Wednesday, November 7, 2007

The Flood from the North

The boreal forest is emptying out – I can see it at our feeders, and every night we see it in our owl nets.

Last year, the forests of eastern Canada provided an incredible bounty of food, in the form of mast – the broad term for nuts, berries, pine and spruce seeds. Many trees "mast" unpredictably, as a strategy for thwarting seed predators like birds and mammals; some years they bear very little, keeping predator populations low, then they will flood the market, so to speak, with vast quantities of mast, so that at least some of the seeds will survive to germinate.

As it happens, 2006 was a year when most of the species in the eastern boreal forest heaped the table with a feast – mast in great abundance. Many species of seed-eating birds, like finches, nuthatches and chickadees, stayed north.

Because the mast provided food for huge numbers of mice and voles, many northern owls also stayed home last fall and winter, and those of us who band migrant saw-whet owls down here in the eastern United States had our worst season ever.

But my, how things have changed. This fall, the trees of eastern Canada shut off the spigot; the mast crop was meager to nonexistent in many areas. Faced with poor prospects, many northern birds have been flooding south in a phenomenon known as irruption (the term is the opposite of eruption – a rushing into, as opposed to an explosion out of, something).

Irruptive birds include purple finches, pine siskins, red and white-winged crossbills, common and hoary redpolls, red-breasted nuthatches and pine grosbeaks. They also include many northern raptors, among them northern shrikes and a variety of owls. Unlike most migrants, which travel predictable routes at predictable seasons, the irruptive species strike like lightning, appearing and disappearing as though on a whim.

(This has nothing to do with the severity of the coming winter, by the way. The last really good winter finch year we had down here, back in the late 1990s, was also one of the warmest on record. It's all about food, not temperature.)

But with the food growing scarce, the birds are getting out of Dodge. Night after night, my crew of owl banders have been catching record numbers of saw-whet owls, up to 75 in an evening. These birds have been in exceptionally good condition (we check their fat stores and muscle mass), but if they'd stayed north, chances are they might have starved.

Birders here in the mid-Atlantic states are also excited by the arrival of large numbers of northern finches and other seed-eating species. We noticed a huge influx of black-capped chickadees and white-breasted nuthatches starting in late August, and in recent weeks additional boreal species have been coming in; this morning, purple finches and a red-breasted nuthatch, both irruptive species, joined the melee at our feeders.

This past week, there have even been growing numbers of reports of evening grosbeaks across Pennsylvania, a stunning species that was unheard of in Pennsylvania prior to 1890, but which began to invade the state in winter more and more frequently, and in greater and greater numbers, in the early decades of the 20th century. Many of us remember fondly their biennial invasions in the 1960s and '70s, when they would crowd feeders, sometimes by the hundreds, consuming vast quantities of sunflower seed and earning the nickname "gross-pigs" from birders who got tired of shelling out for so much food.

Then, they were gone. Since the 1980s, evening grosbeaks have been almost completely absent from our region. In this case, though, there is more at work than simply the boom-and-bust cycle of northern mast production.

Evening grosbeaks nest across the boreal zone from Newfoundland to British Columbia, and south in high conifer forests into southern Mexico. Across much of their range, a favorite summer food is a moth larvae known as the spruce budworm, a cyclical species that can reach plague dimensions in spruce forests.

Many boreal songbirds depend upon spruce budworm, including Cape May and Tennessee warblers. Like the grosbeak, their populations rise and fall in lockstep with the fortunes of the budworm. The last time we had lots of grosbeaks coming south into the eastern U.S. was during a spike in the budworm cycle in the 1960s and '70s, when millions of acres of across Canada were defoliated. But since then the caterpillars have been on the wane, and the populations of many of these budworm specialists have been on the decline as well.

So while the lack of mast this year is obviously driving the grosbeaks south, they will probably not come in the great numbers I remember from my childhood, because there just aren't that many grosbeaks these days.

There may be more at work with the grosbeaks than simply the budworm cycle; while research in eastern Canada suggests evening grosbeaks are strongly linked to budworm numbers, that doesn't appear to be the case in part of the West. And in fact, there is evidence that at least some of the irruptive grosbeak flocks that reached the East in the good old days were actually from the West, crossing the Plains and eating the seeds of box-elders, planted in shelterbelts across the prairies.

There is still a lot to learn about this beautiful finch, but for now, I'm just keeping my fingers crossed, hoping that a few will show up at our feeders.

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