Well, as usual in ecology, that will depend on your study species and the question that you want to answer. How variable is the behaviour you want to study between individuals/treatment groups? If it is very variable, you will need a larger sample size to figure that out. You would want to have enough 'adequate sample size' in each of the categories which have (or have not) different behaviours (sex classes, age classes, study sites/treatments.....). If males move a lot and females don't than 10 of each could be sufficient. If males only move slightly more than females (but for some reason that's biologically meaningful) then you will need a larger sample size to be able to show this difference.

Additionally I would like to mention that especially GPS studies are prone to accuracy problems, so that should be taken into consideration when designing the study (is an error of 50-100m acceptable, can I reach errors of only 4-5m with my study species, environment, GPS tag, or do I need more precision to answer the questions that I have?

And some general questions for the tracking study that should be answered first: what tags can I use with my study species, that will dictate battery life, precision, trackable distance etc. Also, what is the environment you track in: are bouncing signals a problem, dense shrub or canopy cover, are your animals hiding in crevices a lot? Do you really need GPS tags, or can the question be answered (maybe even ebtter answered) with other tagging, genetics, etc? How many locations do I need to get a meaningful answer which is also dictated by battery life needed, environment, animal behaviour? etc.

Good question. Surely, it depends on the study, but I don't think you have to have a large sample size. Yes, you should try to track as many individuals as possible (regarding to the costs of the transmitters, the biology of the tracked species, the risk factors etc.), but there are papers published in fairly good journals (like J Ornith), with data from no more then 3 individuals fitted with geolocators (not satellite tracked, but basically the same thing, with 11 birds fitted with geolocators), see here: http://www.aquaticwarbler.net/download/Pub_various/Salewski_et_al_2012_Unknown_AW_migration_route_%20JOrnithol.pdf

Another study is the ongoing Lesser Spotted Eagle LIFE project from Romania, where from 15 birds fited with satellite tracking 11 died or lost their transmitters (more info on pomarina.ro). So, because of the high risks, I think that journals may accept data from few animals, but you should interpret your data carefully. Hope it helps:)

depends on your species but satellite tracking rarely gives reliability and validity, it is too expensive for many repeats and the animals rarely do the same thing. look at turtle, whale shark or great white shark studies. Fantastic information from 1 individual.

I have a question on a very related topic so I just post it inhere. How much are GPS-trackers? I tried to find that out, but I didn´t find any definite prices on the internet, even though I found a bunch of very interesting products. I know that there is very likely a rather large range. But how much e.g. are high end and current products? And what are good products? Thanks in advance!

In general terms a GPS satellite platafform transmitter terminal (PTT) is around $3.500 per unit. You have to add +-$1.000/year for data retrieval and downloading via Argos system. There are other alternatives such as GPS dataloggers, which you do need to recover the bird. In other cases you can retrieve information by approaching the device (usually within a circle

Otago University (NZ) is currently developing and testing transmitters that use mobile phone tower signals to triangulate the position of animals. If you have mobile phone reception in your area, that might be an option. The transmitters are lots smaller and cheaper than your average GPS transmitter.

I would strongly encourage you to look at using GPS technology with Iridium satellite upload of data. This is the very best current technology and a unit can be obtained for about $2,500 US.

I think the answer to your question depends on the species and your research question. I was able to publish with an n of 5 because I put data loggers on a species simply to test a hypothesis about where the species spent the winter which was more or less unknown. In the process, I found the bird had an unexpected third migration.

Further to the excellent points made so far, I would also be sure to account for ~10% loss rate for marked animals, or more depending on the study area and species. The technology is improving but failures of the technology do occur, animals wander way outside of the study area, transmitters fall off or are ripped off, and animal mortalities from predators, capture effects, hunters, weather etc mean that there will be some attrition in sample size so you should deploy at least 10% more than your required sample size to account for these losses.

It seems we are talking about devices on birds here. I know more about mammals but I would recommend satellite GPS collars (Iriidium --> yes) than mobile phone technology. First, if your species migrates and goes in Africa for example where little (even if developing) mobile phone coverage, you can loose a lot of data compared to satellites GPS: ex. your device acquires 2 months of data but does not enter a mobile phone covered area --> the data is in the collar but if the collar fails, it is lost. So except if you know that your species is and stay in mobile phone covered area, don't use it. Then we had cases (in Africa) where mobile phone company were holding the "collar data" to give priority to real phone data by users and the GPS data was almost lost.

Percentage of loss --> I would plan in advance in my protocol more than 10% loss: account for 30 or 50% loss (or at least think if you can do something with only 50% of the data you want). Our last batch of GPS satellite collars on buffalo (from a well-known company) --> 15/20 failed after 6 months... They will replace them but still... Percentage of loss depends on the species as well.

GPS data loggers --> it can be a trap if you don't know well the behavior of your species (which usually you don't in that kind of study). You need to be close to your collared animal for quite some time depending on the amount of data stored in the collar... It can take time and the animal moves and gets out of the range... It can be a pain.

Finally if you don't yet know about this paper:

Cappelle et al. 2011 in Ostrich.