Population trends of less-common raptors in Boulder County

Another question of possible interest is the distribution of the less common raptors: Bald Eagles, Golden Eagles, and Northern Harriers. Are they significantly more common on one of the routes than the others? The straightforward answer is that the differences aren't large.

There are a few inconsistencies in the giant spreadsheet from which the data were taken to produce these graphs. Two conspicuous cases are both in the Rabbit Mountain data, in which the "By Species" sheet and the "by Survey Route" sheet have different values for Rabbit Mt data -- Bald Eagles in 1990 and Golden Eagles in 2004. These were resolved in producing the graphs below by assuming that the sum over all routes for those years is correct and adjusting those two values accordingly.

We examine first the Bald Eagles. One might expect that they would be more common on the Rabbit Mountain and Boulder Reservoir routes, because the routes pass close to two large reservoirs -- Lagerman Reservoir and Boulder Reservoir. This expectation is borne out by the data.


Golden Eagles have occurred with similar frequency on all three routes.

Golden eagles

Northern Harriers are less common than eagles on all three routes (note the different scale on the graph). They are extremely scarce in the South County, even in the earliest years in the chart. This difference can probably be attributed to limited suitable habitat and to increasing development. On average, the frequency on the other two routes is similar and quite variable.

Northern harrier

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