(A shorter version of this article was originally published in the January/February, 1997, issue of this newsletter.)
In the fall of 1996, my late wife Shani and I went to Kitt Peak, about an hour southwest of Tucson, to do some birding and see the many telescopes located there. The serpentine road that winds its way up the northern end of the Quinlan Mountains on the Tohono O’odham Nation ends at the National Optical Astronomy Observatory (NOAO) that houses one of the most extensive and diverse collections of astronomical observatories anywhere in the world. The 30+ large telescopes occupy the top 300 feet of the 6880’ peak. While there are many other astronomy sites around the world that are located at higher elevations, Kitt Peak is far enough away from Tucson’s light pollution, and the air is generally clean and clear enough most of the year for excellent professional viewing of the night skies. We spent a few hours touring the observatories and the gift shop there. But, it was the possibility of seeing/experiencing an unusual birding occurrence that initially compelled us to take the trip.
Within hours, word had spread throughout the Southern Arizona birding community that an irruption of Clark’s Nutcrackers had invaded the mountaintop. Irruption is a phenomenon that only occurs every several years and is closely associated with a dearth of native foods in a given habitat. The failure of an important food source causes a location’s population of a given species to wander out of their familiar territory or range in search of other foods. Apparently, this was the reason why Kitt Peak was experiencing a highly unusual influx of Clark’s Nutcrackers, members of the jay family that grow to lengths of 12”. The nutcrackers, aptly named, normally occupy high elevation mountain sites that are rich in pine nuts. These birds are usually found in elevations between 8000’ and 12,000’. We had previously seen them in good numbers at Rocky Mountain National Park and other timberline locations in the Rockies. This far south, we had only seen them occasionally at the highest elevations, atop Mount Lemmon and Mount Graham.
Clark's Nutcracker photo by C. Friedman
Seeing them as far south as Kitt Peak, and lower in elevation than we would normally expect, we deduced that the nuts of the Apache and Ponderosa Pines weren’t sufficient to support the community of Clark’s Nutcrackers in their typically higher Southern Arizona habitat. Kitt Peak has Pinyon Pine and oak habitat, to which the nutcrackers seemingly adapted easily. We walked around the entire acreage of the observatory campus and saw more of these birds that day than we have observed in all our years of birding, before or since.
Later that year, in late December and early January, Red Crossbills were reported along the San Pedro and Santa Cruz Rivers, in Patagonia, and even right in the neighborhoods of Tucson that have large, mature pine trees. The crossbills are 6+ inches, large among the finch family, and have mandibles that cross – much like a pair of scissors do. This unusual beak formation allows them to forage easily on the cones of various pine species of the higher, boreal forests. It seemed that the pine seed/nut crop failure affected more than just the nutcrackers. Customers of ours who lived in the Sabino and Bear Canyon areas were seeing flocks of Evening Grosbeaks at their feeders as well. These grosbeaks are rarely seen at those lower elevations in Southern Arizona, unlike the Black-headed and Blue Grosbeaks seen at other times of the year in this region.
Great-horned Owl photo by Diane Poleyquiva
Seeing all three of these species in the same time period provided birders here with direct and first-hand examples of the irruption phenomenon. Irruptions occur when an avian population, which is a permanent resident of one location, moves to another area in a dramatic way, but on an irregular, and sometimes unexpected, timetable. North American species that are known to occasionally irrupt include: Bohemian and Cedar Waxwings; Pine Grosbeaks; Black-capped and Boreal Chickadees; Pine Siskins; Red-breasted Nuthatches; Common and Hoary Redpolls; Purple Finches; Snowy, Great-horned and Short-eared Owls; Rough-legged Hawks and Northern Goshawks; as well as the Red Crossbills, Evening Grosbeaks and Clark’s Nutcrackers mentioned above.
When the seed/nut crops of the high elevation spruces, pines and other coniferous trees fail, the birds that depend upon these foods for their existence are forced to travel/relocate to other lower altitude habitats to survive.
In years blessed with good crops at high elevations, larger populations of the birds that consume them are amply supported. If a year of crop failure follows an abundant one, bird populations may be large enough to seriously stress scarce food resources. This will result in an increased urge to migrate out of their normal range in search of adequate food. While crop failure is a primary reason for irruptions, there can be other factors that will affect populations as well. Avian population density is a secondary reason because other factors, such as insect abundance during breeding season, can have a great impact on the total numbers of a species in any given year. Not all species are affected by a seed/nut crop failure simultaneously. Crop failures can result when forests suffer the effects of droughts, wildfires, habitat fragmentation and loss of habitat.
In my decades living in Southern Arizona, there have been some winters that I noticed an abundance of Pine Siskins at my feeders. During those infrequent times, many customers would call to inquire about these birds that are not common visitors. Casual birders may not even notice them, as Pine Siskins are members of the larger finch family, and they resemble some of the females of other finch species. However, they generally tend to be a little bigger than most finches, and the striping on their breasts and bellies tends to be more pronounced.
What about the raptors listed as North American species known to irrupt? While the success or failure of seed crops has a direct impact on birds of prey like owls and hawks, the number of small mammals and rodents that raptors feed upon could also play a role in their irruptions. Among tundra and grassland rodents, there is a four-year cycle of abundance and scarcity. For snowshoe hares, there is a similar cycle – but it’s a ten year cycle! The exact reasons for the cyclic flux in the numbers of these mammals is not known. What is known is that whenever the population of the rodents or hares “crashes”, their avian predators irrupt to more southern areas where they can find a better and more reliable food supply.
Northern Goshawks feed on hares and rabbits. They irrupt about every ten years when the bottom drops out of the snowshoe hare populations. These irruptions are often exacerbated by dense populations of raptors that occur during the preceding years of abundant prey. Snowy Owls, Northern Shrikes and Rough-legged Hawks all depend on lemmings and voles for their survival. When the population of lemmings and voles crashes about every four years, these predator birds irrupt. It is a big deal in the birding world - even the local news sources tend to take notice - when Snowy Owls, birds of the extreme north, suddenly appear in good numbers in the lower 48. Due to their large size and all-white coloration, as well as their rarity in the states, they quickly become newsworthy, and the general public is alerted to their sudden appearances. As they are rarely seen south of Canada, birders flock to sites where they are reported. New England is perhaps the region of the U.S. where they are more likely to be seen or encountered. They have been recorded in locations as far south as Boston and even St. Louis. Not too many years ago, Washington, D.C. had a Snowy Owl on the banks of the Potomac River. It made the national news! This winter, Wisconsin and Minnesota experienced a major Snowy Owl irruption.
Sometimes only the young birds of a species irrupt. From 1961 to 1963, an invasion of Northern Goshawks occurred on the west shore of Lake Michigan. Most of the birds examined by Mueller and Berger, two ornithologists who investigated the irruption, had hatched in 1961. The young goshawks were hatched shortly after a steep drop in the numbers of snowshoe hares and grouse, the mainstays of their diet. Mueller and Berger thought that this irruption of predominantly young hawks was due to their displacement by well-established, fiercely territorial adult hawks during a time when their normal range could not provide for such large wintering numbers of hawks.
If you notice a bird species, or several birds, that are not usually present at your feeders, give some thought to the possibility of an irruption happening right in front of your eyes. You can call the Rare Bird Alert to hear what unusual species are currently being reported in our area. You can also call the store with your reports. Remember, nature’s ebb and flow is happening all around us, perhaps even now right outside your kitchen window!