Daily RC Article 63

Status Signalling among birds

Paragraph 1

Many birds that form flocks compete through aggressive interaction for priority of access to resources such as food and shelter. The result of repeated interactions between flock members is that each bird gains a particular social status related to its fighting ability, with priority of access to resources increasing with higher status. As the number and intensity of interactions between birds increase, however, so increase the costs to each birds in terms of energy expenditure, time, and risk of injury. Thus, birds possessing attributes that reduce the number of costly interactions in which they must be involved, without leading to a reduction in status, are at an advantage. An external signal, such as a plumage type, announcing fighting ability and thereby obviating the actual need to fight, could be one such attribute.

Paragraph 2

The zoologist Rohwer assented that plumage variations in “Harris sparrows” support the status signaling hypothesis (SSH). He reported that almost without exception birds with darker throats win conflicts with individuals having lighter plumage. He claimed that even among birds of the same age and sex the amount of dark plumage predicts relative dominance status.

Paragraph 3

However, Rohwer’s data do not support his assertions: in one of his studies darker birds won only 57 out of 75 conflicts; within another, focusing on conflicts between birds of the same age group or sex, darker birds won 63 and lost 62. There are indications that plumage probably does signal broad age-related differences in status among Harris sparrows: adults, usually dark throated, have higher status than juveniles, who are usually light throated; moreover, juveniles dyed to resemble adults are dominant over undyed juveniles. However, the Harris sparrows’ age-related plumage differences do not signal the status of individual birds within an age class, and thus cannot properly be included under the term “status signaling.”

Paragraph 4

The best evidence for status signaling is from the greater titmouse. Experiments show a strong correlation between the width of the black breastplumage stripe and status as measured by success in aggressive interactions. An analysis of factors likely to be associated with breast-stripe width (sex, age, wing length, body weight) has demonstrated social status to be the only variable that correlates with stripe width when the other variables are held constant.

Paragraph 5

An ingenious experiment provided further evidence for status signaling in the greater titmouse. One of three stuffed titmouse dummies was mounted on a feeding tray.When a live bird approached, the dummy was turned by radio control to face the bird and present its breast stripe in “display.” When presented with a dummy having a narrower breast stripe than their own, birds approached closely and behaved aggressively. However, when presented with a dummy having a broader breast stripe than their own, live birds acted submissive and did not approach

Topic and Scope:

The status signaling hypothesis; specifically, the extent to which this hypothesis is supported by observable evidence.

Purpose and Main Idea:

The author’s purpose is to describe the extent to which the status signaling hypothesis is supported by observable evidence. Since this passage is descriptive rather than argumentative, the author presents no specific main idea of her own.

Paragraphstructure:

paragraph 1 provides a lot of background information on the relationship between the fighting prowess of birds and their access to resources, like food and shelter. Essentially, this paragraph notes that there’s a positive relationship between the two—the better a bird is at fighting, the greater its access to resources. However, as the paragraph also notes, actual fighting is potentially costly to birds, so they’ve developed external indicators of fighting ability, such as plumage type, which allow them to gain access to the amount of resources their fighting prowess commands without having to engage in actual violence in order to acquire those resources. Up to this point, you can’t really make any firm predictions about where the passage is headed, except to note that it’s likely to continue to probe the relationship between indicators of fighting prowess and access to resources.

At the beginning of paragraph 2, we’re told that this relationship is known as the “status signalling hypothesis” (SSH). The remainder of paragraph 2 and all of paragraph 3 are devoted to a discussion of whether Rohwer’s experiments with Harris sparrows support the SSH. Rohwer, we’re told, believes that his experiments support the notion that a direct relationship exists between indicators of fighting ability and access to resources. The author, on the other hand, contends that Rohwer’s experiments don’t support the SSH.

In paragraphs 4 and 5, the author contends that experiments with the greater titmouse provide much greater support for the SSH. In this case, unlike that of the Harris sparrow, she asserts, there is a strong correlation between plumage type and access to resources.

The Big Picture:

Like most CAT science passages, this one contains a lot of details. On Test Day, when you tackle the science passage, don’t focus on the details. Instead, try to grasp the basic process or relationship—in this case, the SSH—that all science passages are built around. Most of the questions will focus on this process or relationship.
It’s especially important to make a mental roadmap of a science passage. Since the passage will most likely involve rather complex ideas and details, you’ll need to refer back to it regularly; and you don’t want to waste a lot of time by repeatedly scanning the entire passage for specific ideas or details.

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