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Range Expansion of the Pileated Woodpecker in North Dakota


The pileated woodpecker is increasing throughout its range (Sauer et al. 1996, BirdSource 2000b) and is reoccupying areas not inhabited since the early 1900's (Mumford and Keller 1984, Thompson and Ely 1989, Robbins 1991). Breeding Bird Survey data indicated that populations of the pileated woodpecker significantly increased during 1966 to 1991 in North America (Bull and Jackson 1995). CBC data indicated that populations of the pileated woodpecker increased 1.3% per year from 1959 through 1988 (Sauer et al. 1996). Trend analysis for CBC's was conducted only on the 34 states and three provinces for which there were consistent sightings from 1959 through 1988, which did not include North Dakota (J. R. Sauer, U.S. Geological Survey, Laurel, Maryland, pers. comm.). Population increases also are occurring in states adjacent to North Dakota. In Minnesota, numbers of pileated woodpecker increased 1.5% per year from 1959 through 1988 (Sauer et al. 1996). The pileated woodpecker has become established in northeastern South Dakota, and nesting has been documented (Harris 1987, 1995).

Possible explanations for the range expansion of the pileated woodpecker include: an increase in the number of people that enjoy birdwatching and thus observe the species; exclusion of fire leading to an increase in numbers of trees; an increase in dead and dying trees due to expansion of tree diseases, such as Dutch elm disease, or to natural death of old trees along major river systems; an increase in arthropod abundance due to tree diseases; reestablishment of forests that had been cleared in the 1900's; normal fluctuations in woodpecker populations; or responses by pioneering woodpeckers to climate change (Hengeveld 1989, Robbins 1991, Johnson 1994, Bull and Jackson 1995). The surge of sightings west of the Agassiz Lake Plain could be attributed to increased numbers of people observing the species; more plausibly, the pileated woodpecker is expanding beyond its historical range due to population growth and dispersal, or to the introduction of tree diseases into North Dakota.

Although the mechanism or combination of mechanisms responsible for the increase in numbers of pileated woodpecker in North Dakota is unclear, the westward range expansion is irrefutable. The increase in number of sightings in the Agassiz Lake Plain and the reports at new locations west of the Agassiz Lake Plains after 1966 attest to range expansion. Prior to 1986, 184 sightings occurred in the Agassiz Lake Plains, compared to 300 after 1985. Six sightings occurred west of the Agassiz Lake Plains prior to 1986, compared to 54 after 1985.

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