In this chapter, I examine song learning in the oscine passerines (songbirds) from several angles, with special attention to our study species, the song sparrow. I focus on social factors and suggest that previous research on song-learning points to three different hypotheses about their role. According to the simple eavesdropping hypothesis, the young bird need only overhear an adult bird singing to learn song (this situation is mimicked by the classic "tape tutor" design). According to the direct interaction hypothesis, the young bird needs to interact with the song tutor to learn songs (this situation is mimicked by the early "live tutor" designs). And according to the social eavesdropping hypothesis, the young bird learns best by eavesdropping on singing interactions between adult tutors. Thus in this last case, social interaction is critical, but the young bird need not directly participate in it. These hypotheses are not mutually exclusive, but each makes some distinctive predictions, and we have argued that the hypotheses can be rigorously tested using our "virtual tutor" design, in which a computer simulates interacting song tutors and/or interacts directly (sings) with the young bird. We are presently carrying out these studies. I describe our field and laboratory studies of song sparrows. I give particular attention to how the birds use their songs in the normal territorial context because these interactions may be crucial to song learning, especially if the social eavesdropping hypothesis proves to be true. Song sparrows in our population typically share songs with their neighbors, and song sharing is a good predictor of a bird's lifetime territory tenure. I summarize our field studies of song learning in terms of "rules" of song learning (e.g., learn the songs of several, neighboring males, preferentially retain songs these tutor-neighbors share). Our roof-top "semi-natural" studies confirm many of these findings, and make several additional points. First, learning proceeds throughout the first year, right up to the bird's first breeding season, and memorization of new songs can occur at least into the bird's first fall, a much longer learning period than was previously thought. Second, counter-singing between song tutors seems especially important. Third, a song tutor does not have to be seen to be effective. Our subsequent, more analytic lab studies suggest that eavesdropping on singing interactions may indeed be critical in song learning, and they have stimulated us to turn to the "virtual tutor" method to analyze social interaction factors more rigorously. I discuss the two most popular classes of hypotheses of song function-that song repertoire size or song sharing is the target of selection-and consider their relation to song-learning programs. Finally, I return to the question of which aspects of the song sparrow song-learning program-in particular those that seem to lead to song sharing among neighbors-can be considered evolved adaptations. The question is whether song sparrows learn shared songs because their song-learning program, in some way, leads them in that direction (the "sharing hypothesis") or as an incidental consequence of their movements. Doubts about the sharing hypothesis arise from a notable population difference in song: neighbors in western song sparrows share songs, whereas neighbors in eastern song sparrows typically do not. This population difference raises both doubts about the adaptation hypothesis and an opportunity to test it. I suggest that a "common garden" experiment, in which young birds from both populations are raised under a common tutoring regime, would directly address this question. Finally, I suggest that our studies on the proximate mechanisms of song learning may also help us address this question, because when we have adequately characterized the mechanisms of song learning, we may find that they are most consistent with one particular set of hypothesized song-learning adaptations. © 2008 Elsevier Inc. All rights reserved.
