is also vague and misleading. It is an small difference between Spanish and English. Anyway, I guess you can criticize my English just as I have criticized your Spanish.
KokoBeto says it is incorrect to omit the article, and MattyRay84 says you do not need the article. In a way, both are correct - you would usually be understood if you left it out, but if you want to speak and write Spanish that sounds natural and could benefit you most in future, you need the article.
The grammar books are pretty clear. Translate ducks (ducks in general - i.e. when you mean all or almost all ducks) as "los patos". Translate "the ducks" (i.e. the particular set of ducks you are talking about) as "los patos".
Translate "los patos" as "ducks" or "the ducks" - context or sense usually helps to tell you which is intended.
In the case of "los patos son pàjaros", this will almost always be "ducks" - it is hard to think of natural situations where you would want to tell someone that "the (these specific) ducks are birds". Real-world examples are seldom this clear.
Note that, in a classifying sentence like this, this applies only to the subject - so, in the example, pàjaros does not have an article. You could think of this signifying 'an example of birds', not 'all birds', but it is probably easier not to think about it, and just memorise the sample translation pattern:
ducks are birds = los patos son pàjaros
As I understand it, it's proper grammar to incluse the article. If you were not starting a sentence with the article you would certainly use it within a sentence or phrase in the same capacity.
Naw, Matty and learn, while the "meaning" might still be there, it would sound brutish to leave out the article in Spanish, or, as angus points out, in French.
By leaving out the "los" you imply that all birds are ducks. Remember, the verb "is" indicates equivalence. To make clear that all ducks are birds but not all birds are ducks, you must put the article before the noun that is a subset of the other noun. If I were to translate literally "Los patos son pajaros" to English, I would add the adjective "some" before the word "pajaro." Specifically, "Ducks are some birds." However, because the literal translation sounds unnatural to a person whose mother tongue is English, I would reverse the order so that the sentence became "Some birds are ducks."