Hi all,
I'm currently Distance software on Windows to estimate the population densities of 5 species of birds in a certain region that was surveyed using 5-min point counts. But the results that I get look much higher than possible. I have a few questions and would like to seek someone's advice on how to fix the issue of over-estimation.
Each analysis is performed separately on each species individually using CDS. Data is being collected monthly over a span of 2 years. (survey effort = 24 per yr, 1 per mth). Data used for analysis is being sorted by month, then by point. survey effort has been put as 1. (Should survey effort be 1 or 24 if data is sorted by month, not by points? -> I noticed that Point 1.1 is being treated as a different point by software if they aren't arranged together in data. Since it's sorted by months, they are separated. I've tried sorting by points, and survey effort = 24 and I got similar values.) Distance data was binned, 10m, 50m, 100m, 200m. Intervals of 0 - 10m, 10 - 50m, 50 -100m, 100 - 200m were created.
All the possible combinations of Key Functions and Series Expansions have been used. (4 x 3 models). I've selected AICc for model selection, although selecting AIC didn't seem to fix the issue. I have stratified my data by month in the system, adjustment terms were automated selection, with max 5 terms, and Strictly monotonically non-increasing was selected for Constraints. For Cluster Size, means of observed clusters were selected.
Here's the result I got (pooled estimate). I also have the estimates for each month. I've looked at the Chi-square GOF for the model for each month and some of them were p = -1.0000 or p = 0.0000. (indicating over-fitting or perfectly fitting models? Either way is not a good model - please correct me if I'm wrong.)
I've tried truncating my data at 100m (As data is binned, 10% will automatically truncated all 200m data, which is equivalent to truncating at 100m.) But the estimates were no less ridiculous.
I would greatly appreciate any advice on how to fix such problem. Thank you for taking the time to read and troubleshoot!