The master signal for migration is photoperiod — the changing length of the day. As days lengthen in late winter, hormonal changes prepare birds to breed and to travel; as they shorten in late summer, the same internal clock pushes them back toward warmer wintering grounds. Because day length is the same every year, it is a far more reliable cue than weather, which varies. Food availability and temperature then fine-tune the timing.
Migration is enormously demanding, so birds fuel up first. In the weeks before departure many species enter hyperphagia, eating heavily to lay down fat — the energy that powers flight. Some shorebirds nearly double their body weight, then burn it down over thousand-kilometre non-stop flights. This is why stopover sites, where birds rest and refuel, are as vital to a successful migration as the breeding and wintering grounds themselves.
A great many songbirds migrate at night. The cooler, calmer air is easier to fly through, predators that hunt by sight are less active, and the stars give a stable compass. Nocturnal migrants take off shortly after dusk and climb to hundreds or thousands of metres; on big nights, the flow is dense enough that weather-surveillance radar picks up the birds as a bloom spreading out from the ground after sunset.
Weather still rules the calendar night to night. Migrants ride favorable tailwinds — warm southerly winds in spring, cold fronts with northerly winds in fall — and hold back when conditions turn against them. When good flying weather follows a delay, a backlog of birds can pour through all at once, producing the spectacular ‘fallouts’ that birders chase.