Thursday, July 28, 2011

Blinded by science

Actually, the title should read blinded by bad assumptions while in the field performing science! I have previously written of my adventures studying Northern Goshawks in the South Hills of the Sawtooth National Forest of Idaho. In the previous reports I have noted the discovery of a dead male goshawk in one of the nesting territories we were studying. Well, not so fast...

One of the nesting territories of my study remained inaccessible until very late in the season due to snow. Once we did arrive in the territory, I quickly received a response from a adult female goshawk. Her response was consistent with the presence of a nest very close. The presumed nest stand was fairly small, but after numerous searches we failed to find a nest. However, the female goshawk was easy to find in the same stand day after day.

One day Lauren found a pile of feathers about 150m from where we usually found the female. The feathers were definitely raptor feathers and seemed consistent with a goshawk, although we did not have a feather reference with us.

Feathers discovered in the South Hills.

Due to the territory where the feathers were found and the proximity to the female goshawk, we assumed this pile of feathers belonged to the male goshawk of the territory. This one assumption played a critical role in our interpretation of the female goshawk behavior as I will illustrate below. However, upon further inspection...

Returning home after the season, I accessed the online feather atlas to confirm the identity of the feather owner. They unfortunately do not have male goshawk feathers available, but do have adult female feathers.

Hmm. We have a problem here. The colors don't match, but goshawk colors vary significantly. The shape seems a bit off as well. But the size is the real issue. Our collected feathers are longer than these female goshawk feathers. In most raptors, including goshawks, the female is larger than the male. In goshawks the female can be 15% larger. I would expect male goshawk feathers to be shorter... Well, there is only one other gray raptor in this area. The male Northern Harrier.

Well, that is better, but what about those assumptions...

We observed the female goshawk in the stand on six different days. On each occasion we were there for at least an hour searching and watching her behavior. This behavior was interpreted through our assumption of the male being dead. For example, if the male was dead and the female had a nest with live nestlings she should be hunting most of the time. In our case, she was perched in the stand on every occasion of our visit. This was inconsistent with a nest full of hungry nestlings. On the other hand, if the male was out hunting all of the time then the female could remain back to guard the nest stand. This could be consistent with a nest full of nestlings.

Another challenge in the South Hills is that we would occasionally hear a goshawk call which was unfamiliar. This unique call was omitted from most references regarding goshawk communication. On one occasion the female issued this call and flew like a rocket over my head and to the edge of the stand. With the assumption of the male being dead, I could not explain this behavior. Maybe stand defense? I was confident that her response was not directed at me, but I had previously seen her respond to a Common Raven, but not in this manner. Upon further research I have found one description of a female "dismissal call" which is issued by the female to the male, often as the male delivers food. Could I have witnessed a male prey delivery and not even recognized it? This would be a indication, although not a firm confirmation, of a nest with live nestlings in the area. I have yet to find a recording of the "dismissal call" to confirm this, but we have heard three other females give the same call, on each occasion the male was also present. Once again, the assumption of the male being dead and my lack of experience with the "dismissal call" clouded my interpretation in the field. Yes, experience does matter! 

Next week I hope to return to the stand one more time. By this time any nestlings should have fledged, but would still be expected to be in the area. Nestlings are easily identified by their begging calls. Begging calls would allow me to classify the territory are a successful nesting territory even without finding the nest. Otherwise, it will remain as "occupied".

Next year I will definitely have more resources in the field including a printed feather atlas!

1 comment:

Anonymous said...

Fascinating findings and discussion. I enjoy reading and getting the sense that I am observing an excellent logical mind at work.
Dean Lapinel