J. Cully Nordby

Territory tenure in song sparrows is related to song sharing with neighbours, but not to repertoire size

Song repertoires may be a product of sexual selection and several studies have reported correlations of repertoire size and reproductive success in male songbirds. This hypothesis and the reported correlations, however, are not sufficient to explain the observation that most species have small song repertoire sizes (usually fewer than 10, often fewer than five song types). We examined a second important aspect of a males song repertoire, the extent to which he shares songs with his neighbours. Song sharing has not been measured in previous studies and it may be partially confounded with repertoire size. We hypothesized that in song sparrows, Melospiza melodia, song sharing rather than repertoire size per se is crucial for male territorial success. Our longitudinal study of 45 song sparrows followed from their first year on territory showed that the number of songs a bird shares with his neighbourhood group is a better predictor of lifetime territory tenure than is his repertoire size. We also found that song sharing increases with repertoire size up to but not beyond eight to nine song types, which are the most common repertoire sizes in the population (range in our sample 5-13). This partial confound of song sharing and repertoire size may account for some earlier findings of territory tenure-repertoire size correlations in this species and other species having small- or medium-sized repertoires. Copyright 2000 The Association for the Study of Animal Behaviour.

Song sharing in two populations of song sparrows (Melospiza melodia)

Abstract Sharing song types with immediate neighbors is widespread in birds with song repertoires, and sharing songs may confer a selective advantage in some cases. Levels of song sharing vary between different geographical populations of several bird species, and ecological differences often correlate with differences in singing behavior; in particular, males in migratory subspecies often share fewer songs than males in resident subspecies. The song sparrow (Melospiza melodia) appears to fit this pattern: resident song sparrows in western North America generally share 20–40% of their repertoire (of about eight songs) with each neighbor, while migratory subspecies from eastern North America often share 10% or less. We compared song sharing in two populations within a single subspecies of song sparrow (M. m. morphna) in Washington State. These populations, separated by only 120 km, nonetheless differ in migratory tendencies and several other ecological and life history variables. We recorded complete song repertoires from 11 male song sparrows in a high-elevation, migrating population at Gold Creek in west-central Washington, and compared them to two samples (n = 15 and n = 36) from a coastal, resident population at Discovery Park, Seattle, Washington. Despite major differences in habitat, population density, and migratory tendencies, song sharing among Gold Creek males was as high as that among Discovery Park males. In both populations, sharing was highest between immediate neighbors, and declined with distance. We conclude that at the within-subspecies level, neither migration nor population density affect song sharing in song sparrows, a song repertoire species.

Effects of a habitat-altering invader on nesting sparrows: an ecological trap?

Many invading species impact native species through predation, parasitism or competition, while others affect natives indirectly by restructuring their habitat. How invasive plants affect native animals, and to what extent native animals respond to changes in their habitat and the novel selection pressures that follow, is not well known. We investigated the impacts of a habitat-altering invader, the Atlantic cordgrass Spartina alterniflora, on the nesting success of Alameda song sparrows (Melospiza melodia pusillula), a California Species of Special Concern, in tidal marshes in three sites in San Francisco Bay. Date of laying was the most influential factor in determining daily survival rate of nests, but whether the nest was placed in exotic Spartina was the most important ecological variable. Nests placed in exotic Spartina had a success rate that was 30% lower than those placed in native vegetation. Nests in exotic Spartina were significantly more likely to fail due to tidal flooding than were nests placed in native vegetation, because the densest stands of exotic Spartina occurred at significantly lower elevations relative to the tides. Our results suggest that exotic Spartina may be an ecological trap for song sparrows in San Francisco Bay, attracting birds to nest sites that are often destroyed by tidal flooding.

What is the Function of Song Learning in Songbirds

In this paper we approach the question of the function of song learning in songbirds by addressing the more particular question of why a bird chooses the particular songs he does from among the many songs he hears during his song learning period. Our generalizations about song learning are derived from observations of a sedentary (nonmigratory) population of song sparrows (Melospiza melodia). We examine this question in the field, rather than in the laboratory, because we believe that the variables controlling song selection are social and ecological. We proceed from the hypothesis that song learning is an adaptive strategy, and attempt to identify the specific design features and overall function of this strategy in our particular study species. From our studies, we have identified several design features which, taken together, serve to maximize the number of songs the young bird will share with his neighbors, especially his near neighbors, in his first breeding season. Why should it be advantageous for the bird to have songs he shares with his neighbors? We suggest four possible, non-mutually exclusive advantages, and discuss the evidence in support of the fourth one. First, shared songs may be attractive to females. Second, shared songs may provide a mechanism by which two neighbors might effectively codefend their territories against other birds: each bird would effectively be mimicking the other while repelling prospective intruders. Third, shared songs may function as a “badge” of familiarity among territorial males; shared songs are a reliable signal of familiarity since they must be learned in the local neighborhood. Fourth, at least in song sparrows, shared songs appear to facilitate communication among neighboring birds. In particular, a bird uses the songs he shares with a neighbor to direct his song to that bird. We conclude by noting the paradox that songbirds have the ability to improvise new songs (demonstrated in lab experiments), yet in the field birds of most species faithfully copy the songs of their older neighbors. We suggest that song researchers need to ponder this paradox, and figure out why it is so important for a songbird to have the same songs as his neighbors.