What we're learning:
Biennial Irruptions of Pine Siskins Across North America
To view larger versions, click on each map.
The spring migration pathway of the Black-throated Green Warbler.
The year-round resident range of the Tufted Titmouse
has gradually expanded northward over the past 100 years
Even though range maps in most field guides suggest that one could predict exactly where a bird species may or may not be found, for many species the picture is actually more complex. How a bird population moves across the landscape and within and among seasons varies from one year to the next and varies across species. See two examples in the table to the right.
Continentwide monitoring projects such as Project FeederWatch and the Christmas Bird Count allow us to identify patterns in bird population changes. A closer look at one finch species that often visits feeders-the Pine Siskin (Carduelis pinus) has revealed some notable trends.
Pine Siskins are distributed across much of Canada and at higher elevations in the western portions of the United States. Periodically, they will irrupt into parts of the East and into areas of lower elevations in the West. The animated map of Project FeederWatch sightings during the month of January between 1991-1998 demonstrates that for this period, the southern irruption of Pine Siskins is biennial, meaning this species irrupts every other year.
What is peculiar about the biennial irruption of the siskin is that it is asynchronous. That is, when Siskins irrupt into Southern California they are not irrupting into the East. And, when they are irrupting into the East, they are not irrupting into Southern California.
But the asynchronous pattern in the winter movements of Pine Siskin has not always been the norm. As curious as this biennial, "see-saw" pattern is, movements of the Pine Siskin are even more complex when viewed across decades. Christmas Bird Count results indicate that between 1978 and 1992, the pattern of Pine Siskin irruptions was not asynchronous, but much more variable. The graph below comparing Christmas Bird Count results between 1979-1998 from Southern California to reports from North Carolina demonstrates this variability. During this time, the general trend is for biennial irruptions. But sometimes the bird's population numbers remained low in a year that the pattern would prescribe them to be high (see blue arrow). Other times, the bird's population numbers remained high in a year when the pattern would prescribe them to be low (see black arrow).
What is it that drives this pattern of biennial irruptions for Pine Siskins, and why is the pattern occasionally broken? It is unknown precisely why Pine Siskins irrupt some years and not others, but for other closely related finch species there appears to be an association with food production on the wintering grounds. For example, the catkin production cycle in birches (Betula sp.) appears to be correlated with invasions of Common Redpoll (Bock and Lepthien. 1976. Synchronous Eruptions of Boreal Seed-eating Birds. American Naturalist 110: 559-571). When birch catkin production is high, Common Redpolls remain in Canada, but when catkin abundance is low, they irrupt southward into the United States. There is speculation that this variation in food production is an evolutionary strategy that forces these birds south every few years, thereby reducing their long-term impact on the plants. The same may be true for Pine Siskins. In years when Pine Siskins appear in either Southern California or North Carolina, food abundance in their typical wintering grounds may be low.
The data submitted by the participants of long-term bird population monitoring programs such as Christmas Bird Count and Project FeederWatch are what make these fascinating observations possible. Continued long-term monitoring of bird populations on a continentwide scale will only enhance our knowledge and understanding of wild bird populations. We encourage you to get involved.