Tanvir -

I had only heard of the capture - recapture method.  Checking the internet, I just now found this (and I have noted in the past that Pennsylvania State University, PSU, often has nice explanations):

 https://onlinecourses.science.psu.edu/stat506/node/58

Note that they provide a variance estimator.

Perhaps you already considered that method, and decided against it if it is too difficult to be able to tag or somehow reliably determine which animals in the second group had been seen/detected in the first.  However, I do not see how you can otherwise have any chance to collect any reliable information.

This is not one of my areas of expertise, but it looked interesting.   I found the following, apparently open access journal article, which might be of interest to you:

"Comparing opportunistic and systematic sampling methods for non-invasive genetic monitoring of a small translocated brown bear population,"

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2009.01752.x/pdf

It also mentions capture-recapture, but I have not read this article, only glanced at it, and do not know how useful it might be for you. 

I would be interested in what you decide to do, if you post it here.

Hopefully someone with expertise in this area may give you a better answer. 

Cheers - Jim 

Dr. Jim, Thank you very much for your answer.

I am one of the students who knows very little on Biostatistics, so for better understanding the lesson you mentioned i need to meet my advisor and it need some time actually. And I'll to reply you. Thanking you again.

Hi Tanvir

You could use Distance Sampling as one of the methods to estimate the density and population size provided you have also recorded the perpendicular distance while recording your flying squirrel data. It can account for variable efforts of the transects walked.  But if you don't have perpendicular distance recorded,the other alternative if to apply N-mixture abundance models if you have recorded number of individuals sighted. Have a read on papers using N-mixture abundance models and its model assumptions. A recent paper by Ecol Evol. 2014 Jun;4(11):2165-77. doi: 10.1002/ece3.1086. 

Using dynamic N-mixture models to test cavity limitation on northern flying squirrel demographic parameters using experimental nest box supplementation.

Priol P, Mazerolle MJ, Imbeau L, Drapeau P, Trudeau C, Ramière J

should be useful.

You would need to calculate detection probabilities.

Dr Sergio C. Gonzalez,

Thank you too for your opinion. 

It will be pleasing to everyone reading the paper n a quite same topic:https://www.researchgate.net/publication/269872114_Opportunistic_sampling_of_felid_sightings_can_yield_estimates_of_relative_abundance

Article Opportunistic sampling of felid sightings can yield estimate...

Tanvir -

I am glad you found our paper interesting, but I have to add that these are relative densities, which is not the same as absolute densities let alone population size. Relative densities and absence-presence data are important, but in order to get the absolute population size you have to use other methods as indicated in the previous discussion.

Cheers, Andreas

Dr Andreas Sir,

I hoped any analysis by which total population size can be resulted out with the data of opportunistic sighting records, so far i have learned that is not happening.

So, we also will have to figure out the relative abundance most probably like you, because following the loris we are unable to collect more data and other sampling. Distance sampling might possible in last 8 months we surveyed but only 4 months are left of the project. However, thanking you for mentioning about 'relative abundance'.

I followed you for Phayre's Leaf Monkey and during viewing your articles i got the article on felid sightings, made me surprised.

Best Regards

Tanvir Ahmed