Montane InvasionsA Historical Perspective
This winter we enjoyed exceptional numbers of Mountain Bluebirds and Cassin's Finches, a flock of Pinyon Jays drew birders to Lake Cuyamaca, and a couple of Steller's Jays made it to the coast. These events lead us to reflect on the phenomenon of montane invasionsthose irregular variations in the numbers of mountain birds wintering south of their breeding ranges. Every few years birds that feed mainly on fruit or conifer seeds like thrushes, finches, and corvids move out of their usual ranges in numbers much larger than normal. Sometimes invasions of certain species coincide, sometimes not. This ever-changing kaleidoscope of unpredictable combinations is one of the prime factors making birding an endlessly fascinating pursuit.
We learn a great deal about each species by researching its distribution and biology individually. Here, though, I'd like to look at montane invasions historically across speciesa perspective I hope will enhance our understanding of seemingly random events.
Montane invasions have undoubtedly been happening since before there were ornithologists and birders to observe and write about them. The earliest evidence I find in the history of San Diego County dates back to 1876-1877. In that winter pioneer naturalist Frank Stephens saw flocks of Pinyon Jays and Clark's Nutcrackers at Mount Laguna and collected a Red Crossbill at Campo. In the first half of the 20th century there were still few observers in the county and the record is sketchy. Nevertheless, we know that Varied Thrushes invaded in 1906-1907 and 1924-1925, and Clark's Nutcrackers staged their biggest invasion ever in the fall of 1935. Flocks of 50 to 60 were common then in the Volcan and Cuyamaca mountains. Pinyon Jays were reported from Vista at the same time.
In the past 50 years, the record is far more detailed. One of the biggest invasions, involving the Pinyon and Steller's Jays, Clark's Nutcracker, Evening Grosbeak, and even the White-headed Woodpecker took place in the winter of 1955-1956. In 1963-1964 Pine Siskins and Red-breasted Nuthatches invaded, leading to the nuthatches' establishing their isolated breeding colony on Point Loma. In 1966-1967 it was Pygmy Nuthatches and Red Crossbills. Crossbills remained through the spring of 1967 at Point Loma and attempted to nest.
The invasion of 1972-1973 was another big one, with a wide variety of species participating: Clark's Nutcracker, Pygmy Nuthatch, Varied Thrush, Evening Grosbeak, and Williamson's Sapsucker. The only flock of Bohemian Waxwings ever recorded in San Diego County (up to 6 at Yaqui Well) occurred during this invasion. The winter of 1975-1976 brought us Pine Siskins, Red Crossbills, and the largest flock of Cassin's Finches ever recorded in San Diego County.
Golden-crowned Kinglets invaded in 1976-1977 and in several subsequent winters through 1985. Apparently it was these events that led to this species' establishing itself as a rare summer resident on Hot Springs and Cuyamaca mountains. Lewis' Woodpeckers occurred in the highest numbers ever recorded in 1982-1983, when over 60 were on Palomar Mountain. The winter of 1984-1985 witnessed another multispecies event, including Mountain Bluebirds, Purple Finches, and Red Crossbillsup to 25 of the last on Point Loma. Two years later Evening Grosbeaks occurred in record numbers; Ken Weaver encountered up to 40 on Palomar Mountain. In 1989-1990, Varied Thrushes invaded, as did the biggest flocks of Pinyon Jays ever reported in San Diego Countyup to 200 in the Laguna Mountains. An 11-year gap followed until the Pinyon Jay reached San Diego County again this winter.
In the 1990s, the multispecies invasion of 1994-1995 would have been remembered more if it hadn't been eclipsed by that of 1996-1997, the most spectacular since 1972. This was the year that brought three Clark's Nutcrackers to Buddy Todd Park in Oceanside and Red Crossbills and Red-breasted Nuthatches practically everywhere.
Do we see any pattern in these events? Pinyon Jays often coincide with Clark's Nutcrackers, Varied Thrushes with American Robins, Red Crossbills with Pine Siskins. But exceptions are striking. In 1992-1993 Pine Siskins but no other species invaded; the next winter, it was the Mountain Bluebird but no other species. In most cases, it is probably scarcity of food that drives these birds to wander. The food supply may fail in some regions but not others, accounting for the invasions' irregularity. The past three years of drought over much of the western United States, combined with widespread forest fires last summer, may have compelled many birds to flee their usual haunts. Will their populations crash, then rebuild slowly, until the next catastrophe triggers the next invasion? In some cases, we see the invasions as precursors to colonization and range expansion. Clearly, becoming nomads when necessary is for many species a strategy for survival.