Date of this Version
I have tried to present an adequate if far from exhaustive survey of the general biology, ecology, and behavior of all the included species, written so as to be understandable to the interested layman as well as useful to the biologist who might be looking for relevant literature citations or trying to deal with a specific question without resorting to extensive library searches. The larger number of species of North American hawklike birds (31, compared to 19 owl species) has required that I keep the individual species accounts substantially shorter. I have excluded the New World vultures from consideration, primarily because it is now increasingly apparent that these birds are not directly related to any of the other falconiform groups. I have similarly excluded a few vagrant Eurasian or Mexican species that, although they may rarely have occurred on mainland North America north of Mexico, are not yet (as of 1988) known to have nested within these limits. For species whose ranges extend to the United States, however, I have included all the generally accepted races occurring in Mexico, the Greater Antilles, and/or other nearby islands.
In common with my earlier books, I have provided anatomical drawings, measurements, keys, and plumage descriptions (these mainly adapted from those of Friedmann, 1950) to facilitate in-hand species identification and further to assist in aging and sexing living or dead specimens. Except for a few generalities, units of measurement have been presented metrically, or converted if necessary to that system. I have provided North American distribution maps and provided range descriptions (mainly adapted from the American Ornithologists' Union Check-list of North American Birds, but with substantial modifications) for all the included 31 species and their generally accepted subspecies. The taxonomic sequence, species-level taxonomy, and English vernacular names of birds are primarily those most recently (198}) adopted by the AOU. The listed subspecies are those of the fifth edition (1957) of the AOU Check-list, with additions and modifications based on more recent literature or as needed for my somewhat expanded geographic coverage. The nomenclature of nonavian terrestrial vertebrates follows Banks, McDiarmid, and Gardner (1987); that of other organisms follows various standard references.
Field identification criteria and a limited reference series of field-guide-type drawings have been included in this book, although it is not intended to substitute for a good field guide. Indeed, the field identification of many hawk species is one of the more difficult but most fascinating aspects of studying them, owing to their remarkable individual variability associated, for example, with age changes and plumage phases. However, a very good field guide to the North American falconiform birds has recently been published (Clark and Wheeler, 1987) that has an unusually complete illustrative coverage of such plumage variations.