A new approach to evaluate multimodal orientation behaviour

Circular cages, so-called Emlen-funnels, have extensively been applied to study compass orientation and ecological causes of differential orientation in songbirds. The method was developed already in 1966 by Emlen and Emlen and has been applied, for instance, in describing the functional characteristics of the biological compasses used by songbirds. For several decades the circular cage approach has been appreciated, but it has become clear that it suffers from statistical limitations in evaluating the directions of the activity recorded in the cages. The migratory activity has been reported to vary, including complex multimodal orientation of migratory passerines tested in orientation cages irrespective of species studied. A substantial drawback of the currently applied circular statistical methods is that they fail to describe orientation responses differing from unimodal and axial distributions. In a recent paper by Ożarowska et al. (2013) published in Journal of Experimental Biology, we propose a modelling procedure enabling the analysis of multimodal distributions at either an individual or a group level. We have been able to compare the results of conventional methods and the novel modelling approach. Reasons behind developing an alternative method to evaluate orientation cage data is that migratory routes may be more complex than a simple migratory direction, and multimodal behaviour in migratory species at the individual and population levels can be advantageous. For instance, individuals may select the expected migratory direction, but may also return to safer sites en route, i.e. sites already known, which provide food and/or shelter in reverse directions. In individual birds, several directions may be expressed in the same test hour when recorded in the circular cages. At the species level, multimodal orientation may give an opportunity to expand the range or may refer to differential migration route preferences in different populations of birds. A conflicting experimental situation may also result in a different preferential orientation. In this paper we suggest a statistical solution to deal with these types of variations in orientation preference.

/Susanne

Photo: Susanne Åkesson