Tuesday, November 18, 2008

How Redpoll Numbers Fluctuate - 40 Years


A reflection on past winters came up today on the Wisconsin BirdNet. Jerry DeBoer recalled the years when redpolls and some other winter birds seemed to him to be more numerous in southern Wisconsin. This graph shows how Common Redpoll numbers have fluctuated in Wisconsin over the past 40 years. Remember the vertical axis shows "birds-per-party-hour" - this is an excellent way to visualize these data. The numbers across the horizontal axis are count years (as they are identified by N. Audubon - thus, count 107 is the 2006-7 CBC). It's been a reasonably-true (at least much of the time) statement that redpoll numbers show a two-year cycle -- higher one year, lower the next year --but this does not always hold. The gradual difference in amplitude of these changes does seem to indicate that numbers on Wisconsin CBCs are generally lower than they were, four decades ago. Or were several of the late 1960s just banner years for this species?

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