For long distance migrants such as this Whimbrel, which may have wintered along a coast in South America, the timing of their spring migration north to Arctic nesting areas is instinctive and the timing remains static regardless of weather conditions.
Snow and Ross’s Geese that winter along the Gulf Coast tend to begin migrating north as soon as weather conditions permit, stopping short of areas with ice and snow, then continuing northward as warmer weather produces open water and snow-free fields to feed in. Eventually, they make one last push into the Arctic to claim a territory at traditional colony nesting sites.
As North American birds begin their spring migrations northward from wintering areas, stopping at traditional resting and feeding areas for periods of time as they make their way to nesting ranges, our interest in bird migration is piqued. As we see the first migrants appearing and as we anticipate the waves of migrants that will soon follow, let’s examine some simple information about bird migration, especially spring migration, and what triggers birds to make these impressive and dangerous journeys from wintering areas to nesting sites.
Migrating birds can be divided into two groups: obligate migrants and facultative migrants, although these are not black and white divisions – many birds migrate using some aspects of both obligate and facultative migration strategies – but the two definitions are helpful in understanding what triggers a species’ migration.
For obligate migrants, the timing of travel is dictated by instinct. They begin migrating at about the same time each year, regardless of weather. An unusually warm or cool season won’t make them suddenly decide to change their departure dates.
Facultative migrants are more tuned in to the conditions of the moment. They have a standard time period for their migration, but they might alter the timing by days or weeks if the season is warmer than usual, or colder with snow cover later than usual. In short, they are more flexible. Read more
