Meredith J. West

Validating the female bioassay of cowbird song: Relating differences in song potency to mating success

Female cowbirds (Molothrus ater), maintained in isolation from males during the breeding season, respond to the playback of male song with copulatory postures. They respond more often to some songs than others. Song potency can thus be operationally defined by the number of copulatory responses a song elicits. The purpose of the present study was to validate this measure of song potency by investigating its relationship to mating success. We observed two colonies of cowbirds during the breeding season and recorded details of their courtship. In addition, song potency of the males was tested by playback with a different group of captive females. The results indicate a relationship between maximum song potency and mating success: the males that obtained the most copulations had songs of higher maximum potency and were also observed to have been more dominant during the winter and early spring.

A role of her own: female cowbirds, Molothrus ater, influence the development and outcome of song learning.

Previous work has shown that captive female cowbirds, Molothrus ater, can influence the outcome of male song development by affecting retention or deletion of song elements and by stimulating improvization. Here we looked for evidence of female influence during the process of learning, as males progress from subsong to plastic song to stereotyped song. In a longitudinal study, we measured the rate and timing of vocal development in captive, juvenile male brown-headed cowbirds, M. a. artemisiae. Half the young males were housed with female cowbirds from their own population (South Dakota: SD) and half with female cowbirds from a M. a. ater population (Indiana: IN). Both populations of females prefer local songs and differ in the time of breeding, with SD females breeding 2 weeks later than IN females. The results showed significant effects of female presence on the age at which males advanced through stages of vocal development: the SD males with SD females, as opposed to SD males with IN females, developed stereotyped song earlier, reduced motor practise earlier, and produced more effective playback songs. Longitudinal observations of social interactions showed that the two groups of females reliably differed in social responses to males. Degree of social proximity of females to males in the winter predicted song maturity, rate of rehearsal and song potency. Thus, females can stimulate the progression of song learning, as well as prune song content. Copyright 2000 The Association for the Study of Animal Behaviour.

The case for developmental ecology

We call for renewed emphasis on the tasks confronting animals as they develop and learn. We are extending the use of the term ‘developmental ecology’ employed by plant biologists who have studied how fitness can be influenced by the ecological context present during development (Watson et al. 2001, Evolutionary Ecology,15, 425–442). We seek an expanded venue for the term, arguing that for animal behaviourists to understand some of the traits so familiar in behavioural ecology, they must consider the fundamental phenomena of development. Not doing so runs the risk of misidentifying both the proximal and functional causes of traits. For example, without a developmental view, macrogeographical variation in species-typical behaviour may be viewed as evidence of genotypic differences when, in fact, the variation is being produced by developmental contexts. Detailed below are some general issues about how development can be studied if it is to contribute to our knowledge of the adaptive value of behavioural systems. We argue for a prospective and longitudinal orientation, with an emphasis on relatively continuous observation and measurement. Both behaviours and contexts that may only occur during ontogeny are examined, as well as the reproductive outcome of the traits of interest. We present examples from our work on courtship and communication in brown-headed cowbirds, Molothrus ater, to show that a prospective and ecological view of development reveals pronounced variation in patterns of reproductive behaviour that cannot be understood without taking into account developmental ecology.

The context of social learning: association patterns in a captive flock of brown-headed cowbirds

Much work on social learning has involved behaviour transmission between pairs of individuals, but recently the need to examine the social context in which learning occurs has been recognized. Previous studies using small numbers of animals have shown social influence on the behavioural development of juvenile male brown-headed cowbirds, Molothrus ater. Here we looked at the larger social context that forms the framework for such influence in more natural settings. We allowed a captive group of over 70 cowbirds, comprising adult and juvenile males and females, to associate freely in a large complex of connected aviaries. Highly organized social assortment emerged in the group, with individuals associating with others based on similarity in age and sex. Juvenile males that associated more with adult males had higher courtship success. Juvenile males that associated more with females sang less over the year. These results indicate that the social context of social learning for juvenile males is not just random association with all other birds in the social group, but is a selective and structured pattern of interaction. Differences in navigating this social structure correlated with courtship success and vocalization, behaviour known to be affected by social learning. Studies such as this, using large groups with free assortment of individuals, are the first step towards understanding the effects of the larger social context surrounding social learning.

Social Malleability in Cowbirds: New Measures Reveal New Evidence of Plasticity in the Eastern Subspecies (Molothrus ater ater)

The social experiences of young Molothrus ater ater cowbirds were manipulated in a 2-year study. In the 1st year, males were housed with pairs of canaries. The males were tested in 3 social contexts. Also, vocal repertoires were recorded and played back to females. In contrast to a previous study of the M. a. artemisiae subspecies, the males did not vocalize to the canaries in courtship tests (T. M. Freeberg, A. P. King, & M. J. West, 1995) but showed incompetent courtship of female cowbirds. In their 2nd year, half of the males were housed with older males and female cowbirds, and half were housed with only females. Those exposed to older males courted much more successfully than did those deprived of such experience. All males developed new repertoires, and song potencies did not correlate across years. The data reveal intraspecific variation in the ontogeny of mate recognition but intraspecific dependence on social learning to acquire courtship skills.

An experimental investigation of the bioacoustics of cowbird song

SummaryFemale cowbirds (Molothrus ater ater), maintained in isolation from males during the breeding season, respond to the playback of male song with copulatory postures. They respond to some songs more than to others. Cowbird song potency can thus be operationally defined by the proportion of copulatory postures a song elicits across multiple playbacks. The purpose of the present study was to explore whether song potency changes with distance in the field. No field recordings elicited high levels of responding by the females. When songs of known high potency are systematically degraded, the results indicate that female cowbirds are sensitive to small changes in signal to noise ratio and to atmospheric attenuation. The data suggest that cowbird song potency degrades very rapidly with transmission distance in the field.

Searching for the functional origins of song in eastern brown-headed cowbirds, Molothrus ater ater

Abstract Three experiments were conducted to examine the functional properties of vocal precursors to stereotyped song in socially housed captive cowbirds. Previous studies had shown that eastern male cowbirds developed different song repertoires when housed in different social contexts. This paper reports on the acoustic origins of the different vocal outcomes. In addition, a winter roost of cowbirds was studied to investigate the setting in which males naturally develop song. Analyses of the vocalizations of laboratory-housed subjects revealed acoustic differences from the earliest stages of song ontogeny. The field data indicated that free-living and laboratory-housed males shared the same structural categories of song and that the winter roost provided males with opportunities to interact with females. Taken as a whole, the studies suggest that song ontogeny is a dynamic process involving social and vocal interactions between singers and listeners.

Song repertoire development in male cowbirds (Molothrus ater): its relation to female assessment of song potency.

Two studies were conducted to investigate the relation between the male cowbirds (Molothrus ater ater) development of a song repertoire and the female cowbirds assessment of song potency. Male development was assayed by vocal copying and female assessment by copulatory responsiveness to song playback. The results demonstrate that males do not copy most often the particular songs that females respond to most often. Whereas rank orderings of potency were highly correlated across two independent samples of playback females, male and female rank orderings were not significantly correlated. The data highlight the potential significance of social interactions between and across the sexes for repertoire development.

Cultural Transmission of Vocal Traditions in Cowbirds (Molothrus ater) Influences Courtship Patterns and Mate Preferences

In this study, the authors tested the cultural transmission of vocal traditions in cowbirds (Molothrus ater). Young cowbirds from a South Dakota (SD) population were housed over winter with adults of the SD population or with adults from an Indiana (IN) population. Song differences between the original SD and IN adult models were acquired by South Dakota culture (SDC) and Indiana culture (INC) males, respectively, and were transmitted to a 2nd cultural generation of birds. During playback tests of SDC and INC songs, SD females gave more copulatory responses to SDC songs. Finally, males with SD-like songs courted SDC females preferentially in breeding season tests, whereas males with IN-like songs courted INC females preferentially. These results indicate that the transmission of vocal traditions plays a fundamental role in the courtship patterns and mating decisions of cowbirds.

The Inheritance of Niches

The study of genetics overshadows that of exogenetics for three reasons. First, no single unit of exogenetic inheritance exists. Even the most central feature of exogenetic inheritance, the concept of something “outside” the genes, lacks clarity. That something is the environment, but how do we define the environment? To some, the term stands for the complex of biotic elements that surrounds all organisms. To others, the term refers to a particular species’ or population’s habitat.