1. In the given sentence, the phrase Each of the birds requires a singular verb. Although the birds in question are plural, EACH means that the sentence is focusing on one bird and another individually: exactly the same would apply if you were mentioning each of the problems or each of the lions or each soldier in the army. WERE should therefore be WAS.

2. Continuing with the same line of attack, you would be able to eliminate option B because EVERY ONE is singular twice over (the phrase of these birds does not alter the singular nature of the subject), and WERE is plural. Here again, you need WAS.

3. Option C has no problem from this point of view (though the ONE after EACH is not necessary), so it must either be right or there must be some other mistake - and clearly, there is. The peculiar beginning of the sentence - Crested and solitary - gives the reader two adjectives in isolation and then proceeds to a list of species of bird, so we are to suppose that those adjectives describe those species. Fair enough perhaps, but then a problem arises: each one of these birds was very different from the birds in a certain study. Different in which way? The sentence has already spent its ammunition by placing the relevant adjectives at the very beginning rather than keeping them to make the alleged difference clear, so the contrast just doesn't fire. (Furthermore, it's questionable whether it's sound to contrast singular birds - EACH ONE - with a group: the birds in Marshall's study. The solution in the correct answer, on the other hand, contrasts plural with plural, and is definitely not questionable.)

4. The latter objection could also be made to every one of them was crested and solitary in option D. But what you want is something plainly incorrect in an option, and that objection is perhaps debatable. And certainly there is nothing wrong from the point of view of subject-verb agreement in that clause. There is, however, something else: The study's birds. This use of the Saxon Genitive is plainly unidiomatic and illogical: in which way could it be said that a study possesses birds? This error is then compounded by the repetition of the subject in the phrase every one of them. English does not repeat subjects in this way, and the sentence has passed from plural to singular birds to boot: The study's birds ... every one of them was... So option D can be eliminated, leaving only E.

5. The birds (plural) examined in the study (idiomatic) ... were all (plural) crested and solitary (adjectives correctly positioned for the following contrast), very different from the birds (plural) ... It all hangs together.

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