The most important part of planning project with telemetry equipment is to find the RIGHT one for target species. It has to be underlined that tracking equipment MUST be safe for animal. Therefore the GPS logger weight limits should be NOT exceeded especially in case of small species.
Planning a project start from a minimal body weight of species you would like to tag. The weight limit for equipment is 3% (in case of large species it might be up to 5% BUT make sure it is still safe!) thus in case of species below 100g of body weight equipment (in total with harness etc.) should NOT exceed 3g!
Second decision to make is to describe as detailed as possible data to be received. This step is also important in case of species where equipment limits are not strict (lets say birds of 500g of body weight). In some cases it is not necessary to select "all on-board" trackers (GPS-GSM-UHF) when data from breeding season are the most valuable for project. Radio data transmission loggers (GPS-UHF) would be the best option in above case - more frequest GPS fixes (= more data) and no GSM transfer (= less costs). So, the right selection should be made based on planned research - breeding season only or breeding + migration.