Irruptive Migration of Bird Species
The main reason birds migrate is to ensure they have access to food all year round. For some bird species this may mean flying half way around the world, and their migration patterns are so predictable that birders arrange special events to welcome the weary travelers to their wintering ground, as well as to see them off when they depart. For centuries, farmers have looked to the departure and arrival of local birds as an indication of what the seasons hold, while mariners judged ocean currents and the nearness of land by the birds they encountered. Some migrating bird species are somewhat less predictable and these are referred to as irruptive migrants, with their migration habits being referred to as irruptive migration.
Some species may have a cycle of irruptive migration where they visit an area every two or four years and are therefore still predictable, while others are completely random. Reasons for irruptive migration are varied, but the most common cause is a lack of food in their normal wintering grounds. Birds that depend on birch, pine, spruce and maple seeds and catkins are known to irrupt when these trees produce poor crops. As their name suggests, crossbills have unique crossed bill-tips designed to pry conifer cones open and extract the seeds. They do not have the luxury of choosing an alternative food source and are obliged to find food they can access. As these seeds are also rodent food, if they fail to appear or produce an insufficient crop for demand, raptors may also leave the area in search of a more readily available food source.
Irrespective of what the motivating factors are for irrupting, it is virtually impossible to predict which species will irrupt in any given year and where they will migrate to, however the following species have been noted for regularly irrupting: pine siskins, bohemian waxwings, boreal chickadees, evening grosbeaks, purple finches, pine grosbeaks, common redpolls, red crossbills, white-winged crossbills, northern shrikes, hoary redpolls, red-breasted nuthatches, snowy owls, great grey owls and rough-legged hawks.
While birders generally welcome seeing unfamiliar birds at their backyard feeders, they should be aware that sometimes the unexpected visitors, particularly if they arrive in great numbers, can intimidate the locals in their quest for food. This can be overcome by setting out extra feeders and spacing them as far apart as possible. Also ensure that plenty of water is provided and water sources are cleaned regularly. Then sit back, relax, and enjoy the sight of feathered friends from far away.