From page 7 of Hawks from Every Angle:
Several factors influence the migratory pathways of raptors, including geography. Mountainous sites are famous for attracting concentrations of migrating hawks, in part because as wind strikes a ridge it is deflected upward, providing birds with a current of air—or updraft—along the ridge for more energy-efficient means of travel. Raptors can travel along these updrafts for miles, covering great distances with little to no effort.
Hawks from Every Angle: How to Identify Raptors In Flight
Foreword by David A. Sibley
Identifying hawks in flight is a tricky business. Across North America, tens of thousands of people gather every spring and fall at more than one thousand known hawk migration sites—from New Jersey’s Cape May to California’s Golden Gate. Yet, as many discover, a standard field guide, with its emphasis on plumage, is often of little help in identifying those raptors soaring, gliding, or flapping far, far away.
Hawks from Every Angle takes hawk identification to new heights. It offers a fresh approach that literally looks at the birds from every angle, compares and contrasts deceptively similar species, and provides the pictures (and words) needed for identification in the field. Jerry Liguori pinpoints innovative, field-tested identification traits for each species from the various angles that they are seen.
Featuring 339 striking color photos on 68 color plates and 32 black & white photos, Hawks from Every Angle is unique in presenting a host of meticulously crafted pictures for each of the 19 species it covers in detail—the species most common to migration sites throughout the United States and Canada. All aspects of raptor identification are discussed, including plumage, shape, and flight style traits.
For all birders who follow hawk migration and have found themselves wondering if the raptor in the sky matches the one in the guide, Hawks from Every Angle—distilling an expert’s years of experience for the first time into a comprehensive array of truly useful photos and other pointers for each species—is quite simply a must.