The randomness of the computer program! Here is a dictionary in case it happens again.: http://dictionnaire.reverso.net/francais-anglais/oiseau
Even with the wrong sound, if the noun starts with a vowel "le" becomes l' and "la" becomes l'
So, you will know next time. http://french.about.com/od/grammar/a/articles_2.htm
Here is a site to listen to: http://www.forvo.com/search-fr/l%27oiseau/
Conjugations: When a word such as "le," "la," or "je" directly meets another word that starts with a vowel, that word gets conjugated to l' or j'. The reason being, because in French, vowels usually don't bounce on each other, it helps to be able to speak more fluidly. So instead of saying, Je ecris, or le homme, sounding like luh omme, it sounds better and easier to say, l'homme. L'homme, because the "h" is silent. Think of conjugations in English. If English conjugates word, it is perfectly acceptable to say that French can. Conjugation in English is like conjugation in French, but with different rules. Here is a list of some conjugated words that you may know already and will learn later:
C for correct. X Improper.
J'ecris-C Je ecris-X (I write/am writing). L'eau-C La eau-X (The water). J'ai-C Je ai-X (I have). L'éléphant-C Le éléphant-X (The elephant). L'oiseau-C Le oiseau-X (The bird).
The translation for this means, "The bird is red."
The French context looks like, "L'oiseau est rouge."
The la is conjugated to l' because of the next word starts with a vowel, and so does la.
This happens to keep consecutive vowels sounds from happening, and keep a clear distinctive sound when speaking.
L is a contraction of La and/or Le. L as a contraction will always use an apostrophe ( ' ). L' will be used if the word following the definite article Le/La starts with a vowel.
For example: La chapeau. Meaning:
Use the full word "Le" in this sentence because chapeau (the word following Le) does not start with a vowel.
In the sentence above, L' oiseau est rouge, the Le is conjugated because
Oiseau (bird) starts with a vowel.
Le chapeau. (No conjugation.)
The reason for this, which is not mandatory, is to prevent consecutive vowel sounds from bouncing off each other, thus keeping good flowing sentences.
As I said, in verbal context, this is not mandatory, and if you forget/do not follow this 'rule,' you will be understood. For Duolingo, when writing, I am not sure if you must follow this, but it is not that hard, and regardless, is very good practice.