Every fall Croton's jays migrate south. The jays always join flocks of migrating crookbeaks with which they share the same summer and winter territories. If a jay becomes separated from the crookbeaks it is accompanying, it wanders until it comes across another flock of crookbeaks. Clearly, therefore, Croton's jays lack the navigational ability to find their way south on their own.
[editor: text corrected on 7/feb/2011; some words were missing from the passage]

Strengthen the argument:

A. Croton's jays lay their eggs in the nests of crookbeaks which breed upon completing their southern migration.
B. The three species most closely related to crookbeaks do not migrate at all.
C. In the spring, Croton's jays migrate north in the company of Tattersall warblers.
D. Species other than Croton's jays occasionally accompany flocks of migrating crookbeaks.
E. In the spring, crookbeaks migrate north before croton's jays do.

OA is C. I chose A.

How does the CJ migrating north with a warbler strengthen the argument that the CJ lacks the ability to fly south on their own?

Any explanation for this question? I think the same than Arun

Premise 1: Every fall Croton's jays migrate south.

Premise 2: The jays always join flocks of migrating crookbeaks with which they share the same summer and winter territories.

Premise 3: If a jay from the crookbeaks it is accompanying, it wanders until it comes across another flock of crookbeaks.

This implies that a jay needs a flock to migrate south.

Conclusion: Croton's jays lack the navigational ability to find their way south on their own.

Consider the following problem (perhaps not the best analogy)

David goes to the gym every Wednesday. Every time David goes to the gym, Richard goes with him. If Richard stops to buy something, David waits for him. Clearly, David lacks the navigational ability to find the way to his gym.

what would strengthen this argument?

Every time that David needs to go to the University, he calls Richard to find the way. If the author said that David can go on his own to the university this would hamper the conclusion that David lacks navigational ability.

Regarding the problem

(A) Croton's jays lay their eggs in the nests of crookbeaks which breed upon completing their southern migration.

does this account for the lack of navigability of the croton's jay?

Let me know if this helps. I know my explanation might be not really clear

good analogy ...tx

The conclusion is that jay( J) lacks navigation ability.

It is based on the premise that the jays migrate with crookbeaksI( C). and that if some J gets separated from C, it waits to meet another group of C.

Now any example telling that J can go to some place on their own, will weaken the argument. Because it will clearly show that J have some navigation ability.

And any example stating that J need help in going to other place as well will actually strengthen the argument. and thats what C does

excellent explanation.

here's the short version:

takeaway:
in STRENGTHEN/WEAKEN, you should ALWAYS select answers that have as DIRECT an impact on the argument as possible.

we need a choice that DIRECTLY evinces the jay's lack of navigational ability.

* where the jays lay their eggs has nothing whatsoever to do with navigational ability. (this may just mean that they're clever enough to take advantage of others' nests, like cuckoos, rather than going to the trouble of building their own.)

* if the jays ALSO need help finding their way back north, this is much stronger evidence that they can't navigate by themselves.

Ron,

Pardon me to come back on this post but I have a question related to the basic meaning of the sentence .

I was hesitating between C and E because I didnt understand if " the navigational ability to find their way south on their own" :
- should be read as one full sentence meaning that the bird lack a specific ability to find the SOUTH (i dont know maybe reverse magnetic field gives it headaches, or its eyes could be able to see only in the north direction who knows) thus the answer would have been E because the bird can go north alone so his problem would only be to find the south

- or if the sentence should have been read as the bird lacking a global navigational ability and can not find his way , the south in this example but also the north as written in answer C supporting here it navigational skill problem, south being just a colloateral and useless word in the original stem.

Knowing the OA and with your explanation there is no doubt about the meaning of this sentence now , but my problem is how to differentiate for similar questions how to dismiss the useless word. For example how would you have ask the question for my first case with the purpose of asking about the capacity of reaching one precise goal and not about the skill itself ?

Thanks a lot,

John

I agree with John; reference of South in the conclusion threw me off from C.

Ron, your advice on this will be very helpful.

Thanks,

Vineet

Hey guys!
It really doesn't matter in this case whether birds are traveling south or north. Look at the stimulus, the conclusion is - jays can't find a place to spent winter/summer or whatever on their own. Jays need other means of orientation to get to the right place.
Therefore, we need to strengthen the next statement: jays' "navigational ability" is weak.

ok, i see what you guys are saying. so, it's time to apply the reasonableness test - which of these two is more reasonable?

let's simplify this:

which of the following is more likely to strengthen the idea that the jays cannot fly south on their own?

(c) they can't fly north on their own, either.

(e) they CAN fly north on their own.

in this simplified version, it should be obvious that (c) is better than (e).

yes.

I totally understand Arun's confusion. Whoever chooses C makes this assumption: If a bird cannot fly north on its own, it cannot fly south on its own. In other words, assuming that the birds' flying north or south does not make a difference is a bit too far fetched to me.

The only explanation I have for answer C is that: on rare occasions, you encounter a question/answer in GMAT that is not very well thought. You just have to choose the best answer. And in this case, it's C.

your view is a little bit too extreme here.
remember that your job is to STRENGTHEN the argument -- not to prove it.

if you had to find a choice that provided a 100% perfect deductive proof of the conclusion, then, yes, you would have to assume that the direction of flight is completely immaterial.

but that's not how these questions work. (i think your mind is stuck in the mode of "draw the conclusion" questions, which are essentially the only problems in the cr section that are 100% perfectly deductive.)
instead, you are just trying to find the BEST EVIDENCE for the conclusion at hand. as a somewhat close analogy, you can think of it as similar to the task of a trial court: you are never going to be able to prove, deductively/mathematically, that the suspect committed the crime, but you can certainly accumulate a pile of evidence in favor of that conclusion.
given that this is your task, choice (c) is much, much better than the other choices.

I had the answer right, but I struggled on interpreting the meaning of " If a jay from the crookbeaks it is accompanying, it wanders until it comes across another flock of crookbeaks."

Is this a proper English sentence? Seems to me that something is off (My first language is not English).

good find.
i've corrected the text; see the first post.
thanks.