Coordinated visual displays and vocal duetting in different ecological situations among western palearctic non-passerine birds

The biological significance of mutual behaviours, like duet singing and visual displays, is not entirely clear. The main hypotheses (pair bonding and territorial defence) come from studies of tropical bird species. The aim of this work is to identify some behavioural-ecological parameters that characterize the birds of the temperate area showing duets and coordinated-visual displays (CVD). To this end we performed a multivariate statistical analysis considering 122 nonpasserine of the Western Palearctic, 46 of which make coordinated displays. Five main groups have been obtained, two of which are pure vocal duetters (owls/woodpeckers and swifts/shearwaters), and the other three consisting of birds where vocalisations are accompanied by CVD (some marine colonial species, Anatidae and divers). In agreement with previous studies of tropical birds, vocal duetting is typical of monogamous, monomorphic and territorial species. CVD is present in species living in open habitat with nidifugous offspring and migratory habits. It is suggested that in the latter birds, pair-synchronization and pair-strengthening needs are the main ecological pressures selecting for these mutual forms of communication.