To apply some of the concepts and principles noted in previous chapters, biologists typically measure habitat quality for birds for a variety of purposes (Adamus et al. 1987, 1991, Bookout 1994): (i) ornithologists or ecologists studying the biology and ecology of individual species or groups; (ii) ecologists studying how birds are distributed in space and time; (iii) wildlife managers measuring habitat use or habitat quality to assess the effectiveness of some form of management; (iv) consultants selecting alternatives for habitat creation or restoration projects; (v) conservation biologists setting priorities for purchase or lease options to protect species, habitats, or communities; (vi) conservation or regulatory agencies evaluating the effectiveness of a wetland mitigation (i.e., replacement) project and subsequent monitoring of success in meeting goals; and (vii) regulatory biologists and population managers following trends in populations, measuring effects of harvest regulations, or evaluating environmental impacts,

Population size and density often are used as indicators of good or poor habitat, but such data have numerous biases and are expensive to obtain. However gathered, it is also essential to have measurements of habitat features to correlate with the population indices to provide some relative scale of quality. Measurement of habitat condition for a particular animal commonly is termed habitat suitability, but some workers advise against use of the word “suitability” as redundant (i.e., it either is or is not habitat) and recommend the word “quality” instead (Hall, Krausman and Morrison 1997). However, “suitability” is an integral part of a widely used evaluation system (see below) that describes a range of features attractive to and indicative of habitat quality for the species in question.