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Many vertebrates inhabit dynamic and loosely structured groups where group size and social composition continually fluctuates. The ability to sustain nonrandom interaction preferences across group changes is an important aspect in maintaining social organization. In two experiments, we explored the strength and persistence of social preferences for familiar conspecifics in brown-headed cowbirds. In the first experiment, we demonstrated that females preferentially associate with familiar females when introduced into a flock containing novel individuals. In the second experiment we investigated the consistency of familiarity preferences across a series of social introductions. Females maintained preferences to approach familiar conspecifics, and the individual strength of those preferences remained consistent across introductions. Male preferences changed across the introductions. In the first introduction, males showed a significant preference to approach familiar conspecifics, but increased their approaches towards novel conspecifics in subsequent introductions. Our findings suggest that female cowbirds are an important factor in maintaining social organization through their enduring associations with familiar individuals, whereas males facilitate social integration by extending connections towards novel individuals during periods of change. © 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.

Individuals often show predictable variation in social tendencies over time and across contexts. In species that rely on social interaction for the development of courtship behaviours, predictable variation in juvenile social behaviour may influence the ontogeny of courtship skills by exposing young individuals to different levels of contact with conspecifics. During the autumn and winter, brown-headed cowbirds utilize a social display known as the head-down to facilitate close social interaction with others. In this study we investigated whether head-downs rates are correlated across social contexts, and thus constitute a social temperament trait, and whether variation in head-down rates predicts subsequent courtship behaviour in juvenile cowbirds. During the autumn, we used a fissionefusion perturbation on a flock of juvenile females and males, and recorded the number of approaches and head-downs across social contexts. All individuals were then followed into the breeding season, where we recorded male singing behaviour, female chatter vocalization and consort status. We found that female head-down rates were correlated across contexts during the autumn, and that females that engaged in more head-down displays received more songs from males, responded to more songs with chatter vocalizations and were more likely to be in a consortship during the breeding season. As far as we know, this study presents the first demonstration that a social temperament trait can predict competent adult-like courtship behaviour in juvenile birds, and suggests that juvenile sociability may be an important developmental mechanism in the emergence of reproductive competence. The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.

Variation in individual interaction patterns creates different social niches. Social niches afford differential exposure to social information that individuals utilize to develop species-typical behaviours. Brown-headed cowbirds depend on social interaction to develop competent reproductive behaviour. We used a fission–fusion paradigm to perturb two flocks of birds: one adult flock (N = 38) and one juvenile flock (N = 24). We split the two flocks, then recombined them while recording the frequency of individual approaches. Both adult and juvenile females maintained stable social niches, but males did not. Females maintained equivalent levels of gregariousness and attractiveness across perturbations. Juvenile females maintained the highest levels of sex assortment. The consistency of female niche construction may be explained by females’ reliance on interaction with other females to assess males. Early sex assortment may also foster the development of social preferences and influence reproductive choices.

Previous work has found that adult female brown-headed cowbirds, who do not sing, shape male vocal structure when in restricted housing. The present work extends this finding to a flock setting to examine the role of social behaviour in shaping male vocal development. We housed juvenile males with either adult or juvenile females in large flocks. Over the course of a year, we recorded their song and social behaviour. We found in early autumn that juvenile males housed with adult females improvised more song elements than males housed with juvenile females. During late autumn, we switched the males across female age-class conditions for 6 days. The males switched to the adult female flocks increased the number of improvised elements in their vocal repertoires. Analysis of the sequences of female–male social contact revealed that juvenile females, but not adult females, interacted indiscriminately with the males. Social network measures such as betweenness and closeness centrality further showed that adult females are more selective in their interactions with males than juvenile females. This study suggests a systemic role for selective social behaviour in shaping male vocal development.

Male song development has been studied in detail across a range of different songbird species. Very little, however, is known about the development of preferences for songs in females. Here we describe a method to study female preferences outside the breeding season by documenting female responses to playbacks of songs of different developmental stages and of varying quality. We compared ‘wingstroke’ responses of juvenile and adult females in four groups across autumn, winter and spring. We found that adult, but not juvenile, females showed preferences for variable song that was developmentally advanced. We assessed female preferences for the songs in the breeding season by playing back songs in sound-attenuation chambers and recording the number of copulation solicitation displays females gave to each song. We detected a strong relationship between wingstroke patterns in spring and song preferences, but we did not detect this relationship in the autumn or winter. Female preferences, like male songs, appear to show seasonal and developmental plasticity.

We investigated how individual variation in social behaviour among group members interacts to affect social structure and development of reproductive skills. We randomly assigned juvenile male and juvenile and adult female cowbirds to four aviaries and observed patterns of singing and social assortment in each. Although social composition of each aviary was the same, juvenile males in one aviary engaged in more intra- and intersexual affiliation and singing interactions. We designed a series of rotations of individuals among the aviaries to determine which individual traits were responsible for the observed group differences. First, we rotated groups of three males from a less interactive aviary into a more interactive aviary, replacing interactive males. Within 4 days, the rotated males changed their behaviour to match the aviary into which they had been moved (i.e. interactive males became less interactive and vice versa). Next, we rotated juvenile females through the aviaries. After this rotation, the behaviour of the juvenile females remained the same, but the behaviour of the resident males changed, becoming like the males in the females’ former aviaries (i.e. when juvenile females were moved from an interactive aviary into a noninteractive aviary, the males in the new aviary became more interactive and vice versa). Across the aviaries, the amount of female–male associations correlated positively with male–male competition. During the breeding season, males that had experienced more competition over the year received more copulations than males that had experienced little competition. Furthermore, more eggs were produced in aviaries containing competitive males than in aviaries containing less competitive males. Past work has shown that females can influence male vocal development; here we show that they can also influence male social development.

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.

In four large aviaries, we studied social assortment and reproductive behaviour of female brown-headed cowbirds housed with males differing in age class and in corresponding levels of intrasexual interaction. Juvenile and adult females resided with either (1) adult males, (2) juvenile males, (3) adult and juvenile males, or (4) no males. We observed social behaviour of males and females from September 1999 through to the 2000 breeding season. Throughout the year, males in the different conditions showed different levels of social interaction, with adult males being most engaged in male–male interactions and juvenile males being least engaged in male–male interactions. Females in the four conditions differed in their associations with males and with other females. In conditions with adult males, females spent more time near males, approached males more often, and associated with females less. During the breeding season, females in conditions with more adult males copulated more and produced more fertile and viable eggs. In the condition where females were housed with juvenile males, they copulated less, laid fewer eggs and destroyed more of their eggs. Results indicate that throughout the year, females are sensitive to male age and behaviour in their social group, and that this sensitivity can have reproductive consequences. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

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.

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.

We assessed courtship success in 14 adult male brown-headed cowbirds, Molothrus ater. Volumes of song control nuclei and visual nuclei were measured in Nissl stained tissue. Variation in courtship success was found to be related to variation in two areas of the avian brain: the song control nucleus, area X, and the thalamic visual area, nucleus rotundus. Volume of area X was negatively correlated with song potency, as assessed by female playback, and with rate of vocalizing. Volume of nucleus rotundus was positively correlated with song potency, vocalizing to females and courtship persistence. These data are the first to implicate a visual nucleus in the use of song. The data also complement previous findings with cowbirds suggesting that song learning involves visual attention to females. Taken as a whole, these results suggest that use of song depends on integration of auditory, vocal and visual information. In that female songbirds in many species assess multimodal performance of song, these findings with cowbirds suggest that future studies of brain and behaviour include a broader view of possible behavioural and neural correlates. Copyright 1998 The Association for the Study of Animal Behaviour.

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.

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.

The courtship and mating behaviour of captive individuals from allopatric populations of two cowbird subspecies ( Molothrus ater ater and Molothrus ater obscurus ) were observed in a large indoor-outdoor aviary, using a multiple-choice design in order to assess whether they would selectively pair with members of their own subspecies. Twenty-nine of the 42 consortships observed were between consubspecifics, and 10 of the 15 copulating pairs were homogamic. Individuals tended to pair with consubspecifics whether or not they had had prior breeding experience with consubspecifics or winter contact with heterosubspecifics. This demonstration of selective courtship between individuals of widely separated populations is one of the first quantified examples, to our knowledge, of the development of assortative pairing in allopatry for a passerine species. These data place in perspective previous findings on the female cowbird's preferential response to playback of her own subspecies' song, thereby suggesting that song may be an important cue for selective mating between passerine populations.