The Greater Akron Audubon Society Summit Co summer bird counts (GAASSBC) and the Cuyahoga Falls Christmas
bird counts (CBC) substantially census the same area of the county. In past articles I compared the last 25 years of data
from the two censuses for population trends for some year-round resident birds.
The trend data agreed well for Red-bellied Woodpecker (Chasar 2003a) whose population has been growing rapidly and for eight other common resident birds (Chasar 2003b) whose populations appear to be relatively constant.
For all these birds the CBC showed a higher density
(birds/party hour) than the SBC and possible reasons for this were addressed.
Although the Song Sparrow (Melospiza melodia) is classified as a permanent resident in Ohio, it undergoes a definite
migration in the spring and fall, where the last fall lakefront migrants depart by mid-November (Peterjohn 2001).
complicate the issue, it was estimated that approximately 50% of the wintering population around Columbus were visitors from north of Ohio (Nice 1937). Without question the "Silver-tongue" (Dawson 1903) could be the ultimate test species for which to compare population trends between the two counts.
When I look at population trend data I use various methods to see how they compare or how the trends can best be presented. Sometimes a straight trend line or a smoothed curve can be more demonstrative. For the Song Sparrow I used a two-year moving average to compare the data. In a two-year moving average, the density (birds/party hr) is averaged
for year one and two, for two and three, for three and four, and so on. This method tends to smooth out the year to year
fluctuations and can sometimes better reveal subtle changes in a trend.
The accompanying graph shows plots of song sparrow density vs. year using the two-year moving average for each of
the two different county censuses. It is clear that the density is highest for the summer bird count, contrary to that found
for the other resident birds discussed earlier.
More interestingly, however, is that the two trend lines tend to fluctuate up and down in nearly a parallel fashion. A change in density as a result of some factor that increases or decreases the population can be observed in both
When I looked at the straight-line trend for both sets of data, both were slightly down, contrary to the state-wide
Breeding Bird Survey (Earnst 1996) which indicates a slight overall increase. It would appear that the two censuses in
Summit Co are very consistent with one another.