The clear cold weather is upon us. The abundance of bird species has dwindled down to the few hardy souls who’ll stick it out with us in the cold temperatures to come.
Winter means lower temperatures, fewer hours of daylight, and stronger winds. Most birds who spent the warmer months of the breeding season at the higher and middle latitudes have migrated south where finding food is easier.
Migration is carried out by most birds in North America. About 318 North American landbird species migrate to tropical regions of Central and South America.
Migration can take several forms including short distance , long distance, and altitudinal migration.
Of the birds which brave the cold, some stay in an area as long as the food is readily available. They may fly around the in general area but they don’t have to migrate. Or they may migrate short distances (partial migration) of a mile to ten miles away to a food source.
In short distance or partial migration, the entire population of a bird species moves a short distance from the breeding range to winter homes. The distance is usually less than 1.2 miles (2000 km).
In partial migration, some individual birds remain in the breeding grounds while others move to lower altitudes. Adult males often stay close to the breeding range. Staying close to prime breeding and nesting sites makes it easier to claim choice areas in the spring. Females and young birds may move further south.
When I think of short migration, I think of the local American Robin population. The Robins which live in my city neighborhood, move to a local woodland a little over a mile away. The woodland has a year-round running stream, many fruit-laded trees and shrubs, and insect eggs.
Importance of the Boreal Forest and Bird Irruptions (Video)
Ducks and Geese and Partial Migration
You know how you see a flocks of ducks and geese one day and they disappear a day or two later? Ducks and geese migration short distances regularly throughout the winter.
The surface of a body of water freezes in winter from the edges toward the center. As the unfrozen spot grows smaller, the ducks and geese will crowd onto the still open areas of water.
As the open space becomes too crowded they disappear overnight. They are flying to another body of open unfrozen water. They’ll fly south until they find suitable unfrozen water.
If the water unfreezes or conditions improve, the ducks and geese will fly back (often at night) to their previous winter spot.
Long distance migration means the entire species moves more than 1.2 miles (2000 km) often to a different continent. In the case of North America that means moving to Central or South America. We can think if many bird species which fly south for the winter. All the warblers come to mind. But, we are focusing on winter migrations.
Altitudinal migration of a species population moves from higher elevation to lower ones. I think of the Black-capped chickadees which arrived in my area with the fall season.
The chickadees have spent the breeding season in the northern areas of North America. They migrated to lower elevation areas. Philadelphia is at very low elevation. The highest point in the city is just 440 feet above sea level. The lowest point is one or two feet below sea level.
Irruptions and Winter Bird Migration
Migration is a predictable annual event. We know migration will happen each year in a particular season. Irruptions events are unpredictable mass migrations. And the numbers of birds which migrate vary from one winter to the next. An irruptive event is called a flight year.
Perhaps, you wake up one morning and there is a flock of unexpected species gathered at your winter bird feeders. The flock is made up of birds of the far north that you rarely get to see.
Several bird species of the northern boreal forests sometimes appear in more southerly backyards during winter. The species fall into two categories, seed eaters and mammal-eating raptors.
The Irruptive Seed Eaters
The seed-eaters are those species which live in the northern boreal forests of the United States and Canada.
The various species feed on the seeds of trees, flower seed heads (especially composite flowers) and shrubs. Redpolls and goldfinches mainly eat grass seeds.
When seed production is plentiful, bird populations increase and remain in their normal winter ranges.
Birds can tell in late summer and autumn, when seeds production is not what they need to survive the winter. They evacuation when they decide the time is right. The seed eating birds move to areas where food is still available. This is when we see these species moving into our neighborhoods, roadsides, and backyards.
The poor boreal seed production can happen over a wide area of the far Northern regions of the Earth. Irruption events are often the same in both North America and Eurasia coniferous (boreal) forests.
Check out the FInch Network’s Irruption Prediction https://finchnetwork.org/
Here is a link to the 2020-2021 Winter Finch Forecast https://finchnetwork.org/winter-finch-forecast-2020
Which Northern Species Take Part in Winter Bird Irruptions?
As you can see from the following list the irruptive species are small, passerine, seed-eaters. Some species irrupt more often than others depending on their boreal forest food sources.
Look out for these birds around your feeders in winter.
- Black-Capped Chickadee (seeds, arthropods; erupts only in the northern part of its’ range)
- Bohemian waxwing (berries, especially Mountain Ash)
- Boreal Chickadee (conifer seeds, arthropods)
- Common Redpoll (birch and weed seeds)
- Evening Grosbeak (conifer, box-elder, and ash seeds)
- Golden-crowned Kinglet (insects)
- Pine Grosbeak (berries, ash, conifer seeds
- Pine Siskin (birch and alder seeds)
- Purple Finch (bud and seeds)
- Red-breasted Nuthatch (pine and spruce seeds)
- Red and White-Winged Crossbills (conifer seeds)
Not just small seed-eating bird irrupt and move from their far north Arctic and Boreal forest homes.
Raptors (birds of prey) which feed on small mammals and small birds may also have to leave their winter ranges in boreal forests to survive. Some of those raptors may also eat the small birds which irrupt and leave. Those raptors would be following their food.
Arctic rodents experience three to five year cycles of abundant and die-off in their populations. Perhaps this has to do with an increase in populations until the areas can no longer support the population numbers.
Larger raptors which feed on snowshoe hares irrupt in response to the hares approximately ten-year population cycle. Boom or bust. As with the small rodents, the increased numbers hares probably can’t find enough food in their home ranges. The hare population declines and the larger hare-eating raptors must find more food.
What raptors might we see in irruption years? Snowy Owls are the owls most often seen during irruption flight years. Snowy Owls and Short-eared owls venture south in irregular numbers each year. These raptors will return north in the spring and settle wherever their food source is plentiful.
Boreal and Northern Goshawks will irrupt but often stay in the range of the boreal forests.
The larger raptors like Great-horned Owls and Northern Goshawks may irrupt for three years or so as the Snowshoe hare population recovers to numbers which can support the raptors need for food.
The Irruptive Raptor Species
- Snowy Owl (lemmings, voles, hares)
- Great Grey Owl (lemmings, voles, hares)
- Great Horned Owls (small mammals, hares)
- Northern Hawk Owl (lemmings, voles, birds)
- Northern Goshawk (birds, hares, lemmings, voles)
- Rough-legged Hawk (hares, voles, lemmings)
- Northern Shrike (a passerine, not a raptor that feeds on mice and small birds)
I have wanted to learn about irruption for sometime. When I take part in the Christmas Bird Count in the future I’ll be better equipped to identify irruptive species.
Have you seen some of these birds at your winter feeders or in your neighborhood?
I have had some of these birds like Pine Siskins and Purple Finches at my Philadelphia winter feeders in the past. Now, I can’t wait to watch my feeders for signs of irruptive species.
See also: Winter Birding and How to Master It