PrepTest 20, Section 4, Question 21

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 bird 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.

The zoologist Rohwer asserted 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.

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."

The best evidence for status signaling is from the greater titmouse. Experiments show a strong correlation between the width of the black breast-plumage 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.

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.

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 bird 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.

The zoologist Rohwer asserted 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.

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."

The best evidence for status signaling is from the greater titmouse. Experiments show a strong correlation between the width of the black breast-plumage 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.

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.