Winter Bird Irruption

Many species of birds, including raptors, warblers, waterfowl, shorebirds and sparrows migrate twice each year, south in fall, north in spring. Not all birds migrate to the same places each year though. Some birds that breed in the boreal forests of the north come as far south as the Mid-Atlantic states only occasionally, after an absence of several winters. These irregular southward migrations are known as irruptions. Unusually large numbers of several of these “irruptive” species have reached Virginia this winter, including such beautiful birds as purple finches, red-breasted nuthatches, and evening grosbeaks.

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Purple Finch

Other irruptive species include boreal finches, like pine siskin, red and white-winged crossbills, common and hoary redpolls, and pine grosbeaks, as well as certain species of owls and raptors, such as rough-legged hawk, northern goshawk, and snowy owl. Black-capped chickadees also irrupt. These northern species rarely all come south in the same years, but it is not uncommon to have years when multiple species have large coordinated movements.

Irruptive birds move south when there is not enough food to support their populations in the north. The finches, red-breasted nuthatches and black-capped chickadees fly south because of bad conifer cone crops in the boreal forest. Snowy owls, on the other hand, seem to irrupt in years when their prey, lemmings, are particularly abundant. While these irruption triggers appear contradictory, they really are not. In both cases, relative scarcity of prey to predators causes a food shortage. The snowy owl population explodes from the plentiful food to the extent that the lemming population can no longer support them all come winter, so they come south in search of food.

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Pine Siskin. Photo by Drew Chaney.

Irruption is closely tied to a cyclical fluctuation in the seed crops of trees the birds depend on as winter food sources. Birches, alders and conifers in the boreal forests— all important food sources for irruptive songbirds — do not bear equal seed crops every year. “Mast” years when lots of seeds are produced are followed by several years of poor seed production. This appears to be an evolutionary strategy to limit the populations of creatures that eat the trees’ seeds to ensure the maximum possible number of seeds survive. In good seed crop years, seed eaters do well, and their population expands. When the next year’s crop is poor, there’s even less food for each animal than there would have been if the population hadn’t grown the year before, and the population falls. Then, when the next good year comes, there are not enough animals to fully capitalize on the seed crop, and some seeds escape. Oaks, hickories, and beeches in the deciduous forests of the East have similar cyclical fluctuations in seed production.

The winter of 2018-2019 is an irruption year for many species of boreal birds. Red-breasted nuthatches started moving south into Virginia as early as August. I saw my first of the season on August 19th in Sussex County. By September they were abundant throughout the state, with several migrating individuals being observed each day at the Rockfish Gap Hawk-watch in Augusta County. Starting in September and October, the nuthatches were joined by large numbers of purple finches and pine siskins. Numbers of these three species dropped significantly by December, but they remained present throughout the state. Many evening grosbeaks and even a couple of common redpolls have also been found in Virginia this year.

Few of the irruptive finches are as beautifully colored or as interesting as the evening grosbeak. Evening grosbeaks were fairly rare in eastern Canada over 100 years ago. Their population exploded about 60 years ago, which corresponded to a major outbreak of their preferred summer food — the spruce budworm. During the mid to late 20th century, they irrupted in huge numbers, sometimes with hundreds in Virginia at once. Since then, their population in the East has crashed, possibly due to habitat reduction from logging in the boreal forest, increased diseases, or decreased spruce budworm populations. As the evening grosbeak population has declined, they’ve irrupted in smaller numbers. So far, this winter has been relatively great for them though, with many having been seen at feeders around the state.

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Evening Grosbeak

Bird feeders are a particularly good way to see some of the beautiful finches that will be visiting our region this winter. Almost all of them like black-oil sunflower seeds. Pine siskins and redpolls — if we should be lucky enough to have more of them this winter in Virginia — particularly like nyjer or thistle seed. My brother and I recently put up a large platform feeder full of black-oil sunflower seeds in hopes of attracting an evening grosbeak, as they apparently prefer platform feeders to tube feeders. In all probability, we won’t see one on our feeder this winter, but you never know!

Sources

Pittaway R. (2014, September 20). Winter Finch Basics. Retrieved from http://www.jeaniron.ca/2012/winterfinches.htm

Pittaway R. (2018, September 20). Winter Finch Forecast 2018 – 2019. Retrieved from http://jeaniron.ca/2018/wff18.htm

Ehrlich P. R., Dobkin D. S., & Wheye D. (1988). Irruptions. Retrieved from https://web.stanford.edu/group/stanfordbirds/text/essays/Irruptions.html

Shaw D. W. Bird Irruption: A Sudden Surge of Birds. Retrieved from http://www.birdsandblooms.com/birding/birding-basics/bird-irruption-surge/

Cornell Lab of Ornithology. Evening Grosbeak Life History. Retrieved from https://www.allaboutbirds.org/guide/Evening_Grosbeak/lifehistory

Mortensen R. (2012, November 7). Birders Can Contribute to Evening Grosbeak Science Right Now! Retrieved from http://blog.aba.org/2012/11/birders-can-contribute-to-evening-grosbeak-science-right-now.html

Devokaitis M. (2018, November 6). This Could be the Winter You Get Evening Grosbeaks at Your Feeder. Retrieved from https://www.allaboutbirds.org/this-could-be-the-winter-you-get-evening-grosbeaks-at-your-feeder/

Blue Ridge Young Birders Club Field Trip to Rockfish Valley Trail Fall 2018

On October 14th I lead a trip for the Blue Ridge Young Birders Club to the Rockfish Valley Trail, a local birding hotspot in Nelson County.  I had high hopes for the trip, as the Rockfish Valley Trail tends to be a very productive place for finding uncommon and rare migrants in the fall, with Philadelphia vireo and Lincoln’s sparrows often present.  Only a few weeks previously I’d had one of the best mornings of birding in my life there, finding Philadelphia vireo and blue-winged, Connecticut and mourning warblers.  Around this time of year last year, my brother and I had two marsh wrens.  Six young birders attended the trip.

We walked under Route 151, doing our best to avoid getting our feet wet in the water overflowing from the South Fork of the Rockfish River.  The day was cool and cloudy but not unpleasantly so.  We encountered a large flock of song sparrows in a dense tangle of pokeweed and began scanning them for Lincoln’s.   We didn’t find any Lincoln’s in that flock but saw a few swamp sparrows.  As we were walking along a mowed path through a dense, brushy field, a tiny, bright yellow bird dropped out of a tree and into the grass.  Curious about what this could be so late in the year, we went to investigate.  The bird popped up onto a low branch of a black walnut tree for a few seconds, and I saw it was a Wilson’s warbler.

We continued around the loop towards the back of the field, where we encountered more sparrows.  I put my binoculars up to one and saw that it had a yellowish malar, gray supercilium and a yellowish breast covered in super fine, dark streaks — a Lincoln’s sparrow.  I think everyone got on the bird, although it soon hopped back down into the brush.

A flock of purple finches flew over and landed in the branches of a leafless oak.  We soon began hearing more purple finch calls, and several other flocks joined the first.  By the end of the day we counted 34 in small flyover and foraging flocks.  It was still early in the year for purple finches and seeing them in these numbers was encouraging for a good winter for them in our area.

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Purple Finch (Haemorhous purpureus)

As we rounded the bend in the path a flock of birds flew up into a tree.  One appeared to be a Catharus thrush, which my brother got on and said looked like a gray-cheeked.  We slowly crept to the other side of the tree so as not to scare the thrush again and saw that it was indeed a gray-cheeked thrush.  We re-found the Wilson’s warbler and a late Eastern wood-pewee nearby.

Once we got back to the top of the loop where we first saw the Wilson’s warbler, we paused for a bit to listen.  I heard a yellowthroat chipping in a large patch of common mugwort, so I went to investigate.  I found the yellowthroat and a couple of palm warblers, and I was just about to return to the group when Baxter called that he thought he had a Connecticut warbler.  The group assembled behind him and we slowly advanced towards the bird, which was entirely obscured by the dense mugwort.  I got a brief glimpse of the bird through the vegetation and saw a large warbler with a pale gray hood and a thin eye ring.  Suddenly the bird lifted off and flew to the end of the mugwort patch, followed a second later by a similar looking bird.  There were two of them!  Chaos ensued as everyone tried to see the birds while we debated their ID’s.  Eventually we cornered the two birds in a corner of the mugwort patch, and everyone got a decent look.  Their eye rings, although fairly extensive, were not complete, making them mourning warblers, not Connecticuts.  Finding two of them was still extremely exciting, and it was a Nelson County high count.  We photographed a beautiful blue-headed vireo in a willow along the river on our way back towards the cars.

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Blue-headed Vireo (Vireo solitarius).  Photo by Baxter Beamer.

The next location for the trip was State Route 610, a quiet mountain road that can sometimes have good migrants.  When we arrived the area was totally quiet.  We had to walk down the road for five minutes before we even heard a bird, which was a pileated woodpecker.  I did not give up hope, as I’d birded up here many times before when it first appeared very slow but then incredible bird activity occurred in explosive bursts.  Sure enough, a few minutes later the forest suddenly rang with a cacophony of warbler chips, and birds could be seen moving in every tree.  The vast majority of the warblers were blackpoll, but we also found several other species including Tennessee, Cape May, bay-breasted, pine and black-throated green.  Kinglets were also present in high numbers — we had fifteen golden-crowned and five ruby-crowned on a small stretch of road.  I spotted a red-eyed vireo, which was beginning to get late, as well as another blue-headed vireo.

Rockfish Valley Trail and State Route 610 did not let us down!