Friday, February 15, 2008

An Irruption of the Natural World

Red-breasted Nuthatch, photo by David Powell

It was along the Rio Grande, in mid-August, that I saw my first Red-breasted Nuthatch heralding the onset of fall migration 2007. With his characteristic, “Yank, Yank” call, there was no mistaking this bird for anything else. A careful scan of the nearby cottonwoods revealed the little fellow climbing head first down the tree trunk, typical behavior of all nuthatches, picking the choicest of insect morsels from the bark. With a dark gray back, a bold white eyebrow, and brilliant red breast, the Red-breasted Nuthatch is the most colorful of the three North American nuthatches. The other two nuthatches found in New Mexico, are the smaller Pygmy Nuthatch, and the larger White-breasted Nuthatch.

It seemed a little early, but perhaps that was due to the unseasonably warm temperatures that tricked my body and mind into thinking that summer would stretch deliciously on for many more months. When I announced my observation to the rest of the songbird department later that day, they also had a few observations of Red-breasted Nuthatch. Typically a bird of coniferous forest, and one that remains on or near its breeding grounds year-round, we generally detect only a handful of individuals during migration. As days progressed though it began to appear that a veritable herd of little red-breasted birds were making their way south.

I noticed a change in bird numbers and behavior at the cabin too. Following a summer with lower than normal feeder activity, September was marked by packed feeders and what appeared to be voraciously hungry birds. There didn’t seem to be any one dominant species, rather a wide assemblage, all foraging together. The big feeder that holds about 10 pounds of black oil sunflower seemed to be empty every few days. The suet disappeared at record pace. Pine Siskins stuck to the thistle sock like frenzied ants whose nest had been disturbed, aggressively pecking at any errant newcomers vying for a feeding position. All birds seemed to share in a universal need for sustenance, feeding as if they had never before or would never again comfortably fill their bellies.

Being the responsible bird nerd, the seed bills began to mount. First it was the extra 20 pounds of thistle, then the 50 pound bag of black oil sunflower, and the 25 pound bag of raw peanuts in the shell. Suet began to be purchased by the case, and it seemed that every week at least one of the staples in my bird kitchen was missing. The wild bird store received increasingly larger checks, first $25-$40, then rising to $60-$90. Just like a teenager, my feathery friends were never full.

I began to hear stories from afar in early October. Apparently, the change is not unique to New Mexico. From Nebraska, a story about a mountain of Mountain Chickadees hanging out in the grasslands. From Colorado, it was nuthatches arriving in the eastern plains from more mountainous environs. Just what might be going on to drive all these birds to seek new wintering grounds?

Irruptions, or mass movements of certain animals including birds and mammals, to lower altitudes or more temperate climates, are thought to be caused by mass food shortages. These events occur irregularly, likely in response to the failure of a major food crop such as coniferous cones, or other seeds.

Years with high crop production for birds result in greater nesting success and adult survival, thus increasing overall populations. If a crop failure occurred in the year immediately following one of abundant food resources, then the bird populations might be larger than normal, adding pressure on scarce food reserves. This increases the pressure on individuals to migrate in search of a more reliable food source. Often, the birds that leave are the young birds, less experienced at foraging. However, because different species of birds feed on varying foods, crop failure might only impact a few species.

Interestingly, songbirds are not the only birds that might be affected by a failed seed crop. The avian predators that rely on these smaller birds for sustenance may also be driven from their normal range in search of adequate food. Such was the case in the winter of 2005-06, when large numbers of rodent-feeding boreal owls including the Great Gray, Snowy, and Northern Hawk Owl irrupted southward into northern Minnesota. The site was a birder’s bonanza as one could count up to 75 owls in one day near Duluth. Unfortunately for the owls, most were young of the year and starving. Other hazards faced by the owls of the remote Canadian forest were cars, cats, and even the loving humans that traveled so far to view them. Most of these owls had never before been exposed to humans or our accoutrements, and had not yet learned a healthy fear of us. An estimated 62% of the owls that came to Minnesota that winter never saw their homes again.

Irruptions occur only when conditions are so severe that survival is not possible in an animal’s natural habitat. Soon, we will begin to hear of more irruptions, this time from California, a state not known for severe cold during winter, but fire. So, if you notice a few extra birds at your feeders this winter, be generous.

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