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.

The Bald Eagle - A Tumultuous Journey

Photograph by David Powell


“For my own part, I wish the Bald Eagle had not been chosen the representative of our Country. He is a bird of bad moral character. He does not get his living honestly. You may have seen him perched on some dead tree near the river, where, too lazy to fish for himself, he watches the labour of the fishing hawk; and when that diligent bird has at length take a fish, and is bearing it to his nest for the support of his mate and young ones, the Bald Eagle pursues him and takes it from him.”

“With all this injustice, he is never in good case but like those among men who live by sharping and robbing he is generally poor and often very lousy. Besides, he is a rank coward: The little King Bird, not bigger than a sparrow, attacks him boldly and drives him out of the district. He is by no means a proper emblem for the brave and honest Cincinnati of America who have driven all the King Birds from our Country . . .”

“I am on this account not displeased that the figure is not known as a Bald Eagle, but looks more like a Turkey. For the truth, the Turkey is in comparison a much more respectable bird, and withal a true original Native of America . . . He is besides, though a little silly and vain, a bird of courage, and would not hesitate to attack a grenadier of the British Guards who should presume to invade his farm yard with a red coat on.”

Benjamin Franklin
January 26, 1784

So began the rocky road of the National Bird of the United States, the majestic Bald Eagle. Unappreciated and feared, all raptors were viewed as varmints by the general populace, even after the Migratory Bird Treaty Act was passed in 1918, and the 1940 passage of the Bald and Golden Eagle Protection Act. Humans have been the most significant cause of mortality for the Bald Eagle and other raptors, and it has only been in the past 35 years that their persecution has declined. Early settlers shot and trapped Bald Eagles because of their perceived threat to livestock, competition for game, or tradition such as the use of feathers by Native Americans. This practice continued; however, even after the above laws were in place. Shooting, trapping, and intentional or accidental poisonings were responsible for 38% of the mortality of Bald Eagles recovered from 1963 to 1984 (Wood et al. 1990). Over 128,000 bounties were paid in Alaska from 1917 to 1952 because Bald Eagles were thought to impact salmon fishing. Significant numbers of Bald, and especially Golden Eagles, were shot by ranchers in western states earlier in the 20th century because of suspected livestock depredations. One such incident, which occurred in Wyoming, involved the shooting of >770 Bald Eagles for which the shooters were paid $25 per dead eagle.

The eagles were further persecuted by those that collected their eggs, a common practice up until 1940. Bald Eagles also suffered from reduced reproductive success due to eggshell thinning related to DDT poisoning. Population declines of eagles, Osprey, and Peregrine Falcons particularly in the eastern US lead to the ban of this pesticide in 1972, but not before the Bald Eagle (1966 and 1978) ) and Peregrine Falcon were listed under the Endangered Species Act of 1973. Since then; however, population trends have increased for the three species most affected by this chemical. In the early 1980s the nationwide population of Bald Eagles was estimated to be 70-80 thousand birds, and that number had grown to 100,000 by 1999.

This recovery represents on of our nation’s most successful conservation stories. By the late 1990s, Bald Eagles were successfully nesting in all but 2 of the contiguous United States. Interestingly, although most citizens now revere our national symbol, today lead poisoning is considered a significant cause of mortality, and has been reported in 34 states. The source of the lead is the pellets and bullets in hunter- shot waterfowl, deer, and other game species. Other factors continue to affect populations such as collisions with vehicle, electrocution on utility structures, and an unknown number that are killed annually for their feathers which are sold on the illegal feather market.

The adults are easily distinguished by the characteristic white heads and tails, and bright yellow bills and feet. The youngsters; which are very similar in appearance to their cousin, the Golden Eagle, often defy identification. Beginning life as a mostly dark brown bird with a black bill, they undergo a variety of plumage changes before they acquire the adult plumage at age 4, when they are sexually mature. The record longevity in the wild is 28 years, with a captive Bald Eagle surviving until age 36. It is likely that, similar to other raptors, mortality is high the first year, with increasing survival to adulthood.

In New Mexico, we generally see the Bald Eagle only during the winter months, where they are primarily feeding on waterfowl, particularly wintering ducks. The numbers of eagles present here during the winter is largely affected by the weather up north. If those lakes are frozen, forcing ducks and other waterfowl further south, likewise the eagles follow their food source. By the time you read this, most of our birds will be heading north for nesting areas located near large bodies of water. Fewer than 8 pairs of Bald Eagles nest in New Mexico, mostly in the northernmost counties, but during the winter months they might be observed just about anywhere, even far from the nearest river or lake. I’ve seen them calmly roosting in canyon country far from any water source, not surprising for a bird that can go 5-7 days between meals.

Speaking of meals, this article would probably have been much different had Benjamin Franklin has his way with our fledgling country. Thanksgiving might have been celebrated in a very different style!