From page 10 in Hawks from Every Angle:
During spring, many peak flights occur ahead of a warm front, as birds heading north use southerly prevailing winds. Most fall flights occur after the passage of a cold front, when northerly winds that assist birds heading south are most prevalent. Geography, however, determines which specific wind directions will lead birds to each site. The most favorable winds for ridge sites are those that strike the ridge at an angle that produces optimal lift. At coastal and shoreline sites, optimal winds are those that “push” birds towards the shorelines. Even during snow squalls or light drizzle, optimal wind conditions can produce significant hawk flights.
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 at 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, including plumage, shape, and flight style traits, are discussed. For all birders who follow hawk migration and have found themselves wondering if the raptor in the sky does in fact match 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.