Bird Song as an Honest Signal of Sickness

Abstract

When animals have differing or diverging evolutionary interests—as they often do during mate choice—theory predicts they should use honest signals to communicate. Many signals are kept honest through costs that low-quality individuals are less able to pay, and have been shown to be reliable indicators of signaler quality. Yet costs can be incurred at different times during an animal’s lifetime, and it is not clear that a signal that is primarily costly during development will be a reliable indicator of fluctuations in condition or health that occur later in life. In this thesis, I ask whether bird song—an established signal of developmental stress—is also an honest signal of a more recent stressor, an acute immune challenge that simulates a pathogen infection while males are attempting to breed. In Chapter 1, I review the history and current state of honest signaling theory. In Chapter 2, I examine the effects of an immune challenge—induced by exposure to lipopolysaccharide (LPS)—on male song in the swamp sparrow (Melospiza georgiana), asking if and how a recent challenge affects an otherwise reliable indicator of male quality. In Chapter 3, I ask whether female swamp sparrows respond more to song from males undergoing an immune challenge compared to song from the same males recorded during a saline control, in order to determine whether females detect and respond to LPS-induced differences in song that our instrumental analysis in Chapter 2 did not measure. And in Chapter 4, I examine a similar set of questions in the zebra finch (Taeniopygia guttata). Specifically, I test whether an LPS-induced immune challenge affects male zebra finch song in ways that affect female response to that song. In all three chapters, I find evidence that song in these two species of songbird is not being used as an honest signal of male immune status. Though LPS reduced song output in both swamp sparrows and zebra finches, neither swamp sparrow nor zebra finch females avoided song from LPS-treated males or preferred song from saline-treated males. Furthermore, analyses of swamp sparrow song revealed what appeared to be a trade-off between the strength of a male’s immune response and the effect of LPS on song: males who invested more energy into an immune response, as measured by their body mass change, produced lower quality song after LPS injection than after injection with a saline control. In contrast, males who invested less energy into an immune response produced higher quality song after LPS injection than after injection with a saline control. Overall, the findings in this thesis indicate that a signal like bird song, which is an honest indicator of developmental stress, may not necessarily be an honest indicator of current condition.