Design an investigation

Traffic noise and songbirds

Units 2, 7 and 9
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

A four-lane highway was built through a large forest. Ecologists studying a common songbird recorded the noise level and the lowest pitch (minimum frequency) of male birds' songs at different distances from the highway.

Table 1: Noise level and song pitch at four distances from the highway

Distance from highway (m)	Average noise level (dB)	Mean minimum song frequency (Hz)
50	68	2,350
200	58	2,180
400	49	2,050
800	42	2,000

Source: Hypothetical data

Planned 'phantom road' investigation

To find out whether noise alone, and not other effects of the highway, reduces the number of songbirds, the ecologists will choose a quiet forest 20 km from any road. They will mount speakers along a 500 m line through the forest and play recorded highway traffic noise at about 60 dB.

The speakers will be switched on for four days and then off for four days, repeating this cycle through the six-week breeding season. Each morning, the same trained observer will walk a fixed path beside the speakers and count all of the songbirds seen or heard.

Source: Hypothetical study design

Suggested time: 23 minutes

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Part (a)

1 point

Using Table 1, describe the relationship between the noise level and the minimum frequency of the birds' songs.

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Part (b)

1 point

Explain one way traffic noise can reduce the reproductive success of songbirds.

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Part (c)

1 point

Identify the independent variable in the phantom road investigation.

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Part (d)

1 point

Identify the dependent variable in the phantom road investigation.

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Part (e)

1 point

Identify the control in the phantom road investigation.

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Part (f)

1 point

Explain why the phantom road design is a better test of the effect of noise than comparing bird numbers near and far from the real highway.

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Part (g)

1 point

Identify one variable that the ecologists should keep the same each time they count the birds.

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Part (h)

1 point

Explain how the highway can harm forest animal populations by fragmenting their habitat.

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Part (i)

1 point

Propose one realistic way to reduce the amount of traffic noise reaching the forest.

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Part (j)

1 point

Suppose song pitch is partly a heritable trait. Explain how, over many generations, traffic noise could act as a selective pressure on the songbirds near the highway.

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