Hi Eric,
One way to conduct a power analysis would be to use the simulation capacity of MARK. Essentially, make a guess at the parameter values (survival probabilities for the known fate model), simulate many data sets for many different values of n (the number of marked individuals), and compute the proportion that meet your "significance" requirement for each value of n. This will give you a rough idea of how many individuals you will need to mark, and you can vary the parameter values to see how sensitive the choice. Simulations in MARK are discussed in Appendix 1 of the GIM
http://www.phidot.org/software/mark/docs/book/pdf/app_1.pdf.
You will need to think carefully about what you mean by "significance" as it doesn't sound like you have a hypothesis to test. Do you just mean that you want good estimates of the survival probabilities? In this case, you'll have to quantify what good means in terms of the size of the standard errors (equivalently, the width of the confidence intervals) for the survival probabilities or the corresponding beta parameters.
Of course, if your question is whether or not 5 marked individuals is sufficient then you probably don't need to go through all of this. If you simply wanted to estimate the survival probability over a fixed period of time and could assume that the individual all have the same probability of survival, phi, then the number that survive would have a binomial distribution with probability of success phi. The standard error for phi would then be sqrt(phi.hat * (1-phi.hat)/5), where phi.hat is the estimate of phi. In the worst case, if phi.hat=.5 then the se would be .22 and a (very) approximate 95% CI for phi would be (.06,.94), covering almost the entire real line. The situation improves somewhat if phi.hat is close to 0 or 1. E.g., if 4 of 5 birds survived and phi.hat=.8 then the se would be .18 and an approximate 95% CI for phi would be (.44,1.00), but this is still very wide. These calculations are rough because the normal approximation to the binomial is not accurate for such a small sample, but they give the general idea that any estimates of survival will be very imprecise with only 5 marked individuals.
Hope that helps.
Cheers,
Simon