Abstracts of publications from the projects

Effects of social rearing conditions on song structure and repertoire size: experimental evidence from the field Johannessen, L.E., Slagsvold, T. & Hansen, B.T. (2006) Animal Behaviour, 72: 83-95 Through a cross-fostering experiment, we studied song learning of blue tit, Cyanistes caeruleus, and great tit, Parus major, males reared by heterospecific parents. This was done in the wild, and so potential song tutors, territorial neighbours, potential mates and other social factors were all natural and not affected by the treatment. Normally, the song repertoires of the two species are completely discrete. However, the cross-fostered great tit males altered their song in several aspects, including repertoire size and composition, and temporal and frequency parameters, thus becoming intermediate between the normal songs of the two species. For the cross-fostered blue tit song, only repertoire composition was affected. However, an analysis of repertoire size in both species proved this to be larger in cross-fostered than in control males. This increase in repertoire size shows that repertoire size is influenced by social conditions in these species and is not strictly constrained by memory capacity. Furthermore, several of the cross-fostered great tit males included trilled songs in their repertoires. In these two species, trilled songs have been regarded as unique to the blue tit. In conclusion, our results show the importance of early social experiences for song learning in these wild tits.

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Manipulation of male quality in wild tits: effects on paternity loss Johannessen, L.E., Slagsvold, T., Hansen, B.T. & Lifjeld, J.T. (2005) Behavioral Ecology, 16: 747-754 Extrapair paternity (EPP) has proved to be widespread and highly variable among birds and other taxa, including socially monogamous species. A multitude of hypotheses have been put forward to explain this variation, but its occurrence is not fully understood. Male age, social dominance rank, song and breeding density or synchrony have been among the suggested correlates of EPP, but results so far are inconclusive. We interspecifically cross-fostered blue tits (Parus caeruleus) and great tits (Parus major) in the wild, thus manipulating males to exhibit reduced social dominance rank, sing aberrant songs, and consequently be perceived as low-quality males as compared to controls. This allowed us to test if male quality had an influence on loss of paternity. We found no statistically significant differences between cross-fostered and control males of either species, neither with respect to levels of cuckoldry nor proportions of extrapair young (EPY) in the broods. Paternity levels were comparable to other studies on the same species. No effect of density could be detected on levels of EPP either, while an age effect seemed to be present at least in the blue tit, EPY being almost absent in broods of older blue tit males. We conclude that the effects of male quality on paternity loss are minor, if any, in these populations.

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Cross-fostering of pied flycatchers (Ficedula hypoleuca) to heterospecific hosts in the wild: a study of sexual imprinting Slagsvold, T. (2004) Behaviour, 141: 1079-1102 Learning from parents during early development may crucially influence future mate choice decisions of birds. Such sexual imprinting is thought to be important to many fields of evolutionary ecology, including sexual selection, hybridization, speciation, and interspecific brood parasitism. Most results have been obtained from controlled experiments on captive birds. Hence, there is a need to study sexual imprinting and the development of species recognition under more natural circumstances. I have cross-fostered a migratory bird, the pied flycatcher Ficedula hypoleuca, to nests of two resident species, the larger great tit Parus major, and the smaller blue tit P. caeruleus, by adding a single egg to the clutch, or by swapping whole clutches. No cross-fostered bird recruited to the local breeding population from the mixed brood treatment but sample size was small. However, for the whole brood treatment, cross-fostering had no apparent effect, compared to controls, on local recruitment, natal dispersal, mating success, breeding success, or sexual display responses to intruders. Hence, there were no signs that the cross-fostered birds were sexually imprinted on the host. The results are discussed in relation to sexual imprinting, natal dispersal, interspecific brood parasitism, and conservation of endangered birds.

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Manipulation of male quality in wild tits: effects on paternity loss Johannessen, L.E., Slagsvold, T., Hansen, B. & Lifjeld, J.T. (2004) Poster, ISBE 2004. Jyväskylä, Finland, 10-15 July In most bird species some proportion of the females are sexually unfaithful. Why and how this occurs is not known, but some factors have been identified as important, including male quality. We manipulated male quality by cross-fostering tits in the wild, enabling us to study loss of paternity in relation to variation in aspects of male behavior

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Early learning affects social dominance: interspecifically cross-fostered tits become subdominant. Hansen, B. & Slagsvold, T., (2004) Behav. Ecol., 15: 262-268 Social dominance influences the outcome of competitive interactions over limited resources, and may hence be important for individual fitness. Theory thus predicts that its heritability will be low and that non-genetic determinants of dominance should prevail. In this field experiment we reciprocally cross-fostered great tits (Parus major) to blue tits (Parus caeruleus) to investigate the impact of early social experience on dominance status in competition over food during winter. Controlling for potential effects of age, size, sex and site-related dominance, we show that cross-fostered birds of both species were subdominant to conspecific immigrants, while controls originating from unmanipulated broods were dominant to conspecific immigrants. Furthermore, blue tits reared by blue tit parents but with at least one great tit broodmate had lower dominance status relative to conspecific immigrants than did controls. Although great tits generally dominated blue tits, cross-fostered birds of both species initiated marginally more fights against the other species than did their respective controls, suggesting faulty species recognition. Since both social parents and broodmates strongly influence the dominance behavior of offspring later in life, we conclude that social conditions experienced at an early age are crucial for the determination of subsequent social dominance.

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Rival imprinting: interspecifically cross-fostered tits defend their territories against heterospecific intruders Hansen, B. & Slagsvold, T. (2003) Anim. Behav., 65: 1117-1123 Failure to recognize conspecifics in social interactions such as mate choice and aggressive encounters will often result in reduced fitness. Studies on mate choice show that the ability to recognize conspecifics as mates is not universally present at birth, but often needs to be learned. In contrast, little is known about the ontogeny of intrasexual species recognition. To test whether learning influences the recognition of sexual rivals, we compared the aggressive response towards intruders of interspecifically cross-fostered individuals and controls reared by conspecific parents. We simulated territorial intrusion by presenting either a caged individual or playback song near the nest of breeding pairs of great tits, Parus major, and blue tits, P. caeruleus. Great tits reared by blue tit parents responded much more to blue tit stimuli than did great tit controls, and furthermore showed stronger responses to blue tit stimuli than to those of their own species. Blue tits reared by great tits responded much more to great tit stimuli than did blue tit controls. In contrast, blue tits cross-fostered to coal tits, P. ater, did not respond more to coal tits than did blue tit controls. There was a species difference in the response to conspecifics: blue tits cross-fostered to great tits responded more to conspecifics than did cross-fostered great tits. The results were similar for males and females. We conclude that learning influences intrasexual species recognition in these tits.

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Early learning in birds: implications of interspecific cross-fostering in the wild for species recognition, social dominance and the evolution of brood parasitism Hansen, B. (2003) Dr. scient. thesis, Department of Biology, University of Oslo. 102 pp. This thesis concerns the impact of early social experiences on the behaviour of great tits (Parus major), blue tits (Parus caeruleus) and pied flycatchers (Ficedula hypoleuca). Imprinting is a form of learning that takes place early in life and is retained by the animal for a long time. Although laboratory experiments show that birds learn to recognize potential mates by sexually imprinting on parents and siblings, experimental evidence of this effect in the field is scarce. By manipulating the rearing environment of free-living birds through interspecific cross-fostering, i.e. swapping young between nests owned by different species, I wanted to investigate the effect of sexual imprinting and other imprinting-like phenomena in the field. Great tits were cross-fostered to blue tits, blue tits were cross-fostered to great tits and coal tits (Parus ater), and pied flycatchers were crossfostered to great tits and blue tits. The behaviour of these cross-fostered individuals was compared to that of unmanipulated controls resident in the same study area. Judging by fledging weights, morphometric measures and local recruitment, cross-fostered individuals generally thrived in the nests of heterospecifics. However, cross-fostered great tits and blue tits became sexually misimprinted, as evidenced by heterospecific pairings in the subsequent breeding season, each pair consisting of a cross-fostered great tit male and a cross-fostered blue tit female. Moreover, cross-fostered great tits hardly engaged in conspecific pairings as yearlings, further suggesting sexual misimprinting. Cross-fostered blue tits, on the other hand, did not suffer from a significantly lower conspecific pairing success than unmanipulated controls. Pied flycatchers did not seem affected by crossfostering, since cross-fostered and control individuals invariably and to the same extent paired conspecifically. Sexual misimprinting has been thought to constrain the evolution of brood parasitism. Although the effects of cross-fostering were severe for great tits, sexual misimprinting does not seem to represent an absolute obstacle to the evolution of broodparasitic behaviour in this strongly imprintable species, since a few cross-fostered great tits managed to mate with conspecifics. The results suggest that other factors may be more constraining on the evolution of brood-parasitic behaviour, such as the virtually complete absence of intraspecific egg dumping in great tits, which may be a necessary initial step for interspecific brood parasitism to evolve. In fact, some sort of imprinting on the host may facilitate the evolution of interspecific brood parasitism, since recognition of good host species for future exploits may increase the reproductive success of brood parasites. I also investigated whether blue tits and great tits learn to recognize potential sexual rivals, by sequentially presenting a heterospecific and a conspecific stimulus near the nestbox of breeding controls and cross-fostered birds. Visual (a live, caged bird) and auditory (song playback) stimuli were presented in separate experiments, and yielded similar responses. Cross-fostered birds of both species responded aggressively to simulated territorial intrusion by members of their foster species, while controls did not respond aggressively to heterospecific intrusion. Controls and blue tits cross-fostered to great tits responded aggressively to conspecific intrusion, while great tits cross-fostered to blue tits did not. In another experiment, I estimated the dominance status of cross-fostered and control birds from aggressive interactions over food during winter. Cross-fostered blue tits and cross-fostered great tits both had lower dominance status than their respective controls in interactions with conspecific immigrants. Cross-fostered birds both initiated less aggression against and received more aggression from conspecific immigrants than did controls. Blue tits reared by blue tit parents but with at least one great tit broodmate also had a lower dominance status relative to controls in interactions with conspecific immigrants. Hence, early learning seems to affect subsequent dominance status. Crossfostered individuals tended to initiate more aggression against members of their foster species than did controls against heterospecifics, indicating faulty species recognition among cross-fostered birds. I conclude from these experiments that early learning is important for species recognition in great tits and blue tits, since both the recognition of potential mates and sexual rivals were affected by cross-fostering. Pied flycatchers seem to rely less on sexual imprinting in mate recognition, although a direct comparison with the tits is unwarranted. Several experiments indicate that siblings as well as parents may influence species recognition through learning. The effects of interspecific cross-fostering also reach beyond sexual behaviour, since cross-fostered birds attain a low dominance rank in competition over food. Several lines of evidence also indicate that there may be a difference in the impact of imprinting between great tits and blue tits. I discuss various possible explanations for this difference.

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Mate choice and imprinting in birds studied by cross-fostering in the wild Slagsvold, T., Hansen, B.T., Johannessen, L.E. & Lifjeld, J.T. (2002) Proc. R. Soc. Lond. B 269: 1449-1455 Sexual-selection theories generally assume that mating preferences are heritable traits. However, there is substantial evidence that the rearing environment may be important for the development of mating preferences, indicating that they may be learnt, or modified by experience. The relative importance of such sexual imprinting across species remains largely unexplored. Here, we report results of a large-scale crossfostering experiment in the wild in which nestling birds were raised by parents of a different species. We show that resulting sexual imprinting may have a negative effect on pairing success in one species (the great tit, Parus major), but not in two other species (the blue tit, P. caeruleus and the pied flycatcher, Ficedula hypoleuca). A remarkable variation thus seems to exist, even between species that are congeneric and have similar breeding ecologies. The cross-fostering resulted in heterospecific pairings between the two tit species (female blue tit breeding with male great tit), which has never, to our knowledge, been previously documented. However, the chicks fledging from these nests were all blue tit.

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Do interspecifically cross-fostered tits obtain territories of inferior quality? Johannessen, L.E., Hansen, B. & Slagsvold, T. (2002) Poster, ISBE 2002. Montreal, Canada, 7-12 July Background: Earlier studies have shown that great tits (Parus major) and blue tits (P. caeruleus) become imprinted on their foster species the two species, and that this imprinting influences negatively on various behavioural and life-history aspects, such as preferences and reproductive success (Slagsvold & Hansen 2001; Slagsvold et al. 2002; Hansen & Slagsvold unpublished) birds seem to be subdominant to controls leads to the prediction that they might also end up in territories of In the present study we use popularity and onset of laying during all the years a nest box has been in place to evaluate and cross-fostered birds, and we also test whether the latter initiate laying later than controls. Conclusions: Based on these findings we can conclude that cross-fostered great tits and blue tits seem to suffer from misimprinting also when it comes to choice of nesting site. They end up in somewhat less popular nest boxes than controls, this being particularly clear for the great tits, and blue tit males. There is also a tendency for nest boxes inhabited by them to have slightly later average dates of first egg laying than those of controls, but here variation is too great to conclude. However, data from the field season of 2002 will probably be supportive of both trends identified here, as well as of the clear tendency of cross-fostered great tit females to initiate laying late.

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Rival imprinting – interspecifically cross-fostered tits defend their territories against heterospecific intruders Hansen, B.T. & Slagsvold, T. (2002) Poster, ISBE 2002. Montreal, Canada, 7-12 July Background: Correct species recognition is particularly important when choosing a mate and when defending a resource such as a territory. While there is abundant evidence that learning may be important in mate recognition (Kendrick et al. 1998; Ten Cate & Vos 1999), little is known about the ontogeny of intrasexual species recognition. In this study, we show that the recognition of sexual rivals may be learnt. Conclusions: Since cross-fostered great tits and blue tits respond aggressively to the species of their foster parents while controls of both species respond mostly to conspecifics, we conclude that: 1) Rival recognition may be highly influenced by learning in great tits and blue tits. 2) Males and females appear to be similarly affected (females only participated in the cage experiment). 3) Both visual and vocal recognition cues may be learnt. 4) There seems to be a species difference in species recognition even among the closely related great tit and blue tit, since cross-fostered blue tits respond to conspecifics as well as heterospecifics, while cross-fostered great tits respond mostly to heterospecifics. Blue tits cross-fostered to coal tits do not respond to coal tits, possibly because coal tits are more distantly related to blue tits than are great tits, and/or because there are very few coal tits in the study area relative to great tits and blue tits. The results coincide with findings on the ontogeny of mate preferences (Slagsvold et al. 2002).

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Sexual imprinting and the origin of obligate brood parasitism in birds Slagsvold, T. & Hansen, B.T. (2001) The American Naturalist 158(4): 354-367 We discuss two pathways along which obligate brood parasitism (OBP) may evolve and examine some of the critical steps that must be passed by letting great tits Parus major be reared by blue tits Parus caeruleus in a field experiment. The cross-fostered chicks survived well in blue tit nests, but their local recruitment and reproductive success was much lower than that of controls. The effect was strongest when great tits grew up with siblings of the host species rather than with conspecific siblings in blue tit nests. The low success seemed to be caused by misimprinting because the cross-fostered birds behaved like blue tits in several aspects (species association, alarm calls, and aggressive response by resident females to caged intruders). Some birds of both sexes were apparently so strongly imprinted that they did not attract or accept a social mate of their own species. We conclude that imprinting may be necessary for OBP to evolve in birds because the parasite must be attracted to the nests of the host species to add eggs and thereby continue the parasitic life cycle. However, strong imprinting may also prevent OBP from occurring if parasitic offspring seek a mate from the host species.

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On the origin and rarity of interspecific nest parasitism in birds Slagsvold, T. (1998) The American Naturalist, 152(2): 264-272 Interspecific nest parasitism is surprisingly rare in birds given the potential advantages for the parasite of exploiting the parental care of other species. One possibility is that chicks will not thrive with the parental care and food of heterospecifics. I simulated parasitism in nonparasitic congeners by switching eggs between nests of three species of titmice (great tit Parus major, blue tit Parus caeruleus, and coal tit Parus ater). The experiment showed that compatibility of parental care was not a constraint preventing parasitism. I also used the model system to compare fitness consequences of inter- and intraspecific nest parasitism, addressing the problem of which form is ancestral. Fledging success (body mass, survival) was higher when an egg was added to the nest of a smaller species than to the nest of a conspecific and also higher when the parasitic chick hatched early rather than late relative to host chicks. This suggests that interspecific nest parasitism may not require a stage of intraspecific nest parasitism before evolving but may start from a larger species directly exploiting the parental care of a smaller species or a species with shorter incubation period directly exploiting a species with longer incubation period.

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