PrepTest 150, Section 3, Question 2
The Common Loon is a migratory bird that(83 more words not shown)
Which one of the following,(11 more words not shown)
- AMost of the(13 more words not shown)
- BContests for occupied(13 more words not shown)
- CLoons that intrude(15 more words not shown)
- DLoons frequently determine(17 more words not shown)
- ELakes that are(21 more words not shown)
Reading the argument
Why fight for a lake when there's an empty one nearby?
At first, competition doesn't seem surprising. If good breeding lakes are scarce, the returning loons may have to displace another pair to find a place to breed. But the last sentence takes that explanation away: there are vacant, perfectly suitable lakes nearby. The birds still choose an occupied lake almost half the time.
Maybe they have a strong preference for a particular place. Perhaps they're returning to the lake where they were born rather than simply choosing any suitable lake. That's a possible explanation, not something we've been told.
What we’re looking for
The question asks us to explain their behavior. We need a reason to prefer fighting for the occupied lake even though an available alternative would work. We shouldn't explain away the fact that the vacant lakes are suitable; that's part of the puzzle.
Working the answers
- A
This makes the alternatives look even more reasonable to us: they have successfully supported breeding before. It doesn't explain why the returning birds pass them up. The puzzle remains why an occupied lake is attractive enough to fight over when these good empty lakes are available.
- B
Either sex can start the contest, but why start one? Identifying who might initiate the fight doesn't supply a reason to choose an occupied territory over a vacant suitable one. The same puzzle would remain if only males or only females initiated contests.
- C
Winning half the contests tells us the takeover sometimes works. It doesn't explain why the birds choose a contest with a substantial chance of failure when they could occupy a suitable empty lake. A possible payoff from fighting isn't yet a reason to prefer fighting over the stated alternative.
- DCorrect
The occupied lake tells the returning loons something they want to know: another breeding pair is already using it, so it looks like good breeding territory. That gives them a reason to select it and try to remove the residents.
This is different from my guess about returning to a familiar lake. The issue is how the birds recognize suitability. We know the vacant lakes are suitable, but that doesn't mean the loons have recognized them as suitable. An existing pair gives the occupied lake a visible recommendation that the empty lake lacks.
- E
This explains what makes a lake suitable: food, nesting space, and shelter for chicks. But we already know the vacant lakes meet the suitability requirement. E doesn't explain why the birds avoid those lakes or how they choose between a vacant lake and an occupied one.
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