Mixed-species flocks: an example of niche construction?

Mixed-species flocks result from coevolved relationships between participants: interspecific gregariousness that varies in strength in different species pairs or groups. Such interspecific associations of birds are a characteristic of most avian communities, observed in diverse habitats from forests to grasslands and from estuaries to the open ocean, both at high latitudes and in the tropics. We explore the consequences of mixed-species flocks in shaping the selection environment and discuss whether the participants in flocks should be identified as niche constructors. As a result of the decision to associate with other species, a bird alters its relationship with competitors, potentially gains access to resources, becomes safer from predators and may change microhabitat use. The recurrent patterns in the behaviour of disparate unrelated species active in mixed-species flocks have led investigators to conclude that similar selection pressures have shaped their behaviour. However mixed-species flocks are variable in their characteristics, the birds active in them are diverse in form and behaviour, and the selection pressures that have shaped their propensity to join mixed-species flocks must be various. In forming mixed-species flocks, species with specialized roles at the centre of flocks are creating a complex social environment that represents an important biotic selection force shaping their own ecology and behaviour, and that of other species within the avian community. In this review, we consider how the spectrum of interspecific relationships in mixed-species flocks makes them an interesting focus for further studies of niche construction by relocation.