I am trying to compare if the proportion of one bird subspecies in a roosting area is different now from over 20 years ago.
My dataset looks like following:
-In 1996 birds were counted at a roosting site and were identified as one of two possible subspecies. These counts were done during 3 two-week periods.
-In 2019 birds were again counted at the same roosting site and the same 3 two-week periods, and were also identified as one of two subspecies.
I want to know if:
a) Are the subspecies' proportions different for the different two time periods?
b) Are the subspecies' proportions different in 2019 from 1996?
c) Is there an interaction between a) and b)? I do not expect this but I don't know if this should be included in any potential model.
I have considered using a GLM, but I'm not very familiar with how they work so I thought I'd better double check first. I am trying to do the statistics using R.
Thank you very much for your answers!