Birds and Habitat: Habitat quality and habitat occupancy by birds in variable environments

It is unwise to be dogmatic about the habitat associations of any bird species. Even the classification of species as habitat generalists or specialists requires careful consideration. Perceptions of specialisation depend on how niche breadth is measured and on locational, spatial and temporal context (Devictor et al ., 2010; Barnagaud et al ., 2011). Close examination of most species reveals variation across individuals in their occupancy and use of habitats. This chapter focuses on the diverse processes affecting habitat choices made by individual birds that determine which patches and habitat types are actually occupied at any particular place and time. Ecologists have given much attention to concepts of habitat quality – these are fundamental to any discussion about the processes of habitat choice. Many of the mechanisms discussed here were initially proposed many years ago (see Hilden, 1965) but, for brevity, I generally cite the recent evidence. The broad limits of what constitutes potential habitat are set by a combination of the individual’s functional needs and the mechanisms by which it recognises its habitat (Chapter 1). In reality most species, most of the time, do not occupy all potentially suitable habitat. Moreover, for various reasons, individuals may occupy, even select, poor-quality habitats. Habitat occupancy can, very broadly, be thought of as the outcome of an interaction between: (i) habitat quality, (ii) constraints that preclude settlement in part of the potential habitat spectrum, (iii) other factors that may stimulate settlement in certain parts of the potential habitat spectrum and (iv) phenotypic flexibility. This balance of diverse factors determines the observed distribution of birds across habitats. Habitat quality forms a logical starting point.