COAT's prediction for the willow ptarmigan populations in Finnmark corresponds quite well with FeFo's assessments

Published on: 30. August 2022

COAT has modeled the population fluctuations of willow ptarmigan in Finnmark and looked at what influences changes in the population. The modeling is the result of a collaboration between COAT and several stakeholders, including the landowner Finnmarkseiendommen (FeFo).

Now we have the results of this year’s FeFo assessment (i.e. line-transect surveys) to compare with our predictions which was done already in the beginning of July.

COAT's prediction for the willow ptarmigan populations in Finnmark corresponds quite well with FeFo's assessments

Willow ptarmigan mating. Photo: Valeri Belov

This year's predictions (figure below) showed that we expected a further increase in ptarmigan density in 2022 on the Finnmark estate. This was due to several factor, such as a relatively normal start to the winter in the autumn of 2021, a late winter with slightly more carcasses than normal, in addition to a quite hot period in July this year, when the chicks were newly hatched. In addition, an increase in small rodent densities (increase phase) has likely resulted in reduced predation on ground breeding birds in the system. As the development of small rodent populations are difficult to predict, especially in increase years, and the fact that they often show large density variation in space, generated some excitement with regards to the quality of our predictions. We now have the results from FeFo’s assessments, based on surveys from line transects spread throughout Finnmark (Hønsefuglportalen), and it seems that we correctly predicted an increase in willow ptarmigan density (Figure 1), for the second year in a row. However, from FeFo’s own density estimates it seems that our model predicted a substantially lower density than the assessments show (see figure 1), though the density estimates from Finnmark show quite large heterogeneity in space.

The figure shows the estimated density of the willow ptarmigan population in Finnmark in the period 2000 - 2022 (red line, estimates from Hønsefuglportalen) compared with the predictions from COAT's food web model in the last 10 years (orange dots). Hence, the last orange dot (i.e. for 2022) was our prediction for this years average ptarmigan density on the Finnmark estate (FeFo), conducted already on the 8th of July. Note that our model predicts the average observed number of ptarmigan per km2(without correcting for variation in detectability), while the assessment results (red line) are given as the estimated number of ptarmigan per km2 from a model that corrects for detectability (i.e. a distance model).