Irrupting Birds

Every winter birdwatchers hope for an irruption of boreal birds from the northern forests. This “irruption” or irregular migratory movement southward of birds that ordinarily live and breed in Canada and Alaska include glamour species such as pine and evening grosbeaks, purple finches, red and white-winged crossbills, pine siskins, common and hoary redpolls, red-breasted nuthatches, snowy owls, northern shrikes, northern goshawks and rough-legged hawks.

The songbirds are dependent on the seeds of conifers and a few hardwood species, mainly alder and birch, and when the seed crop fails, as it does periodically, and bird numbers are high, they are forced to head south in search of food. The same is true of the birds of prey, including the meat-eating northern shrikes, all of which prey on lemmings, voles, or snowshoe hares or a combination of all three. When those populations crash, their predators must also migrate south in search of food.

Some of these movements have been absolutely stupendous. One flight of red-breasted nuthatches over Fire Island Beach, New York, as recorded by William Dutcher back in 1906, lasted from September 21 to 23. “At the height of the migration,” he wrote, “nuthatches were seen everywhere–on the buildings, on trees, bushes and weeds and even on the ground…Every tree had its nuthatch occupant, while many of them evidently found food even in the bushes and larger weeds. On a large abandoned fish factory at least 50 of these birds were seen at one time.”

In the 25 years we have lived on our central Pennsylvania mountaintop, we have had occasional visits from northern shrikes, evening grosbeaks, purple finches, white-winged crossbills, and northern goshawks, but we have only witnessed the irruption of three songbird species–pine siskins, common redpolls and red-breasted nuthatches–and all within the last nine years. The pine siskin irruption years were 1987-88, 1989-90, and 1995-96, common redpolls appeared in 1993-94 and 1995-96 and red-breasted nuthatches in 1995-96.

The pine siskin irruption in 1987-88 was one of the largest in living memory. At least 95 million siskins appeared at feeders all over North America.

I well remember my first sighting of the small, brown-streaked, sharp-billed birds. On October 26, 1987 at the Far Field I heard and then saw a flock of 20 pine siskins eating black birch seeds from a small tree at the edge of the field. Pine siskins (Carduelis pinus) both sound and fly like American goldfinches (Carduelis tristus) to whom they are closely related. But they are bolder than goldfinches and ignored me as I crept close and sat on the ground to watch them. After 15 minutes they whirled off.

A snowstorm in early November brought them to our feeders for the first time. I looked out almost in disbelief as more than 80 siskins descended, settling on saplings, the ground, back steps, porch floor and feeders. But they flew off in a few minutes.

Throughout the winter during stormy days the siskins came as a body to the feeders–up to 100 at a time–and gobbled up sunflower seeds. But since it was a mild winter, they spent most of their time in the forest eating black birch seeds. And I spent a lot of time watching them. On a sunny December day at the Far Field thicket, 60 of them twittered softly as they fed in a black birch tree. Then most of them flew down to a fallen tree trunk to eat snow. Fifteen of them lined up almost beak to beak. Others ate snow from tree branches and on the ground. After that they returned to birch seed eating.

At the end of February I found 80 siskins running over the snow-free ground at the base of Sapsucker Ridge. The golden wingbars of some of the males were prominent and in addition to their usual goldfinch-like calls, they also made buzzy sounds that resembled those of blackbirds. Occasionally they swooped up into saplings in response to warning calls, but I never did see what startled them. Mostly, though, they ran over the ground feeding on fallen tree seeds, sometimes coming within ten feet of where I was standing.

I continued to see them in the woods until the end of April and then they were gone, off to nest in high altitude coniferous forests from Alaska east to Newfoundland and as far south in the eastern United States as northern Pennsylvania. Now siskins are one of Pennsylvania’s rarest breeding birds, but ornithologists think that they were more abundant here before our coniferous forests were cut.

During the 1989-90 siskin irruption, they spent more time at our feeders. Otherwise, I found them in the hollow eating hemlock seeds with American goldfinches. For sheer entertainment at the feeders that winter I couldn’t beat the antics of pine siskins. Although there were less of them (40) than in 1987-88, they totally dominated the feeders whenever they came in, even pushing aside the hoards of house finches. These littlest creatures on the feeders threatened every bird that came too close by running toward them, sharp beaks open. In addition, the males flashed their yellow wing patches like caution lights. House finches, goldfinches, dark-eyed juncos, American tree sparrows, black-capped chickadees and even tufted titmice fled.

The siskins gobbled pounds of sunflower seed every day. One of them even blundered inside the tube feeder when I forgot to close the top after refilling it. I found the bird flopping around, trapped by its own piggishness. When I reached in to lift it out, it seemed to understand what I was doing and did not struggle in my hand as most birds do nor did its heart beat any faster. Siskins, it seems, are too self-confident and scrappy to be scared of a mere human.

Other observers have found them to be downright tame. Back in 1925 Edwin Russell Davis of Leominster, Massachusetts had 100 pine siskins at his feeder throughout the fall and winter. “Their extreme tameness,” he wrote, “made them easy to photograph, the only adverse circumstance being their insatiable curiosity, for no sooner would I appear with my camera than they would perch on it…Whenever I would appear at the window, or step outside the door, down they would come and, settling upon my head, shoulders, and arms, would peer anxiously about for the food that they had learned to know I held concealed from them in a box, dish, or other receptacle.”

Common redpolls (Carduelis flammea), in the same genus as goldfinches and siskins, are similarly unafraid of humans. They also like birch seeds and breed even further north than siskins–from the southern edge of the Arctic tundra south into coniferous forests from Alaska to Newfoundland. They and their close relatives, hoary redpolls (Carduelis hornemanni), can survive colder temperatures than any other songbirds, probably, in part, because of a special storage pouch in their esophagus which they fill with food just before night falls and then digest over night.

So winter-hardy are they that Maurice Braun, when he was Curator of Hawk Mountain Sanctuary back in 1947, reported watching 300 common redpolls bathing and wading in icy brook water. “These are the only birds that I have ever seen bathing–really soaking–in mid-winter,” he wrote.

Both species have bright red caps on their foreheads (“redpoll” means “red cap”) and black chins and male common redpolls have pink breasts as well. While hoary redpolls’ rumps and breasts are frosty-white, common redpolls are brown-streaked as are the backs and wings of both species.

I have never seen a hoary redpoll and until the 93-94 irruption had seen only an occasional common redpoll in First Field over the previous 22 years. So, on January 10, 1994 I could hardly believe my eyes when, in the midst of the house finches, I spotted a nervous common redpoll on the outskirts. Within a couple hours it was joined by seven others. All through the bitter cold and snow of January and part of February I could expect to see as many as ten but their last day at the feeder was February 18.

Having waited so long for my first redpoll irruption, I was amazed to witness another one last winter. Four common redpolls first appeared at the feeders on December 9, 1995 when the thermometer stood at seven degrees. The pine siskins had already been in since November 12, but their numbers were low. Fourteen was the highest count we had all winter. Both species came and went irregularly until the middle of March with common redpoll numbers peaking on March 9 at 32.

The redpolls proved to be as feisty as the siskins. One afternoon I watched a pugnacious redpoll hold the wooden bird feeder against all comers. In less than a minute it chased each interloper that landed by rushing at it and chittering loudly–downy woodpecker, tufted titmouse, black-capped chickadee, dark-eyed junco, and American tree sparrow, all of which fled in the face of its naked aggression.

Unlike the siskins and redpolls, the red-breasted nuthatches were peaceful in their close association with a huge flock of black-capped chickadees among the hemlocks along our hollow road. They also are not flocking birds during winter irruptions. Instead they live solitarily or in small stable groups on relatively small home ranges (about 15 acres). Even though home ranges may overlap during an irruption, they exhibit no aggression toward each other and, unlike resident red-breasted nuthatches, pair bonds are not important.

Smaller than white-breasted nuthatches, red-breasted nuthatches have a broad black line through their eyes with a white line above it and rusty-colored breasts. They live mostly in coniferous forests and eat pine and spruce seeds.

Here in Pennsylvania they will nest in Norway spruces and other ornamental conifers and were more common nesters before our Black Forest was cut. During the Pennsylvania Breeding Bird Survey one of the biggest surprises was the discovery of confirmed nesting in a wide range of coniferous tree plantations as far as the Maryland state line and west to Ohio.

Not only do they eat the seeds of conifers, they also use conifers’ pitch to smear around their nest hole entrances, probably as a defense against predators. According to William Brewster of Maine, “they brought it on the tips of their bills in little globules, alighted against the lower edge of the hole, and then tapped it on in various places as low as they could reach, but without shifting their foothold.”

Another behavioral pattern, one that they share with white-breasted nuthatches, is the ability to cache food. Usually they cache it under tree bark or in cracks and other interstices. But in Montana ornithologists watched a male red-breasted nuthatch flip through needle litter on the ground. Next he flew to the upper portion of a steep dirt cut bank carrying a pine seed. He probed in the dirt five or six times and he put the seed in the ground as deeply as the length of his bill. Then he picked up a small pebble and tamped it into the hole with a few beak jabs and repeated the same action with a similar-sized second pebble before flying to a nearby pine and foraging on the tree trunk.

My own observations of red-breasted nuthatches were not so dramatic. During the last week in December and first two weeks in January I found a red-breasted nuthatch every time I walked down the hollow. My first sighting occurred on a windy, cold December 21. It was silent in the hollow until I reached the hemlocks. They were filled with a merry band of chickadees eating hemlock seeds. As I stood watching them, I was thrilled to see a red-breasted nuthatch land on a hemlock trunk about six feet from me. After giving me the longest, closest view I had ever had of a red-breasted nuthatch, it flew to a fallen log spanning the stream and foraged beneath it. The red-breasted nuthatch was as quick and energetic as the chickadees, flitting from tree to tree, up, down and around at twice the speed of a white-breasted nuthatch.

During a red-breasted nuthatch irruption in the winter of 1972-73 in New Hampshire, ornithologist Lawrence Kilham also watched red-breasted nuthatches associate with chickadees in hemlock trees. Both ate hemlock seeds but only on dry, cold and windy days when hemlock cones opened. On warmer and more humid days the cones closed and the birds foraged on hemlock seeds that had previously fallen to the ground.

The hollow chickadee flock broke up by mid-January when the hemlock seed crop was exhausted, but we still had red-breasted nuthatches in the forest until May. I even watched a pair eating Norway spruce seeds in our own small plantation in late April and wondered if they would set up housekeeping there. So far, though, I have not discovered a nest.

I was not the only person in our area who watched red-breasted nuthatches. Most of the birdwatching members of our Juniata Valley Audubon Society similarly found them in their yards, woods, and even at their feeders. The same was true for common redpolls and pine siskins.

While last year’s irruption was weak in numbers it was the richest we have had in species’ diversity. To look out at my feeders and see both pine siskins and common redpolls and to walk down our hollow road on a cold, windy day and watch red-breasted nuthatches was a joy and a privilege that brightened what turned out to be the longest winter of our lives.