Typical versions of learned swamp sparrow song types are more effective signals than are less typical versions.

The learned songs of songbirds often cluster into population-wide types. Here, we test the hypothesis that male and female receivers respond differently to songs depending on how typical of those types they are. We used computational methods to cluster a large sample of swamp sparrow (Melospiza georgiana) songs into types and to estimate the degree to which individual song exemplars are typical of these types. We then played exemplars to male and female receivers. Territorial males responded more aggressively and captive females performed more sexual displays in response to songs that are highly typical than to songs that are less typical. Previous studies have demonstrated that songbirds distinguish song types that are typical for their species, or for their population, from those that are not. Our results show that swamp sparrows also discriminate typical from less typical exemplars within learned song-type categories. In addition, our results suggest that more typical versions of song types function better, at least in male-female communication. This finding is consistent with the hypothesis that syllable type typicality serves as a proxy for the assessment of song learning accuracy.

Brain development, song learning and mate choice in birds: a review and experimental test of the "nutritional stress hypothesis".

The nutritional stress hypothesis explains how learned features of song, such as complexity and local dialect structure, can serve as indicators of male quality of interest to females in mate choice. The link between song and quality comes about because the brain structures underlying song learning largely develop during the first few months post-hatching. During this same period, songbirds are likely to be subject to nutritional and other stresses. Only individuals faring well in the face of stress are able to invest the resources in brain development necessary to optimize song learning. Learned features of song thus become reliable indicators of male quality, with reliability maintained by the developmental costs of song. We review the background and assumptions of the nutritional stress hypothesis, and present new experimental data demonstrating an effect of nestling nutrition on nestling growth, brain development, and song learning, providing support for a key prediction of the hypothesis.

Experimental confirmation of the polygyny threshold model for red-winged blackbirds.

The polygyny threshold model assumes that polygynous mating is costly to females and proposes that females pay the cost of polygyny only when compensated by obtaining a superior territory or male. We present, to the authors' knowledge, the first experimental field test to demonstrate that females trade mating status against territory quality as proposed by this hypothesis. Previous work has shown that female red-winged blackbirds (Agelaius phoeniceus) in Ontario prefer settling with unmated males and that this preference is adaptive because polygynous mating status lowers female reproductive success. Other evidence suggests that nesting over water increases the reproductive success of female red-winged blackbirds. Here we describe an experiment in which females were given choices between two adjacent territories, one owned by an unmated male without any over-water nesting sites and the other by an already-mated male with over-water sites. Females overwhelmingly preferred the already-mated males, demonstrating that superior territory quality can reverse preferences based on mating status and supporting the polygyny threshold model as the explanation for polygyny in this population.

Response of sparrows to songs of deaf and isolation-reared males: further evidence for innate auditory templates.

Adult song sparrows (Melospiza melodia) were tested for response to songs of conspecific males that had been reared in acoustic isolation or deafened early in life. Territorial males responded more aggressively to playback of songs of isolated males than to songs of deafened males but did not discriminate between songs of deafened males and heterospecific songs. Captive females showed an identical pattern when their response was measured in terms of the number and intensity of courtship displays evoked by song. The results support the auditory template theory of song development in demonstrating that species-identifying features are present in songs of isolates and in showing that males unable to hear their own song production do not incorporate these species-identifying features into their songs.

A test for responsiveness to song structure and programming in female sparrows.

Female song sparrows, primed with implants of estradiol, gave the solicitation display for copulation in response to acoustic stimulation with song. This technique demonstrated that female song sparrows respond more strongly to conspecific song than to alien songs, that they discriminate on the basis of both overall temporal pattern and syllabic structure, that they respond more to several song types than to repetitions of one song, and that they are most responsive to several song types if the songs are organized in bouts of a single type, as they are normally delivered by a male song sparrow. These results demonstrate a substantial correspondence between the structure and programming of the singing behavior of male birds and female responsiveness to song.