Strange Visitors from the North: Irruptive Migrants

By Gabriel Willow

[b]White-Winged Crossbill[/b][br]© Ken JanesWhite-Winged Crossbill
© Ken Janes

"...there was greate plenty of strang birds, that shewed themselves at the time the apples were in full rype, who fedde upon the kernells onely of these apples, and haveinge a bill with one beake wrythinge over the other... The oldest man living had never heard or reade of any such like bird..." [Ian Newton, Finches, 1640]

Close observers of the natural world have always followed patterns of bird movement and migration with interest. Mariners looked to birds as indicators of oceanic currents and the proximity of land. Farmers noted seasonal disappearance or reappearance of certain species as harbingers of seasons to come. Naturalists were once mystified by some birds’ seasonal absence (Aristotle believed that swallows hibernated in the mud of ponds like frogs, and ascribed the similar seasonal disappearances of doves and kites to hibernation as well). Even more mysteriously, amateur naturalists and scientists alike have been puzzled for centuries by the occasional sudden arrival of unfamiliar species from lands unknown.

The majority of bird species undergo some form of seasonal movement. The most familiar type of movement is migration, where a bird species travels in a predictable annual, seasonal fashion (in our hemisphere this is generally south for the colder months and north for the warmer months). People often think birds migrate to escape the cold, but this is only true in the broadest sense. Birds are incredibly cold hardy (think of penguins surviving in the coldest places on Earth). More specifically, birds migrate to ensure continued access to their food. This is why soil-probing and insectivorous birds are typically the first to depart as the weather cools: they have the furthest to fly, and their food will be entirely inaccessible in the winter (with a few notable exceptions, such as woodpeckers, which due to their specialized feeding techniques can access beetle larvae in trees all winter).

This classic type of migration, where the entire population of a species moves seasonally from a breeding range to a wintering range, is known as complete, or obligate, migration. There are also some species in which portions of the population migrate and others remain on their breeding grounds. This is known as partial migration. Tufted titmice and blue jays are well-known examples of partial migrants. Our local NYC blue jays generally remain year-round, but anyone who has participated in a migratory bird count in the fall will have noticed dozens or even hundreds of jays flying south in this area as well; these are generally individuals from more northerly populations heading southward in search of more abundant sources of food.

There is a third type of seasonal movement, found among certain nomadic species of birds, typically species of the far north. These mysterious birds appear in our region in great numbers in certain winters, only to disappear again in the spring; years or even decades may pass before the species is sighted again. These species are known as irruptive migrants, and 2012 is shaping up to be one of the most remarkable irruptive years in recent memory.

Irruptive migration is not seasonally or geographically predictable; it is not a true form of migration. For irruptive migrants, in contrast to regular migrants, site fidelity is low, and few individuals return to the same breeding areas in successive years. Having neither a fixed breeding ground nor wintering ground, these species are the vagabonds of the bird world.

Notable irruptive species are many northern finches, such as redpolls and crossbills; waxwings; and northern birds of prey such as snowy owls and rough-legged hawks. They undergo their periodic irruptions for a variety of reasons, but all are tied to food shortages. The availability of food is irregular, and not tied to season, hence the irregular migratory behavior of these species.

[b]Snowy Owl[/b][br]© Steve NanzSnowy Owl
© Steve Nanz

In a classic example, snowy owls largely rely on lemming populations for food, and when the lemmings undergo their periodic population explosions, the owls have plenty of food and raise larger broods of young. When the lemming population subsequently crashes (not, as popularly believed, due to mass cliff-jumping), the owls are forced to wander hundreds or even thousands of miles in search of food, and wander unpredictably when this occurs (a banded snowy owl in Europe was found over 1,200 miles from its previous wintering location in a subsequent winter).

[b]Pine Grosbeak[/b][br]© Jean-Guy DallairePine Grosbeak
© Jean-Guy Dallaire

The irruptive northern finches, like most finches, are seed-eaters (although some eat fruit as well: Pine grosbeaks are very fond of hawthorn berries and overwintering apples). The most specialized species are the crossbills. Their unique crossed bill-tips are tools evolved to pry open conifer cones to extract the seeds beneath the scales. White-winged crossbills have more delicate bills and prefer smaller cones such as hemlock, larch, and spruce; red crossbills have varying bill-size depending on subspecies but can usually pry open bigger, tougher cones, such as those on true pines. Pine siskins and pine grosbeaks, as their names indicate, are also tied to conifers and their cone-crops.

Conifers undergo periodic bumper cone-crops and also years where they produce none. This can vary from species to species and region to region, and the underlying causes are poorly understood. Regardless of the cause, when cones are in short supply, finches and other cone specialists must travel in search of food.

This year has been an incredible example of this phenomenon: Red and white-winged crossbills have been sighted in almost all larger city parks and cemeteries with suitable coniferous trees. Red-breasted nuthatches and pine siskins, both conifer-seed specialists that are generally rare in the New York City area have been near-ubiquitous this year. On a single morning in October, over 20,000 Pine Siskins were seen flying offshore from a bird observatory in Fire Island. In the past few months, they have been spotted throughout the US, as far south as Florida.

Of course, pine cone crops and lemming population fluctuations may or may not be linked (although they could be responding to shifts in climate or precipitation for example); thus a raptor irruption year may not coincide with a finch irruption year.

But with snowy owls already being spotted on Long Island and red crossbills in Central Park, this may be one of those wondrous winters for bird-watchers, when these peripatetic creatures from the north alight on a sand-dune or pine-top near you. It’s an added incentive to get outside on a cold winter day!

One great way to find some irruptive migrants and other winter birds is to join NYC Audubon on one of our winter birding tours, such as our trip to Floyd Bennett Field and Fort Tilden on January 26, 2013. For more details on all of our upcoming trips and classes, visit our Events & Adventures pages.

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