Mixed-species flocks: an example of niche construction?

Mixed-species flocks result from co-evolved relationships between participants: inter-specific gregariousness that varies in strength in different species pairs or groups. Such inter-specific associations of birds are a characteristic of most avian communities, observed in diverse habitats from forests to grasslands, estuaries to the open ocean, worldwide in both high latitudes and in the tropics. We explore the consequences of mixed-species flocks in shaping the selective 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 selective 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 selective pressures that have shaped their propensity to join mixed-species flocks must be various. In forming mixed-species flocks, species with specialised roles at the centre of flocks are creating a complex social environment that represents an important biotic selective 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 inter-specific relationships in mixed-species flocks make them an interesting focus for further studies of niche construction by relocation.