Behavioural integration associated with the fusion of two flocks is analyzed in captive siskins (Carduelis spinus) by quantifying changes in social behaviour with time since joining. In general there was an increase in the incidence of tolerant behaviour, supplanting attacks and hopping withdrawals with time since fusion of the flocks. However, the number of displays and flights showed the opposite, negative, trend. Taking dominance status into account, the greatest change in behaviour with time since joining is an increase in tolerance by dominants of new flock companions. Factorial analysis of correspondences was used to study how different birds changed their behaviour with time since joining a flock. This analysis showed that the introduction of new birds did not disrupt relationships with familiar birds, and that residents are dominant in interactions with the incoming new flock companions. The analysis also demonstrated that relationships within the new flock had stabilized 20 days after the flocks had joined. The characteristics of the socially integrated group of siskins are quite similar to those described by Rohwer & Ewald (1981) in their shepherds hypothesis: dominants tolerate their subordinates feeding in close proximity, offering them a profitable feeding area, but also supplant them to obtain food; both dominants and subordinates benefit from being in a flock. As a consequence, constant changes of membership in flocks is costly not only because birds lose dominance status, but also the advantages of clear dominant and subordinate roles.

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According to current theory, the development of different conspicuous ornaments may provide multiple, redundant or unreliable information. However, multiple visual and acoustic traits have not often been tested at the same time. In this study, we analyse whether black tie size, yellow breast colour indexes (chroma and hue) and song parameters (repertoire size, strophe length, maximum and minimum frequency) are inter-correlated in male great tits (Parus major). We found that males with larger black ties sang greater repertoires and had higher song frequencies, providing redundant information. However, song parameters did not correlate with hue and chroma of the yellow breast coloration, supporting the idea that they convey different units of information. Results therefore stress that since different plumaged patches can transmit very different units of information, bird song will correlate to plumage coloration traits depending on the patch and the colour considered, an aspect rarely taken into account in previous studies relating song to plumage colour traits.