Spring (late February to mid-May): Spring works its way up from the desert lowlands to the sky island mountains between late February and May. Wintering birds begin leaving the lowlands in late February, and though many resident desert birds are nesting in March the overall birding experience is best between mid-April and mid-May, when spring migration kicks into high gear as millions of songbirds make their way north. Nesting specialties such as trogons, warblers, and most hummingbirds and flycatchers are usually present by mid-April. Owling is usually most productive from late March through May, before family responsibilities distract the birds from vocally advertising their territories. Spring weather is as variable here as anywhere. Typical conditions are cold to cool nights and cool to warm days, often with high winds, but late winter storms can bring cold rain to the valleys and snow to the mountains as late as early May.
Dry Season (May to early July): In Arizona, summer is really two distinct seasons. Most “tropical” specialties are present and nesting during the dry season, but it can be dangerously hot at midday in the lower elevations. If your visit falls between late May and early July, be prepared to get out by 6 AM, birding the lower elevations first then retreating to the higher elevations during the heat of the day (or take a siesta to prepare for owling).
Rainy Season (mid-July to mid-September): The rainy season, which usually begins by mid-July, is characterized by powerful but localized afternoon thunderstorms which often develop with amazing speed. A rapid greening follows the rains, and many local birds delay nesting or nest a second time to take advantage of this short-term bounty. Mexican species often wander up from the south on the wet winds, while from the north come southbound migrants such as Lazuli Bunting, Yellow-headed Blackbird, American Avocet, and Baird’s Sandpiper. Since the rains produce a bounty of wildflowers, this “second spring” is also the time of greatest diversity and abundance of hummingbirds and butterflies. Hummingbird diversity usually peaks between late July and mid-August, while butterfly diversity is often better between late August and late September. A few early migrants, including Sulphur-bellied, Cordilleran, and Buff-breasted flycatchers, are already headed south by early September. In most years the regular afternoon storms bring a welcome drop in temperature just as the heat is building to uncomfortable levels, and there is a jump in bird activity as soon as the storm passes.
Fall (September to early December): September is fair to good for birding in the lowlands, particularly around wetlands as waterfowl and shorebirds stream southward. Mixed flocks of songbirds, particularly warblers, can sometimes be found in the higher elevations through mid-September, but birding in the mountain canyons is often disappointing by early October (butterfly diversity is usually good well into October). Birding usually remains relatively quiet until November, when early storms push good numbers of sparrows, raptors, and Sandhill Cranes southward. The fall colors of aspens, cottonwoods, sycamores, and maples from late October through late November will keep the eyes busy even when the birds are few and far between. Fall weather is typically lovely, with mild, sunny days and crisp nights, but tropical storms and early winter storms can bring heavy rain to the valleys and snow to the mountains.
Winter (December through February): Though most of the “tropical” specialties are absent, diversity remains high in winter; the Ramsey Canyon (San Pedro Valley) Christmas Bird Count regularly has one of the highest inland totals in the U.S., with 150+ species. Lowland areas, including the Sonoran Desert around Tucson, the San Pedro River, and the Sulphur Springs Valley, support the greatest variety of both resident and wintering birds. Arid grasslands and desert scrub support raptors and large flocks of sparrows, while wetlands host waterfowl, cranes, and a few shorebirds and songbirds. By contrast, the mountain canyons can seem eerily quiet in winter as even resident birds move to lower, warmer elevations. Winter days are usually sunny and mild, but occasionally an arctic storm system will bring cloudy skies and rain or snow.