Social learning in birds and its role in shaping a foraging niche

Tore Slagsvold () 1 Karen L. Wiebe 0 0 Department of Biology, University of Saskatchewan , 112 Science Place, Saskatoon, SK , Canada S7N 5E2 1 Department of Biology, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo , PO Box 1066, Blindern, Oslo NO-0316 , Norway We briefly review the literature on social learning in birds, concluding that strong evidence exists mainly for predator recognition, song, mate choice and foraging. The mechanism of local enhancement may be more important than imitation for birds learning to forage, but the former mechanism may be sufficient for faithful transmission depending on the ecological circumstances. To date, most insights have been gained from birds in captivity. We present a study of social learning of foraging in two passerine birds in the wild, where we cross-fostered eggs between nests of blue tits, Cyanistes caeruleus and great tits, Parus major. Early learning causes a shift in the foraging sites used by the tits in the direction of the foster species. The shift in foraging niches was consistent across seasons, as showed by an analysis of prey items, and the effect lasted for life. The fact that young birds learn from their foster parents, and use this experience later when subsequently feeding their own offspring, suggests that foraging behaviour can be culturally transmitted over generations in the wild. It may therefore have both ecological and evolutionary consequences, some of which are discussed. 1. INTRODUCTION Social learning is widely used among vertebrates to acquire information about a fluctuating environment [1 3]. In birds, there is ample opportunity for social learning because parents may provide care for their offspring for several weeks, and because birds may join conspecific or mixed species flocks, allowing both vertical and horizontal transmission of behaviour. Here, we briefly review the literature on social learning in birds. Most insights on social learning in animals come from studies in captivity, and the role of social learning in nature remains rather poorly understood [4,5]. We therefore conducted a study of social learning in the wild, where we cross-fostered eggs of blue tits, Cyanistes caeruleus, to nests of great tits, Parus major, and vice versa, enabling us to quantify the consequences of being reared in a different social context but in an environment otherwise natural to the birds. To our knowledge, the study is the most extensive of its kind of any animal group. Here we report on foraging behaviour and on the foraging niche. (a) Social learning in birds Social learning may be involved in migration of some, but not all, avian species, where inexperienced birds may be guided by adults to find suitable migration routes and wintering areas. This knowledge has been successfully used to train naive, captive-reared endangered birds during migration using a microlight aircraft as foster parents [6]. Social learning early in life may also be important in habitat selection [7,8], and birds may use public information, viz. the breeding performance of other species, in choice of habitats and nest sites [9]. On the other hand, little social learning seems to be involved in nest building. Some species build quite complex nests, yet conditioning probably is a sufficient developmental mechanism, with only a limited repertoire of stereotyped movements needed [10]. In a few birds, males may build courtship display sites (bowers) and for these, there is some circumstantial evidence that social learning is involved in the choice of decoration objects [11]. Social learning has a role in the recognition of predators [12,13] and brood parasites [14]; conservation strategies for endangered species attempt to teach inexperienced birds [13]. Alarm calls are considered to be quite stereotypic. Social learning may be involved in responding to such calls [13,15], but not necessarily in producing them [16], although great tits seem to learn from foster parents of another species (blue tits) in the wild [17]. Convincing examples of social learning in birds come from studies of song. Some details about the ontogeny of song have been well studied such as the brain pathways for vocal learning, the sensitive period during which a juvenile must learn songs, and which individuals the juvenile learns from [18,19]. Apparently, the strongest cases of production learning come from vocal mimics copying calls of other species and environmental sounds [20]. Such mimicry is not confined to song birds [19]. Most experiments with song have been done in captivity but song copying also occurs in the wild, e.g. many birds have song dialects, and the dialects in a given local area may change over time. A few experimental studies on song copying in the wild suggest horizontal transmission, e.g. adult pied flycatchers, Ficedula hypoleuca, may pick up and sing novel playback songs [21]. Social learning is also important for mate choice in birds, primarily by vertical transmission through sexual imprinting early in life [22,23]. Most data are from studies in captivity, but there is some support from experiments in the wild [24 26], where sexual imprinting as a juvenile lasts for life [27]. However, sexual imprinting does not seem to be equally important in all species: pied flycatchers did not imprint on blue tit or great tit foster parents in the wild [28]. Mate choice and species recognition seem to have a strong genetic basis in the flycatcher and may be linked to the sex chromosomes [29]. A challenge is to understand the variation in the degree of sexual imprinting among species [28]. Interestingly, male pied flycatchers raised by tits did include tit song elements in their song repertoire, suggesting that different mechanisms are involved in the development of mate choice preferences and song acquisition [30]. In addition to sexual imprinting early in life, using parents as role models when choosing a mate, birds may use social information gained later in life such as public information and eavesdropping by observing the mating choices of other individuals [31]. Costs and benefits may differ between the sexes, and so also their use of information [32], e.g. females may modify their song preferences by attending to the vocal behaviour of other females [33]. There is some evidence for mate choice copying in polygynous birds but results are more mixed in monogamous birds [34 36]. Many bird species also depend on social learning to learn aspects of foraging such as feeding sites [8,37], food items [38 40], hunting skills [41], handling and feeding techniques [42,43], and tool use [44]. Social learning of foraging may take place early in life, using parents as role models [8,40,45], and later in life by observing others [20,43,46]. The efficiency of learning may depend on whether the demonstrator is rewarded and on which other birds are present [47], e.g. in the presence of familiar birds, males and females (...truncated)