It must be "ARE"...
it's similar to "A number of", which is plural, and "the number of",which is singular.

Hi SRFN,

As you know, the noun and verb in any sentence must agree. That means if the noun is singular, then the verb must be singular.

The noun we are concerned with in this sentence is animals, not percentage (because it is the animals that [IS or ARE] going to migrate, not the percentage). In this sentence, animals is plural, so we need the plural form of the verb, are.

The tricky bit in this kind of question is to figure out which noun and verb to focus on. Once you have the right ones, it's easy!

I hope that helps.

I understand that, but what makes it confusing is, as mentioned, the word "group" in the same sentence would always be singular, but it follows that exact same logic. Yes, in both cases, it's "the animals" that are carrying out the verb, but in one you can say "A group [of nouns] is..." and the other is "A percentage [of nouns] are...". Both "percentage" and "group" are singular.

Also, in this sentence there are one main subject and one main verb, right? The verb is certainly "migrate". But - wouldn't the subject be "percentage"? I was under the impression that the main subject of a sentence was NEVER inside of a prepositional phrase (e.g. "of animals"). No?

Now we're getting into juicy language territory!

Group is a weird word that can correctly affect a singular or plural verb, depending on the interpretation of meaning and context in the sentence. I know, that sounds a little like weasel words. Bear with me for a minute...

Group can be used as a singular or plural noun. Here is an example of group used as a singular noun: A group of people is gathering around the scene of a horrible car accident. It's singular (is gathering) here because the people form a single cluster.

And here is an example of group used as a plural noun: The grade six scout group receive badges for participation. It's plural (receive) here because each member of the group (more than one person) gets a badge.

There are many words like group (class, flock, bag, etc.) that refer to a multitude of things.

In the original question, the "large percentage of animals" refers to a significant number of individual animals (many of them, i.e. plural), which are migrating. In this case, the many individuals is clearly plural.

English has quite a few of these wacky examples. They are wacky because they don't fit into easy rules. I kind of like them, even though they are wacky, because they make our language interesting (if extremely difficult to learn, even to native speakers/readers/writers).

So I get that "group" goes either way, and that we would say "a large percentage of animals ARE". Thank you very much for the explanation.

A follow-up question I have has to do with which noun becomes the subject of the verb "to be". If we say "a large percentage of the birds are migrating", I understand that we use "are" because its "the birds" who are doing the "migrating". But what about the following example:

"The group of people is/are happy." It seems to me that there's an argument for both here -- the "group" IS indeed "happy", but certainly so ARE the people in the group.

Which is correct?

Look at the focus of the sentence. Here are two close examples:

The group of people IS happy. The people in the group ARE happy.

In the first sentence, which perhaps is discussing the emotional state of people at an event, we are focussed on a singular thing - the group is a singular collection (of more than one individual). In the second sentence, our focus is on a plural - numerous individuals in a group.