Advice on a Survivorship Model for the African Fish Eagle

[aeichenwald]

Hey everyone. I have a question I was hoping someone could help me with. I’m new to program MARK – I’ve run through the Gentle Introduction book to get familiar with it – and I need to use the program to make a survivorship model for a population of African Fish Eagles I’ve just begun to study.

Based on the data I have available, I was thinking that a mark-resight model would work best. I was thinking of using the individually identifiable marks model with (zero-truncated) Poisson log-normal estimator [(Z)PNE. However, I’d like to get another opinion from the experts. Is there something else that I should try instead? Or am I approaching the problem in the wrong way?

Here’s a little background information: African Fish Eagles have defined territories, are very easy to spot, and can sit in one place for extended periods. Therefore, I have accurate surveys of the fish eagle population in my study area going back to 1968 (with some sizable gaps when there were years that surveys were not performed). A small number of birds in the population were banded in 1997, 2009, and 2010. The method of “recapture” is to throw a fish into the lake, which will tempt the eagle into flying out over the water to grab a free fish. When it does this, it reveals the band on its leg (which is then captured by a camera). However, the birds don’t always fly to grab the fish, and so there are periods where many of the surveyed birds may be banded without the knowledge of the scientists performing the survey. In addition, the resighting protocol is not performed every time the surveys are conducted, and fish eagles don’t necessarily keep their same territories from year to year. Furthermore, we aren’t sure if the population is closed or open. We think it is closed, but one of the goals of the model is to determine if the fish eagle population is stable or is acting as a sink (which would imply that the study area is a place where birds come to die).

I look forward to hearing what you think! Again, I’m very new at this so any help you can provide would be welcome, including pitfalls you think I should watch out for. Also, if there is any more information I can provide to clarify anything, let me know.