| The House Finch is a native of western North America, generally west of the prairies of the Great Plains. In the 1940's several cage birds were released on Long Island, New York. Their spread throughout the eastern half of the continent has been previously documented. Here I have drawn from Ohio Christmas Bird Count data to generate an animation of the expansion of this species into the state. | Although not a scientific document, this exercise has revealed a pattern of expansion within the state worthy of comment. An ad hoc analysis accompanies the animation but a few caveats are appropriate here. Although the CBC data set generally lends itself well to presence/absence type analysis, some problems involving the House Finch should be kept in mind. It is quite likely that at the earliest appearance of the species it was passed over for the much more frequent and very similar appearing Purple Finch - possibly delaying the recorded presence of the House Finch in an area. Working against this scenario is the common endeavour by volunteers on this annual census to seek out the unusual and thereby pick up on the outlier that may or may not indicate the establishment of a population in that area. For the former there is no data manipulation solution, however for the latter source of error I have chosen to record presence, NOT as first sighting but from the year at which the species is present and ALL years subsequent to that. For example, Dayton records the earliest CBC date for Ohio in 1974 but does not record it again for several years. Therefore 1974 is not the record of presence, but treated as an outlier. That is not to suggest that I consider the record in error nor should the possible significance of that record be diminished, it simply does not suit the general purpose here of showing range expansion at the scale of one state (at larger geographical scales such outliers do become interesting). Go to animation |
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