Probability of No Nest

[Rodders]

I watch birds and search electricity pylons for signs of nesting birds - sad eh!. I reckon a pylon does NOT have a nest (they are not visible fom the ground) if i watch continuously for an hour and i see no activity. If there is a nest i reckon on seeing some activity within 1 hour, may be at minute 1 or in the 60th minute or anywhere in betweeen. Yesterday i could only watch a pylon for 1/2 an hour. I saw nothing so i think i am 50% certain there is no nest. If i went today, watched for 1/2 hour and saw nothing how certain could i then be that there is no nest. I guess 75%, but can't work out a proof. Then, how many 1/2 hour random negative visits would i need to make to be at least 90% sure that there is no nest? NB This is now a mathematical question only, nothing to do with birds per se.
Thanks in advance.

 

[Dr.Peterson]

I think in order to answer, you would have to have some sort of model for the distribution of visits to the nest. For example, if these were robot birds that make visits to their nest exactly one hour apart, you could give a definite answer, but it probably wouldn't be valid for actual birds.

 

[Rodders]

The premise is that visits by me and the birds are totally random, i.e. i may turn up at any time, and the birds make a movement at any time within a 60 minute period after i arrive.
Thanks for taking the time.