PrepTest 148, Section 4, Question 4
Ecologist: Before finding a mate, male starlings decorate(60 more words not shown)
Which one of the following,(10 more words not shown)
- AAdult starlings are(7 more words not shown)
- BMale starlings do(13 more words not shown)
- CNestlings grow faster(12 more words not shown)
- DMale starlings tend(21 more words not shown)
- EThe compounds in(16 more words not shown)
Reading the argument
Why would the plants stop protecting the nest just because the male stopped adding more? The existing fragments might keep killing insects after egg laying begins. If so, the male could have finished supplying protection rather than abandoned that purpose.
The timing still gives us something to think about. The males decorate before finding mates and stop adding greenery once egg laying starts. That fits an attraction story. But the plants also contain insect-killing compounds, and the threatened young might benefit from plants already in place. Both explanations deserve a look.
What we’re looking for
To strengthen the attraction explanation, I'd look in either of two directions. Show that females respond favorably to these decorations, or show that the insect-killing effect doesn't last once the male stops adding plants. The first would support attraction directly; the second would weaken the lingering-protection explanation we just considered.
Working the answers
- A
Adult birds' ability to defend themselves doesn't tell us whether the decorations protect the nestlings. The competing explanation concerns the young, not just the male who brings the plants. A also gives us no connection between the decorations and attracting a female.
- B
Read the location carefully: males don't decorate in areas with unusually few parasites. That makes decoration track the need for parasite protection. If there is little insect threat, they don't bother with the greenery. This fits the rival protection explanation, rather than strengthening the claim that attraction is the function.
- C
Faster-growing nestlings suggest that the plants can benefit the young. That gives us another reason the birds might incorporate them for their offspring's sake. It doesn't establish that the plants attract females, and it supplies a benefit compatible with the rival explanation.
- DCorrect
When a female is placed nearby, males increase their decorating. That links the behavior to the presence of a potential mate. It isn't exactly the evidence I predicted about the female's response; it concerns how the male responds to her presence. But that response supports the idea that the display is connected to attracting females.
This doesn't prove the male's purpose or eliminate every protective benefit of the plants. It gives the ecologist additional evidence for the attraction explanation, which is all a strengthener needs to do.
- E
Being harmless to nestlings doesn't explain what the decorations are for. The compounds could be harmless to the young while killing parasites, or while being used in a display that attracts females. E leaves the competing explanations in place without favoring attraction.
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