As I approach my second field season studying Northern Goshawks in the Sawtooth National Forest, it is difficult for me to tell if I am more excited than last year. Last year, I was filled with the general uncertainty about the field season, my methods, etc. This year, while I know the methods, the territory, etc, I have a whole new set of items to think about. Along with preparing lists of gear to organize, preparing field data sheets, printing maps, etc, there are three larger issues which consume my thoughts. These three are fun to think about, but well outside of my control.
Belding's Ground Squirrels (Urocitellus beldingi). My first year study of Northern Goshawk diet within the Minidoka Ranger District of the Sawtooth National Forest has shown that Belding's Ground Squirrel make up the majority of the diet of breeding goshawks. At least 2/3 of total biomass consumed came in the form of Belding's Ground Squirrels. It is likely that goshawks could not breed in the area without this resource providing key nutrition during the high growth phase of the nestlings. My estimation based on size of the prey delivered to the nest, was that approximately 95% of the ground squirrels consumed were juveniles. As male ground squirrels generally disperse from their natal den, many were probably juvenile males. The females tend to stay in or near the colony they were born in and thus have more protection in numbers (detection of predators).
Anyway, last year the field season was hampered by late spring snow. In fact it snowed on us nearly every day in the field for the first 2 weeks of my season! While further exploring the life history of ground squirrels, I discovered a paper indicating that late spring snow can completely suspend the breeding season of Belding's Ground Squirrels (Morton and Sherman 1978). If the breeding season did not occur last year, no young would have been hatched underground, and none will emerge this spring. The results would be catestrophic for the goshawk breeding season. The ground squirrels were so abundant last year that it seems unfathonable that there would be a prey shortage, however a shortage of young naive dispersing ground squirrels in unknown territory could make all of the difference. The adults are likely much more difficult to catch. Only time will tell.
Spring Weather. On the topic of spring weather, the jury is out on what this season will bring. Will there be heavy snow in May, or will the current trend of a warmer than average spring continue? The chart below shows the snow pack we dealt with last year(blue) and what we have seen so far this year (red). Last year the snow was 20% higher than normal and melted off about 3-4 weeks later than average over the past 40 years. In average conditions, the snow pack would be gone at about my start of the field season. The trend is clearly moving toward that, but the next month will reveal the secrets.
Crew. This year I will have two new crew members. My field tech from last year decided to move on to work on a project much more aligned with the future direction of her studies. I can't blame her for that! This means that I will have two new people to hire, train, and rely upon to get the work done.
All of these items plus the daily challenges presented by field work in general make this an exciting project. I can't wait to get out there and see what the season presents. In the mean time, its back to logistics.
Morton, M.L. and P.W. Sherman. 1978. Effects of a spring snowstorm on behavior, reproduction, and survival of Belding’s ground squirrels. Can. J. Zool. 56: 2578–2590.