when owl populations crash: the irruption of 2000/2001
With winter barely upon us, there are compelling signs that a significant movement of northern owls is underway throughout eastern North America. Unprecedented numbers of boreal, northern hawk, great gray, and snowy owls have been observed in locations well beyond their familiar haunts in the boreal forest and tundra, sending the birding world into an unabashed tizzy. Driven south by cyclical crashes in small mammal populations, owls move to where the food is-they have no choice. In most cases, however, their appearance in the landscape is short-lived and mortality of irruptive owls is one of the unpleasant realities of their biology. For several months they will excite the masses, all the while serving as the paradigm for the cliche' of "feast or famine".
That a food crunch is driving owls towards the sun belt is obvious. Nocturnal species such as boreal and great gray owls are active during daylight hours, oblivious to the slowing traffic and spotting scopes directed at them. Their purpose is singular, their focus downward on grasses and rushes, waiting for the vole or mouse that makes the wrong move at the wrong time. As temperatures plummet and snows accumulate, each successful hunt merely means that death is delayed, not cheated.
Because the fecundity of northern latitude owls is correlated with prey availability (more prey means more young) several observations made during my spring field season hinted at the harshness of the upcoming winter. Male boreals were successful in enticing females onto territories but unsuccessful in convincing them to stay. By late March, I was monitoring five saw-whet nests. By mid-April, there were none left to monitor. By the end of April, great gray owls were seemingly omnipresent, always hunting, always moving. Birding hotlines were abuzz with owl sightings and the assumption that it was a "good year" for owls. I, however, felt otherwise.
In June, I returned to northeast Minnesota for the Boreal Birding Festival . The species generating the most conversation among birders was not the black-backed woodpecker or the parula warbler, it was the great gray owl; visible and active during daylight. Curiosity being the driving force that it is, I visited the same boreal and saw-whet cavity trees that had supported courtship and nesting activities during the spring. My hope was that the owls who had earlier experienced nest failures had recycled, and that young-rearing had been given another chance. Had "recycling" been observed, then my theories and hunches would have been dashed and I would take solace knowing that it was, in fact, a "good year" for owls. With climbing spikes on, I ascended two saw-whet nest trees and found the desiccated remains of young saw-whets, no more than a week old. In a typical nest during a typical year, prey items line the nest in varying degrees of decomposition or digestion. These nests, however, were clean.
When the long days of summer turned to fall, I set up mist nets in my Maine back yard and gathered more anecdotal evidence that a noteworhy winter was pending. Over 90% of the 120 saw-whet owls trapped were adults, with hatch year birds few and far between. The implication here is that even in the northeast, owl reproduction was limited. Furthermore, my observations suggested that we were not speaking merely of a localized, or regional crunch on owl populations, as was suggested by field results in Minnesota. Instead, we were perched on the front porch of a large-scale, continent-spanning restructuring of owl populations. In early November a boreal owl, an absolutely rare species in Maine, was trapped < 5 miles from my home and the owl crunch was on.
The irony of irruptions, from my perspective, is that they place an undue emphasis on the observation of owls in areas beyond their breeding distribution, and overlook the issue that ultimately will have the most long-lasting impacts on owl populations: the loss of critical habitat. With each old forest tract that is removed from the boreal forest zone, cavity trees are lost, roosting and foraging habitats are altered, and competition for remaining resources is magnified. In my study area in northeast Minnesota, the signs that such a change in the environment are underway are readily apparent: boreal owl numbers have declined since the late-1980's, a mere fraction of historic cavity trees remain standing, and the distribution of the owls has decreased markedly. Throw in global warming and scientific predictions that the boreal forest zone is receding to the north and the periodic winter irruptions are (and will be) relegated to nothing less than a birding novelty.
Old growth and old forest habitats are critical to all northern forest owls. Without decadent trees, cavities are not excavated and obligate secondary cavity nesters move elsewhere. And it isn't just the owls. Small mammal populations have been shown to be up to 5 x's higher in intact old forest and old growth tracts. Irruptive owls move south, but for the survivors, the return north increasingly means fewer cavity trees, fewer forests, and fewer resources.
As the winter progresses, owl sightings will increase and so too will the number of owls that meet their demise in habitats far removed from their natal forests and bogs. When spring comes again, the wintertime observations will subside, birders will anxiously await the arrival of neotropical migrants, and owl survivors will move north to sing and hoot in familiar tracts of boreal forest. There, the true struggle will begin and it is there where my attention will focus. Mortality of owls brings attention, reproduction brings hope.